Tutorial is intended for students of the direction "Biology" of all profiles of training, all forms of education for theoretical preparation for classes, tests and exams. The manual covers the main sections of structural biochemistry: structure, physico-chemical properties and functions of the main classes of biological macromolecules. Much attention is paid to a number of applied aspects of biochemistry.
Nucleotides and nucleic acids
Structure of nucleotides and nitrogenous bases
Nucleotides are involved in many biochemical processes and are also monomers of nucleic acids. Nucleic acids provide all genetic processes. Each nucleotide is made up of three types chemical molecules:
nitrogenous base;
Monosaccharide;
1-3 residues of phosphoric acid.
Unlike monosaccharides, nucleotides as monomers are complex molecules consisting of structures belonging to different classes. chemical substances, therefore, it is necessary to consider the properties and structure of these components separately.
Nitrogenous bases
Nitrogenous bases are heterocyclic compounds. In addition to carbon atoms, the composition of the heterocycle includes nitrogen atoms. All nitrogenous bases included in nucleotides belong to two classes of nitrogenous bases: purine and pyrimidine. Purine bases are derivatives of purine - a heterocycle consisting of two cycles, one five-membered, the second six, the numbering is as shown in the figure. Pyrimidine bases are derivatives of pyrimidine and consist of one six-membered ring, the numbering is also shown in the figure (Figure 31). The major pyrimidine bases in both prokaryotes and eukaryotes are cytosine, thymine and uracil. The most common of the purine bases are adenine and guanine. Two others – xanthine and hypoxanthine– are intermediates in the processes of their metabolism. In humans, the oxidized purine base acts as the end product of purine catabolism. - uric acid. In addition to the five main bases mentioned above, less widely represented minor bases are also known. Some of them are present only in the nucleic acids of bacteria and viruses, but many are also found in pro- and eukaryotic DNAs and transport and ribosomal RNAs. Thus, both bacterial DNA and human DNA contain significant amounts of 5-methylcytosine; 5-hydroxymethylcytosine was found in bacteriophages. Unusual bases were found in messenger RNA - N 6 -methyladenine, N 6 , N 6 -dimethyladenine and N 7 -methylguanine. Bacteria also have a modified uracil with an (α-amino, α-carboxy)-propyl group attached at the N 3 position. The functions of these substituted purines and pyrimidines are not fully understood, but they can form non-canonical bonds between bases (this will be discussed below), providing the formation of secondary and tertiary structures of nucleic acids.
Figure 31. Structure of nitrogenous bases
A series of purine bases with methyl substituents was found in plant cells. Many of them are pharmacologically active. Examples include coffee beans containing caffeine (1,3,7-trimethylxanthine), tea leaves containing theophylline (1,3-dimethyl-xanthine), and cocoa beans containing theobromine (3,7- dimethylxanthine).
isomerism and physicochemical properties of purine and pyrimidine bases
A nitrogenous base molecule forms a system of alternating single and double bonds(system of conjugated double bonds). Such an organization forms a rigid molecule, without the possibility of conformational transitions. As a result, one cannot speak of a change in the conformation of nitrogenous bases.
For nitrogenous bases, only one type of isomerism, the keto-enol transition or tautomerism, has been identified.
Tautomerism
Due to the phenomenon of keto-enol tautomerism, nucleotides can exist either in lactim or lactam forms, and in physiological conditions the lactam form predominates in guanine and thymine (Figure 32). The importance of this circumstance will become clear when discussing the processes of base pairing.
Figure 32. Nucleotide tautomerism
Solubility
At neutral pH, guanine has the least solubility. Next in line is xanthine. Uric acid in the form of urates is relatively well soluble at neutral pH, but very poorly soluble in liquids with more low values pH, such as urine. Guanine is normally absent in human urine, and xanthine and uric acid are its usual components. The last two purines are often found in urinary tract stones.
light absorption
Due to the system of conjugated double bonds, all nitrogenous bases absorb in the ultraviolet part of the spectrum. Absorption spectrum - a graph of the distribution of optical density depending on the wavelength. Each nitrogenous base has its own absorption spectrum, it can be used to distinguish solutions of various nitrogenous bases or compounds that include a nitrogenous base (nucleotides), but the absorption maximum for all coincides at a wavelength of 260 nm. This allows you to easily and quickly determine the concentration of both nitrogenous bases and nucleotides and nucleic acids. The absorption spectrum also depends on the pH of the solution (Figure 33).
Figure 33. Absorption spectra of various nitrogenous bases
Functions of nitrogenous bases
Nitrogenous bases are practically not found in the free state. The exceptions are some alkaloids and uric acid.
Nitrogenous bases perform the following functions:
They are part of nucleotides;
Some of the alkaloids are nitrogenous bases, for example, caffeine in coffee or theophelin in tea;
Intermediate metabolic products of nitrogenous bases and nucleotides;
Uric acid is the cause of urolithiasis;
Nitrogen is excreted in the form of uric acid in some organisms.
