What is nadf n. Example of a biochemical reaction involving NAD

Dehydrogenases are enzymes of the class of oxidoreductases that catalyze reactions that remove hydrogen (i.e., protons and electrons) from a substrate, which is an oxidizing agent, and transport it to another substrate, which is reduced.

Depending on the chemical nature acceptor with which dehydrogenases interact, they are divided into several groups:

1. Anaerobic dehydrogenases, which catalyze reactions in which the hydrogen acceptor is a compound other than oxygen.
2. Aerobic dehydrogenases, which catalyze reactions where the hydrogen acceptor can be oxygen (oxidases) or another acceptor. Aerobic dehydrogenases belong to flavoproteins, the reaction product is hydrogen peroxide.
3. Dehydrogenases, which transport electrons from a substrate to an electron acceptor. The cytochromes of the mitochondrial respiratory chain belong to this group of dehydrogenases.
4. Dehydrogenases, which catalyze the direct introduction of 1 or 2 oxygen atoms into the substrate molecule. Such dehydrogenases are called oxygenases.

The function of primary acceptors of hydrogen atoms cleaved from the corresponding substrates is performed by two types of dehydrogenases:

• pyridine-dependent dehydrogenases- contain the coenzymes nicotinamide (NAD +) or nicotinamide adenine dinucleotide phosphate (NADP +).
• flavin-dependent dehydrogenases, the prosthetic group of which is flavin adenine dinucleotide (FAD) or flavin mononucleotide (FMN).

The coenzymes NADP+ (or NAD+) are loosely bound to the apoenzyme and therefore can be present in the cell either in a state associated with the apoenzyme or be separated from the protein part.

Pyridine-dependent dehydrogenases are of the anaerobic type - water-soluble enzymes that oxidize polar substrates. The reaction catalyzed by pyridine-dependent dehydrogenases general view given in the following equations:

SH2 + NADP+ → S + NADPH + H+

SH2 + NAD+ → S + NADH + H+

Working structure in a molecule NAD+ or NADP+ is a pyridine ring, nicotinamide, which adds one hydrogen atom and one electron (hydride ion) during an enzymatic reaction, and the second proton enters the reaction medium. Pyridine-dependent dehydrogenases are very common in living cells. They abstract protons and electrons from many substrates, reducing NAD + or NADP + and subsequently transferring reducing equivalents to other acceptors. NAD-dependent dehydrogenases mainly catalyze redox reactions of oxidative metabolic pathways - glycolysis, Krebs cycle, β-oxidation fatty acids, mitochondrial respiratory chain, etc. NAD is the main source of electrons for the electron transport chain. NADP is used mainly in the processes of reductive synthesis (in the synthesis of fatty acids and steroids).

Flavin-dependent dehydrogenases- flavoproteins, the prosthetic groups in which FAD or FMN are derivatives of vitamin B2, which are tightly (covalently) associated with the apoenzyme. These dehydrogenases are membrane-bound enzymes that oxidize nonpolar and low-polarity substrates. The working part of the FAD or FMN molecule, which participates in redox reactions, is the isoaloxazine ring of riboflavin, which accepts two hydrogen atoms (2H+ + 2e-) from the substrate.

General equation reactions involving flavin-dependent dehydrogenases look like this:

SH2 + FMN → S + FMN-H2

SH2 + FAD+ → S + FADH2

In biological oxidation processes, these enzymes play the role of both anaerobic and aerobic dehydrogenases. Anaerobic dehydrogenases include NADH dehydrogenase, an FMN-dependent enzyme that transfers electrons from NADH to the more electropositive components of the mitochondrial respiratory chain. Other dehydrogenases (FAD-dependent) transfer electrons directly from the substrate to the respiratory chain (eg, succinate dehydrogenase, acyl-CoA dehydrogenase). The transport of electrons from flavoproteins to cytochrome oxidases in the respiratory chain is provided by cytochromes, which, in addition to cytochrome oxidase, are classified as anaerobic dehydrogenases. Cytochromes are iron-containing proteins of mitochondria - hemproteins, which, due to the reverse change in the valency of heme iron, perform the function of transporting electrons in aerobic cells directly in the chains of biological oxidation: cytochrome (Fe3 +) + e → cytochrome (Fe2 +).

The mitochondrial respiratory chain includes cytochromes b, c1, c, a and a3 (cytochrome oxidase). In addition to the respiratory chain, cytochromes are contained in the endoplasmic reticulum (450 and b5). Aerobic flavin-dependent dehydrogenases include L-amino acid oxidases, xanthine oxidase, etc.