Nucleotides and nucleosides
Nucleoside molecules are built from a purine or pyrimidine base, to which a carbohydrate (usually D-ribose or 2-deoxyribose) is attached (by a β-bond) in the N 9 or N 1 position, respectively. Thus, adenine ribonucleoside (adenosine) consists of adenine and D-ribose attached at the N 9 position; guanosine- from guanine and D-ribose in position N 9; cytidine- from cytosine and ribose in position N 1; uridine- from uracil and ribose in position N 1. Thus, in purine nucleosides (nucleotides), the nitrogenous base and sugar are linked by a 1-9 β glycosidic bond, and in pyrimidines, by a 1-1 β glycosidic bond.
The composition of 2'-deoxyribonucleosides includes purine or pyrimidine bases and 2'-deoxyribose attached at the same N 1 and N 9 atoms. Attachment of ribose or 2′-deoxyribose to the ring structure of the base occurs through a relatively acid-labile N-glycosidic bond (Figure 34).
Nucleotides are derivatives of nucleosides phosphorylated at one or more hydroxyl groups of a ribose (or deoxyribose) residue. So, adenosine monophosphate (AMP or adenylate) is built from adenine, ribose and phosphate. 2'-deoxyadenosine monophosphate (dAMP or deoxyadenylate) is a molecule composed of adenine, 2'-deoxyribose, and phosphate. Usually, ribose is attached to uracil, and 2-deoxyribose is attached to thymine. Therefore, thymidylic acid (TMF) is composed of thymine, 2′-deoxyribose, and phosphate. In addition to the above forms of nucleotides, nucleotides of an unusual structure were also found. So, in the tRNA molecule, a nucleotide was found in which ribose is attached to uracil in the fifth position, i.e. not by a nitrogen-carbon bond, but by a carbon-carbon bond. The product of this unusual addition is called pseudouridine (ψ). tRNA molecules also contain another unusual nucleotide structure - thymine, combined with ribose monophosphate. This nucleotide is formed already after the synthesis of the tRNA molecule by methylation of the UMP residue with S-adenosylmethionine. Pseudouridylic acid (ψMP) is also formed as a result of rearrangement of UMP after tRNA synthesis.
Figure 34. Structure of purine and pyrimidine nucleosides and nucleotides
Nomenclature, physicochemical properties and functions of nucleosides and nucleotides
The position of the phosphate group in the nucleotide molecule is indicated by a number. For example, adenosine with a phosphate group attached to the 3rd carbon of the ribose would be designated 3'-monophosphate. The prime after the number is placed in order to distinguish the carbon number in the purine or pyrimidine base from the position of this atom in the deoxyribose residue. When numbering the carbon atoms of the base, the prime is not put. The nucleotide 2'-deoxyadenosine with a phosphate residue at carbon-5 of the sugar molecule is referred to as 2'-deoxyadenosine-5'-monophosphate. Nucleosides containing adenine, guanine, cytosine, thymine, and uracil are usually denoted by the letters A, G, C, T, and Y, respectively. The presence of the letter d (or d) before the abbreviation indicates that the carbohydrate component of the nucleoside is 2′-deoxyribose. Guanosine containing 2′-deoxyribose can be designated dG (deoxyguanosine), and the corresponding monophosphate with a phosphate group attached to the third carbon atom of deoxyribose can be designated dG-3′-MF. As a general rule, when a phosphate is attached to carbon-5 of ribose or deoxyribose, the symbol 5′ is omitted. For example, guanosine 5′-monophosphate is commonly referred to as GMP, and 2′-deoxyguanosine 5′-monophosphate is abbreviated as dGMP. If 2 or 3 phosphoric acid residues are attached to the carbohydrate residue of the nucleoside, the abbreviations DF (diphosphate) and TF (triphosphate) are used. Thus, adenosine + triphosphate with three phosphate groups in the 5′ position of the carbohydrate will be designated ATP. Since phosphates in nucleotide molecules are in the form of phosphoric anhydrides, i.e., in a state with low entropy, they are called macroergs (having a large reserve potential energy). Hydrolysis of 1 mole of ATP to ADP releases 7.3 kcal of potential energy.
Figure 35. Structure of cAMP
Physicochemical properties of nucleotides
Since nucleotides contain nitrogenous bases, such properties as tautomerism and the ability to absorb in the ultraviolet part of the spectrum are also characteristic of nucleotides, and the absorption spectra of nitrogenous bases and nucleotides containing these bases are similar. The presence of sugar and phosphoric acid residues makes them more hydrophilic than nitrogenous bases. All nucleotides are acids because they contain phosphoric acid residues.
Functions of Natural Nucleotides
Nucleotides are monomers of nucleic acids (RNA, DNA). The composition of DNA includes deoxyribonucleotide phosphates - derivatives of adenine, thymine, guanine and cytosine. Also, some molecules of guanine and cytosine in DNA are methylated, that is, they contain a methyl group. As the main monomers, RNA contains ribonucleotide phosphates - derivatives of adenine, uracil, guanine and cytosine. RNA also contains nucleotides containing various minor nitrogenous bases, such as xanthine, hypoxanthine, dihydrouridine, etc.
Nucleotides are monomers of coenzymes (NAD, NADP, FAD, co-enzyme A, methionine-adenosine). As part of coenzymes, they participate in enzymatic reactions. This function will be discussed in more detail below.
Energy (ATP). ATP performs the function of the main intracellular carrier of free energy. The concentration of the most abundant free nucleotide in mammalian cells, ATP, is about 1 mmol/L.
Signal (cGMP, cAMP)(Figure 35). Cyclic AMP (3-, 5-adenosine monophosphate, cAMP), a mediator of various extracellular signals in animal cells, is formed from ATP as a result of a reaction catalyzed by adenylate cyclase. Adenylate cyclase activity is regulated by a complex of interactions, many of which are initiated through hormone receptors. Intracellular concentration cAMP (about 1 µmol/l) is 3 orders of magnitude lower than the concentration of ATP. Cyclic cGMP (3-, 5-guanosine monophosphate, cGMP) serves as an intracellular conductor of extracellular signals. In some cases, cGMP acts as a cAMP antagonist. cGMP is formed from GTP by the action of guanylate cyclase, an enzyme that has much in common with adenylate cyclase. Guanylate cyclase, like adenylate cyclase, is regulated by various effectors, including hormones. Like cAMP, cGMP is hydrolyzed by phosphodiesterase to the corresponding 5'-monophosphate.
Regulatory (GTP). The activity of a group of proteins (G-proteins), which mainly perform a regulatory function, depends on which nucleotide they bind. In an inactive form, these proteins bind GDP; when the protein is activated, GDP is replaced by GTP. When performing its function, the protein hydrolyzes GTP to GDP and phosphate, the released energy is spent on the functioning of the protein.
Activation during lipid and monosaccharide metabolism (UTP, STP). Derivatives of uracil nucleotides are involved as activating agents in hexose metabolism and carbohydrate polymerization reactions, in particular, in the biosynthesis of starch and oligosaccharide fragments of glycoproteins and proteoglycans. The substrates in these reactions are uridine diphosphate sugars. For example, uridine diphosphate glucose serves as a glycogen precursor. Also, the conversion of glucose into galactose, glucuronic acid, or other monosaccharide derivatives occurs as a conjugate with UDP. CTP is required for the biosynthesis of certain phosphoglycerides in animal tissues. Reactions involving ceramide and CDP-choline lead to the formation of sphingomyelin and other substituted sphingosines.
Participation in the deactivation of various alcohols and phenols(UDP-glucuronic acid). Uridine diphosphate glucuronic acid - acts as an "active" glucuronide in conjugation reactions, for example, in the formation of bilirubin glucuronide.
Nucleotides in coenzymes
Coenzymes are low molecular weight compounds associated with enzymes (see the "Enzymes" section) directly involved in a biochemical reaction, in other words, this is another substrate that does not enter the environment.
Coenzymes are divided into two groups:
carriers of protons and electrons, these coenzymes are involved in redox reactions;
carriers of all other groups except for protons and electrons, these coenzymes are involved in transferase reactions.
The mechanisms of these reactions can be considered in more detail in the chapter "Enzymes".
Some coenzymes contain nucleotides in their composition. They are also divided into the same two groups.
coenzymes that carry protons and electrons
These coenzymes are involved in redox reactions, where adenosine performs only a structural function, nucleotides containing other types of bases enter into the reaction, two types of such coenzymes are distinguished: nicotinic and flavin. They differ not only in active grouping, but also in the type of reactions they carry out.
Nicotinic coenzymes
Figure 36. Nicotinic coenzymes. A-structure of NAD, B-structure of NADP, C-mechanism of activity of nicotinic acid, D-mechanism of work of nicotinic coenzymes
Nicotinamide adenine dinucleotide (NAD+) is the main electron acceptor in the oxidation of fuel molecules. The reactive part of NAD+ is its nicotinamide ring. When the substrate is oxidized, the NAD+ nicotinamide ring adds a hydrogen ion and two electrons, which are equivalent to the hydride ion. The reduced form of this carrier is NADH. During this dehydrogenation, one hydrogen atom of the substrate is directly transferred to NAD + , while the second is transferred to the solvent. Both electrons lost by the substrate are transferred to the nicotinamide ring. The role of the electron donor in most processes of reductive biosynthesis (plastic exchange); performs the reduced form of nicotine amide adenine dinucleotide phosphate (NADPH). NADPH differs from NAD by the presence of a phosphate esterified to the 2'-hydroxyl group of adenosine. The oxidized form of NADPH is referred to as NADP+. NADPH carries electrons in the same way as NADH. However, NADPH is used almost exclusively in reductive biosynthetic processes, while NADH is used primarily for ATP generation. The additional phosphate group of NADPH is the site responsible for carrying out the intended purpose of the molecule, which is recognition by enzymes.
Flavin coenzymes
The first flavin coenzyme (flavin mononucleotide FMN) was isolated by A. Szent-Gyorgyi from the heart muscle in 1932, R. G. Warburg and V. Christian at the same time obtained the first flavoprotein containing FMN as a coenzyme from yeast. The second most important flavin coenzyme, flavin adenine dinucleotide (FAD), was isolated by them as a cofactor of D-amino acid oxidase in 1938. Due to the redox transformation of the flavin ring, flavin coenzymes carry out redox reactions as part of many important enzyme systems: oxidases (in particular, oxidases of D- and L-amino acids, monoamine oxidase, which regulates the level of catecholamines in the blood) and dehydrogenases (often with the participation of nicotinamide adenine dinucleotide and ubiquinones).
Figure 37. Flavin coenzymes. A-structure of FAD, B-mechanism of nicotinic acid activity, C-mechanism of flavin coenzymes
The second main electron carrier in the oxidation of fuel molecules is flavin adenine dinucleotide. The abbreviations used for the oxidized and reduced forms of this carrier are FAD and FADH 2 , respectively. The reactive part of FAD is its isoalloxazine ring. FAD, like NAD + , adds two electrons. However, FAD, unlike NAD +, adds both hydrogen atoms lost by the substrate.
End of introductory segment.
A biochemical blood test is a laboratory test that can help identify problems with functioning internal organs. So, if it is necessary to assess the state of health of the liver, kidneys or pancreas, a study is carried out on the level of the enzyme gamma glutamyl transferase. This specific substance is also found in other internal organs, but an increase in its level rarely occurs due to their dysfunction. Most often, deviations from the norm are caused by problems with the work of the liver and gallbladder.
What is gamma HT in a blood test, what are its functions, and when is a clinical study needed to determine the level of its content in the human body? Everyone should know this.
What is GGT and what functions does it perform?
First of all, let's figure out what GGTP is in a biochemical blood test.
On a note. Gamma glutamyl transpeptidase and GGT - gamma glutamyl transferase are identical concepts to each other, therefore the use of the first or second term is equally correct.
Gamma glutamyl transferase is an enzyme that has protein structure and taking an active part in amino acid metabolism. It accelerates the process of transfer and exchange of amino acid compounds in the cells of the body, and after their destruction enters the bloodstream. Since throughout the entire period of the functioning of the body there is a regular renewal of its cells, human blood always contains a certain amount of this protein.
However, when there is a malfunction in the work of the internal organs, the process of cellular decay is disrupted, resulting in a sharp increase (sometimes a decrease) in the level of gammaglutamine transferase in the blood plasma. Only a clinical blood test for liver enzymes, especially for GGT, can reveal a deviation from the norm.
Why are these proteins being studied? This is due to the fact that they are more sensitive to damage to liver cells, for example, with hepatitis. For this reason, a study on the GGT enzyme in a biochemical blood test is often prescribed by narcologists to patients suffering from alcohol dependence.
When alcohol enters the body, a more intense destruction of liver cells occurs, therefore, the GGTP protein is released into the blood much more. If at least 30 days have passed since the last use of alcohol, the level of this substance will decrease by 2 times.
GGT in blood biochemistry
Based on the foregoing, it is not difficult to understand what GGT is in a biochemical blood test. This is an indicator of amino acid metabolism in the body. The level of this protein indicates the activity of the blood serum, which increases sharply with the destruction of healthy body cells.
In simple terms, what is GGTP, then this is a blood plasma enzyme, the level of which shows whether there is a malfunction of the liver or other internal organs, and how serious it is.
An analysis to measure the level of this protein is carried out only if indicated.
Indications for a blood test
A blood test for GGTP is mandatory for patients complaining of:
- frequent bouts of nausea;
- profuse vomiting;
- decreased or complete loss of appetite;
- feeling of pressure and pain in the right hypochondrium.
A gastroenterologist or nephrologist (in case of problems with the kidneys) must send the patient for a blood test for the level of gamma HT if there are suspicions of the development of:
- cholecystitis;
- cholelithiasis (cholelithiasis);
- cholestasis;
- cholangitis;
- hepatitis C.
In these cases, it is not enough to pay attention to the presence of alarming symptoms, since the clinical manifestations of the above pathologies are very similar to each other. Only such a clinical study as a blood test for glutamyl transpeptidase will help determine exactly which disease is tormenting the patient.
Conducting this analysis is also important to determine the reasons why the patient experienced a process of bile stasis. This contributes to the appointment of the correct treatment and the prevention of consequences that are dangerous to the health of the patient.
An analysis for gamma glutamyl transpeptidase is mandatory for people suffering from chronic alcoholism. Under such conditions, it is important to accurately determine the degree of dependence of the patient on ethanol, as well as to understand how badly the liver is affected. Sometimes this helps not only to preserve the health of a person, but also to save his life.
A blood test for GGTP is also indicated for:
- the development of an allergy to medications, which is accompanied by a general intoxication of the body;
- preventive or control evaluation of liver health;
- the need to assess the functioning of the liver or kidneys after surgery.
But this is not all indications for referring a patient to this study. Biochemistry of GGT is performed in case of:
- blockage of the bile ducts;
- the presence of suspicious formations in the pancreas;
- problems with kidney function.
On a note. Donating blood for analysis requires preliminary preparation, so the patient must be instructed by the attending physician on what and how to do it correctly to obtain reliable data from a biochemical study.
The reasons for the increase in GGT, or gamma glutamyltransferase, can be not only problems with the liver, kidneys, gallbladder or pancreas, but also with the heart. Heart failure or previous myocardial infarction are the main indications for this study.
Gamma glutamyltransferase norms
The norm of GGT varies depending on the age and gender of the patient. So, the indicators for men and women do not differ very much, which cannot be said about the normal level of GGTP in older children and newborn babies.
The normal level of ggt in a biochemical blood test in adults is considered to be indicators ranging from 6-70 units per 1 liter of blood. This necessarily takes into account the fact that in women the norm of this enzyme is much lower than in men.
In babies, the rate of gamma glutamyltransferase in the blood is different. So in newborns it can correspond to indicators of 185 units per 1 liter of blood, in six-month-old babies - about 200 U / l. If in the form with the results of blood tests of a newborn such high numbers, do not worry - in infants, the liver cannot yet produce this enzyme on its own, so the placenta performs this function instead.
Interesting fact. In a blood test for gamma glutamyltransferase in dark-skinned people, the concentration of this enzyme significantly exceeds that of a white-skinned person. Therefore, we can say that the level of this protein also depends on the race of the patient.
The norm in women
The norm of GGT in the blood in women is directly dependent on age. The table below will help you figure out which indicators in the analysis results should not cause you concern.
The norm of GGTP in women may vary depending on:
- equipment used;
- units of measurement (IU/l, IU/ml, etc.);
- the race of the patient.
As for the level of gamma HT in expectant mothers, it depends on the gestational age:
- In the first trimester, GGTP fluctuates in the range of 0-17 units.
- The second trimester is accompanied by an increase in the level of this substance up to 33 units.
- In the third trimester, the amount of gamma HT in the blood plasma decreases by 1 unit, and is 32 units per 1 liter.
Sometimes the level of this protein in the blood of pregnant women can rise sharply, but if this is a temporary phenomenon, you should not be afraid of it. Such jumps in GGT can occur against the background of the use of vitamin complexes by the expectant mother and a large number of foods enriched with vitamins.
The norm in men
The norm of GGTP in the blood in men, as already noted, differs from the female in a big way. Permissible indicators are displayed in this table.
The norm of GGT indicators in men is higher than in women due to the high concentration of enzymes in the prostate gland. But if there is a significant jump in the level of this substance in the blood of a patient, he should be immediately examined for possible deviations in the functioning of the internal organs.
Causes of ggt deviation from the norm
A blood test for ggt requires taking a sample of material from a peripheral vein. The research process can take from several hours to several days. The results obtained are recorded on a special printed form, after which they are interpreted by the attending physician.
Of great importance is the correct interpretation of the GGT analysis, which depends on how accurately the rules for preparing the patient for the blood sampling process were followed. Deviations from the prescribed norms may be the result of:
- long-term intake of vitamin complexes containing a large amount of ascorbic acid;
- use of paracetamol or aspirin;
- abuse of oral hormonal contraceptives;
- taking antidepressants, antibiotics, histamine blockers.
Deviation from the norm of the content of gamma glutamitransferase in the body in a big way can indicate:
- hepatitis;
- pancreatitis;
- infectious mononucleosis, which gave a complication to the liver;
- autoimmune pathologies;
- diabetes mellitus;
- oncological process occurring in the prostate gland or mammary glands;
- rheumatoid arthritis etc.
If the level of GGTP in the blood is low, this may be due to:
- hypothyroidism;
- treatment of chronic alcoholism with certain types of medications;
- taking statins (drugs that lower blood cholesterol).
Deciphering a blood test for GGT should be carried out exclusively by the attending physician. Even if the patient understands what the norm should be and what indicators are a deviation from it, he cannot independently prescribe treatment for himself. Often, after the data obtained from a biochemical study, the patient is given a number of additional instrumental and laboratory tests that contribute to an accurate diagnosis.
Most of the pathologies detected or confirmed when deciphering a blood test for gamma HT require immediate hospitalization of the patient in a hospital and round-the-clock medical supervision, which the patient himself cannot provide for himself at home.
4.2.1. Primary structure of nucleic acids called sequence of mononucleotides in a DNA or RNA chain . The primary structure of nucleic acids is stabilized by 3",5"-phosphodiester bonds. These bonds are formed by the interaction of the hydroxyl group in the 3 "-position of the pentose residue of each nucleotide with the phosphate group of the adjacent nucleotide (Figure 3.2),
Thus, at one end of the polynucleotide chain there is a free 5'-phosphate group (5'-end), and at the other end there is a free hydroxyl group in the 3'-position (3'-end). Nucleotide sequences are usually written in the direction from the 5" end to the 3" end.
Figure 4.2. The structure of a dinucleotide, which includes adenosine-5"-monophosphate and cytidine-5"-monophosphate.
4.2.2. DNA (deoxyribonucleic acid) contained in the cell nucleus and molecular weight about 1011 Yes. Its nucleotides contain nitrogenous bases. adenine, guanine, cytosine, thymine , carbohydrate deoxyribose and phosphoric acid residues. The content of nitrogenous bases in a DNA molecule is determined by the Chargaff rules:
1) the number of purine bases is equal to the number of pyrimidine ones (A + G = C + T);
2) the amount of adenine and cytosine is equal to the amount of thymine and guanine, respectively (A = T; C = G);
3) DNA isolated from cells of different biological species differ from each other in the value of the specificity coefficient:
(G + C) / (A + T)
These patterns in the structure of DNA are explained by the following features of its secondary structure:
1) a DNA molecule is built from two polynucleotide chains interconnected by hydrogen bonds and oriented antiparallel (that is, the 3 "end of one chain is located opposite the 5" end of the other chain and vice versa);
2) hydrogen bonds are formed between complementary pairs of nitrogenous bases. Adenine is complementary to thymine; this pair is stabilized by two hydrogen bonds. Guanine is complementary to cytosine; this pair is stabilized by three hydrogen bonds (see figure b). The more DNA in a molecule steam G-C, the greater its resistance to high temperatures and ionizing radiation;
Figure 3.3. Hydrogen bonds between complementary nitrogenous bases.
3) both DNA strands are twisted into a helix having a common axis. Nitrogenous bases face the inside of the helix; in addition to hydrogen interactions, hydrophobic interactions also arise between them. The ribose phosphate parts are located along the periphery, forming the backbone of the helix (see Figure 3.4).
Figure 3.4. Diagram of the structure of DNA.
4.2.3. RNA (ribonucleic acid) is contained mainly in the cytoplasm of the cell and has a molecular weight in the range of 104 - 106 Da. Its nucleotides contain nitrogenous bases. adenine, guanine, cytosine, uracil , carbohydrate ribose and phosphoric acid residues. Unlike DNA, RNA molecules are built from a single polynucleotide chain, which may contain sections complementary to each other (Figure 3.5). These sections can interact with each other, forming double helixes, alternating with non-spiralized sections.
Figure 3.5. Scheme of the structure of transfer RNA.
According to the features of the structure and function, three main types of RNA are distinguished:
1) messenger (messenger) RNA (mRNA) transmit information about the structure of the protein from the cell nucleus to the ribosomes;
2) transfer RNA (tRNA) carry out the transport of amino acids to the site of protein synthesis;
3) ribosomal RNA (rRNA) are part of ribosomes, participate in protein synthesis.
The materials are published for review and are not a prescription for treatment! We recommend that you contact a hematologist at your healthcare facility!
GGT or gamma-glutamyltransferase is a protein that is directly involved in amino acid metabolism in body cells. For the most part, it is found in the cells of the kidneys, pancreas and liver. With an increased content of this protein, specialists can determine the degree of damage to liver cells.
general information
A small amount of gamma-glutamyltransferase is found in the brain, heart, spleen and intestines. The enzyme is located directly in the cell itself, however, when it is destroyed, it penetrates into the bloodstream.
The activity of this enzyme in small amounts is quite normal, due to the constant renewal of cells. However, due to the death of a significant number of cells, the activity of the protein increases significantly. Therefore, an excessive amount of GGT in a biochemical blood test indicates the presence of problems in the patient's body.
Important! The main source of serum GGT activity is the hepatobiliary system. Therefore, deviations of this indicator from the norm allow diagnosing various diseases liver.
Most often, GGT increases against the background of the course of such diseases:
- Obstructive lesions of the liver.
- cholestasis.
- Cholangitis, cholecystitis.
- Jaundice.
Important! In the last three cases, it is studies on GGT that give more accurate results, since it tends to manifest itself earlier than other liver enzymes, remaining for a longer time.
- Cirrhosis caused by excessive alcohol consumption.
- Drug intoxication.
- Fatty degeneration of the liver, in which this indicator increases several times.
- Pancreatitis.
Important! The development of infectious hepatitis leads to the fact that GGT rises up to five times. Therefore, experts in this case are more likely to focus on.
In addition, ggt in a biochemical blood test increases for reasons not related to liver problems, including:
- Alcoholism.
- Oncology of the prostate and pancreas.
- Taking paracetamol, phenobarbital and similar drugs.
In what cases is a study on GGT performed?
Most often, the biochemistry of GGT is prescribed by a narcologist, due to the sensitivity of the liver to alcohol. Also, the analysis is carried out in such cases:
- If you are preparing for the operation.
- If it is necessary to diagnose problems with the liver.
- If the patient develops symptoms of hepatitis or cirrhosis.
- If the patient complains of weakness, pain in the abdomen (right).
- In case of vomiting, nausea.
- If you are checking for newly discovered malignant diseases.
Features of the analysis
Having dealt with what a GGT blood test is, let's consider how it is carried out. This study belongs to the group of liver tests and is carried out during a biochemical blood test. Blood sampling is most often done from a vein. In this case, the patient should properly prepare for the test:
- Since blood sampling must be done on an empty stomach, the last meal is allowed at least 8 hours before the test.
Note. Before donating blood, the patient can drink a small amount of water.
- Fatty foods and alcohol should be excluded a couple of days before blood sampling.
- You should also refrain from heavy physical exertion and sports.
- The patient must be sure to notify his attending physician if he is taking any medications at the time of the test. It is advisable to temporarily stop taking them.
- If the patient is scheduled for fluoroscopic or ultrasound examinations on the day of the test, they should be carried out after blood sampling.
- Physiotherapy procedures (some types) are also prohibited, which the specialist must inform the patient about.
A few words about normal indicators
The norm of indicators may vary depending on the patient's gender, age, and even his belonging to a particular race.
In the male population, GGT rates are relatively higher, since a certain amount of these enzymes is present in the prostate gland. Also high are the rates in infants, since at first this enzyme is found in the placenta, and only over time its production begins in the liver.
Important! In pregnant women, these indicators are largely determined by the duration of pregnancy.
What can influence the result
The results of the analysis may change under the influence of such factors:
- A decrease in performance may occur due to the intake of ascorbic acid.
- GGT is increased by aspirin, paracetamol, antibiotics, antidepressants, etc.
- Overestimated rates are also observed in patients prone to obesity.
Important! The assessment of the dynamics of changes should be carried out only taking into account other blood parameters. These include ALT, AST, LDH lipase and others. For the final diagnosis, the ratio of the parameters of this enzyme with other parameters is very important.
Why does GGT increase?
If GGT is elevated, then specialists first of all pay attention to the diagnosis of liver diseases. However, the problem may lie elsewhere. Most often, an increase in the level of this enzyme occurs for the following reasons:
- Heart failure.
- If high rates are present against the background of alkaline phosphatase, then this may indicate the development of autoimmune diseases.
- Mammary cancer.
- Problems with the biliary tract.
- Diabetes.
- Arthritis.
- Hyperthyroidism.
- Myocardial infarction, etc.
What are the reasons for the decrease in performance?
There can be three main reasons:
- Hypothyroidism.
- Taking certain medications.
- If a patient is being treated for alcoholism, then after a month of such therapy, his GGT can be significantly reduced. This decrease is explained by the absence of ethanol, which stimulates the synthesis of this enzyme in liver cells, to which the body develops addiction.
Hormones affect target cells.
target cells- These are cells that specifically interact with hormones using special receptor proteins. These receptor proteins are located on the outer membrane of the cell, or in the cytoplasm, or on the nuclear membrane and other organelles of the cell.
Biochemical mechanisms of signal transmission from the hormone to the target cell.
Any receptor protein consists of at least two domains (regions) that provide two functions:
hormone recognition;
conversion and transmission of the received signal to the cell.
How does the receptor protein recognize the hormone molecule with which it can interact?
One of the domains of the receptor protein contains a region complementary to some part of the signal molecule. The process of binding a receptor to a signal molecule is similar to the process of formation of an enzyme-substrate complex and can be determined by the value of the affinity constant.
Most of the receptors are not well understood because their isolation and purification are very difficult, and the content of each type of receptor in cells is very low. But it is known that hormones interact with their receptors in a physicochemical way. Electrostatic and hydrophobic interactions are formed between the hormone molecule and the receptor. When the receptor binds to the hormone, conformational changes in the receptor protein occur and the complex of the signal molecule with the receptor protein is activated. In the active state, it can cause specific intracellular reactions in response to the received signal. If the synthesis or ability of receptor proteins to bind to signaling molecules is impaired, diseases arise - endocrine disorders.
There are three types of such diseases.
Associated with insufficient synthesis of receptor proteins.
Associated with a change in the structure of the receptor - genetic defects.
Associated with the blocking of receptor proteins by antibodies.
Mechanisms of action of hormones on target cells.
Depending on the structure of the hormone, there are two types of interaction. If the hormone molecule is lipophilic (for example, steroid hormones), then it can penetrate the lipid layer of the outer membrane of target cells. If the molecule is large or polar, then its penetration into the cell is impossible. Therefore, for lipophilic hormones, the receptors are located inside the target cells, while for hydrophilic hormones, the receptors are located in the outer membrane.
In the case of hydrophilic molecules, an intracellular signal transduction mechanism operates to obtain a cellular response to a hormonal signal. This happens with the participation of substances, which are called second intermediaries. Hormone molecules are very diverse in shape, but "second messengers" are not.
The reliability of signal transmission provides a very high affinity of the hormone for its receptor protein.
What are the mediators that are involved in the intracellular transmission of humoral signals?
These are cyclic nucleotides (cAMP and cGMP), inositol triphosphate, calcium-binding protein - calmodulin, calcium ions, enzymes involved in the synthesis of cyclic nucleotides, as well as protein kinases - protein phosphorylation enzymes. All these substances are involved in the regulation of the activity of individual enzyme systems in target cells.
Let us analyze in more detail the mechanisms of action of hormones and intracellular mediators.
There are two main ways of transmitting a signal to target cells from signaling molecules with a membrane mechanism of action:
adenylate cyclase (or guanylate cyclase) systems;
phosphoinositide mechanism.
adenylate cyclase system.
Main components: membrane protein receptor, G-protein, adenylate cyclase enzyme, guanosine triphosphate, protein kinases.
In addition, ATP is required for the normal functioning of the adenylate cyclase system.
The receptor protein, G-protein, next to which GTP and the enzyme (adenylate cyclase) are located, are built into the cell membrane.
Until the moment of hormone action, these components are in a dissociated state, and after the formation of the complex of the signal molecule with the receptor protein, changes in the conformation of the G protein occur. As a result, one of the G-protein subunits acquires the ability to bind to GTP.
The G-protein-GTP complex activates adenylate cyclase. Adenylate cyclase begins to actively convert ATP molecules into cAMP.
cAMP has the ability to activate special enzymes - protein kinases, which catalyze the phosphorylation reactions of various proteins with the participation of ATP. At the same time, phosphoric acid residues are included in the composition of protein molecules. The main result of this phosphorylation process is a change in the activity of the phosphorylated protein. In different cell types, proteins with different functional activities undergo phosphorylation as a result of activation of the adenylate cyclase system. For example, these can be enzymes, nuclear proteins, membrane proteins. As a result of the phosphorylation reaction, proteins can become functionally active or inactive.
Such processes will lead to changes in the rate of biochemical processes in the target cell.
The activation of the adenylate cyclase system lasts a very short time, because the G-protein, after binding to adenylate cyclase, begins to exhibit GTPase activity. After hydrolysis of GTP, the G-protein restores its conformation and ceases to activate adenylate cyclase. As a result, the cAMP formation reaction stops.
In addition to the participants in the adenylate cyclase system, some target cells have receptor proteins associated with G-proteins, which lead to the inhibition of adenylate cyclase. At the same time, the GTP-G-protein complex inhibits adenylate cyclase.
When cAMP formation stops, phosphorylation reactions in the cell do not stop immediately: as long as cAMP molecules continue to exist, the process of protein kinase activation will continue. In order to stop the action of cAMP, there is a special enzyme in cells - phosphodiesterase, which catalyzes the hydrolysis reaction of 3',5'-cyclo-AMP to AMP.
Some substances that have an inhibitory effect on phosphodiesterase (for example, the alkaloids caffeine, theophylline) help maintain and increase the concentration of cyclo-AMP in the cell. Under the influence of these substances in the body, the duration of activation of the adenylate cyclase system becomes longer, i.e., the action of the hormone increases.
In addition to the adenylate cyclase or guanylate cyclase systems, there is also a mechanism for information transfer inside the target cell with the participation of calcium ions and inositol triphosphate.
Inositol triphosphate is a substance that is a derivative of a complex lipid - inositol phosphatide. It is formed as a result of the action of a special enzyme - phospholipase "C", which is activated as a result of conformational changes in the intracellular domain of the membrane receptor protein.
This enzyme hydrolyzes the phosphoester bond in the phosphatidyl-inositol-4,5-bisphosphate molecule, resulting in the formation of diacylglycerol and inositol triphosphate.
It is known that the formation of diacylglycerol and inositol triphosphate leads to an increase in the concentration of ionized calcium inside the cell. This leads to the activation of many calcium-dependent proteins inside the cell, including the activation of various protein kinases. And here, as in the case of activation of the adenylate cyclase system, one of the stages of signal transmission inside the cell is protein phosphorylation, which leads to a physiological response of the cell to the action of the hormone.
A special calcium-binding protein, calmodulin, takes part in the work of the phosphoinositide signaling mechanism in the target cell. This is a low molecular weight protein (17 kDa), 30% consisting of negatively charged amino acids (Glu, Asp) and therefore capable of actively binding Ca + 2. One calmodulin molecule has 4 calcium-binding sites. After interaction with Ca + 2, conformational changes in the calmodulin molecule occur and the Ca + 2-calmodulin complex becomes able to regulate the activity (allosterically inhibit or activate) of many enzymes - adenylate cyclase, phosphodiesterase, Ca + 2, Mg + 2-ATPase and various protein kinases.
In different cells, when the Ca + 2-calmodulin complex is exposed to isoenzymes of the same enzyme (for example, adenylate cyclase different type) in some cases, activation is observed, and in others, inhibition of the cAMP formation reaction. Such different effects occur because the allosteric centers of isoenzymes can include different amino acid radicals and their response to the action of the Ca + 2-calmodulin complex will be different.
Thus, the role of "second messengers" for the transmission of signals from hormones in target cells can be:
complex "Sa-calmodulin";
diacylglycerol;
inositol triphosphate.
cyclic nucleotides (c-AMP and c-GMP);
The mechanisms of information transfer from hormones inside target cells with the help of the listed mediators have common features:
one of the stages of signal transmission is protein phosphorylation;
termination of activation occurs as a result of special mechanisms initiated by the participants in the processes themselves - there are mechanisms of negative feedback.
Hormones are the main humoral regulators of the physiological functions of the body, and their properties, biosynthetic processes, and mechanisms of action are now well known.
The features by which hormones differ from other signaling molecules are as follows.
Synthesis of hormones occurs in special cells of the endocrine system. The synthesis of hormones is the main function of endocrine cells.
Hormones are secreted into the blood, more often into the venous, sometimes into the lymph. Other signaling molecules can reach target cells without being secreted into circulating fluids.
Telecrine effect (or distant action)- hormones act on target cells at a great distance from the site of synthesis.
Hormones are highly specific substances with respect to target cells and have a very high biological activity.