Dehydrogenases that catalyze the incorporation of one or two oxygen atoms into a substrate molecule are called oxygenases. Depending on the number of oxygen atoms that interact with the substrate, oxygenases are divided into 2 groups:

• Dioxygenases
• Monooxygenases

Dioxygenases catalyze adds 2 oxygen atoms to the substrate molecule: S + O2 → SO2. These are, in particular, non-heme iron-containing enzymes that catalyze reactions of homogentisic acid synthesis and its oxidation along the path of tyrosine catabolism. Iron-containing lipoxygenase catalyzes the incorporation of O2 into arachidonic acid, the first reaction in the synthesis of leukotrienes. Proline and lysine dioxygenases catalyze the hydroxylation reactions of lysine and proline residues in procollagen. Monooxygenases catalyze the addition of only 1 atom of the oxygen molecule to the substrate. In this case, the second oxygen atom is reduced to water:

SH + O2 + NADPH + H+

SOH + H2O + NADP+

TO monooxygenases belong to enzymes that participate in the metabolism of many medicinal substances through their hydroxylation. These enzymes are localized predominantly in the microsomal fraction of the liver, adrenal glands, gonads and other tissues. Since most often the substrate in monooxygenase reactions is hydroxylated, this group The enzymes are also called hydroxylases.

Monooxygenases catalyze the hydroxylation reactions of cholesterol (steroids) and their conversion into biologically active substances, including sex hormones, adrenal hormones, active metabolites of vitamin D - calcitriol, as well as detoxification reactions by hydroxylation of a number of toxic substances, medicines and products of their transformation for the body. The monooxygenase membrane system of the endoplasmic reticulum of hepatocytes contains NADPH + H+, flavoproteins with the cofactor FAD, a protein (adrenotoxin) containing non-heme iron, and heme protein - cytochrome P450. As a result of hydroxylation of non-polar hydrophobic substances, their hydrophilicity increases, which contributes to biological inactivation active substances or neutralizing toxic substances and excreting them from the body. Some medicinal substances, such as phenobarbital, have the ability to induce the synthesis of microsomal enzymes and cytochrome P450.

There are monooxygenases that do not contain cytochrome P450. These include liver enzymes that catalyze the hydroxylation reactions of phenylalanine, tyrosine, and tryptophan.

Good to know

• D-dimer is a marker of fibrinolysis (during pregnancy - normal, increased - with thrombosis, CHF, oncological processes)

The name of vitamin PP comes from an Italian expression preventive pellagra– preventing pellagra.

Sources

Good sources are liver, meat, fish, legumes, buckwheat, and black bread. Milk and eggs contain little vitamin. It is also synthesized in the body from tryptophan - one out of 60 tryptophan molecules is converted into one vitamin molecule.

Daily requirement

Structure

The vitamin exists in the form nicotinic acid or nicotinamide.

Two forms of vitamin PP

Its coenzyme forms are nicotinamide adenine dinucleotide(NAD) and ribose phosphorylated form – nicotinamide adenine dinucleotide phosphate(NADP).

Structure of the oxidized forms of NAD and NADP

Biochemical functions

Transfer of hydride ions H – (hydrogen atom and electron) in redox reactions.

The mechanism of participation of NAD and NADP in biochemical reaction

Thanks to the transfer of hydride ions, the vitamin provides the following tasks:

1. Metabolism of proteins, fats and carbohydrates. Since NAD and NADP serve as coenzymes of most dehydrogenases, they participate in the reactions

• during the synthesis and oxidation of carboxylic acids,
• during the synthesis of cholesterol,
• metabolism of glutamic acid and other amino acids,
• carbohydrate metabolism: pentose phosphate pathway, glycolysis,
• oxidative decarboxylation of pyruvic acid,

Example of a biochemical reaction involving NAD

2. NADH does regulating function, since it is an inhibitor of certain oxidation reactions, for example, in the tricarboxylic acid cycle.

3. Protection of hereditary information– NAD is a substrate of poly-ADP-ribosylation during the process of stitching chromosomal breaks and DNA repair.

4. Defence from free radicals – NADPH is an essential component of the cell’s antioxidant system.

5. NADPH is involved in reactions

• resynthesis tetrahydrofolic acids (vitamin B9 coenzyme) from dihydrofolic acid after the synthesis of thymidyl monophosphate,
• protein recovery thioredoxin during the synthesis of deoxyribonucleotides,
• to activate “food” vitamin K or restore thioredoxin after reactivation of vitamin K.

Hypovitaminosis B3

Cause

Nutritional deficiency of niacin and tryptophan. Hartnup syndrome.

Clinical picture

Manifested by the disease pellagra (Italian: pelle agra – rough skin) How three D syndrome:

• dementia(nervous and mental disorders, dementia),
• dermatitis(photodermatitis),
• diarrhea(weakness, indigestion, loss of appetite).

If left untreated, the disease is fatal. Children with hypovitaminosis experience slow growth, weight loss, and anemia.

In the USA in 1912-1216. the number of cases of pellagra was 100 thousand people per year, of which about 10 thousand died. The reason was the lack of animal food, people mainly ate corn and sorghum, which are poor in tryptophan and contain indigestible bound niacin.
It's interesting that the Indians South America, whose diet has been corn since ancient times, pellagra does not occur. The reason for this phenomenon is that they boil the corn in lime water, which releases the niacin from the insoluble complex. The Europeans, having taken corn from the Indians, did not bother to borrow the recipes either.

Antivitamins

Isonicotinic acid derivative isoniazid, used to treat tuberculosis. The mechanism of action is not exactly clear, but one hypothesis is the replacement of nicotinic acid in the reactions of nicotinamide adenine dinucleotide synthesis ( iso-NAD instead of NAD). As a result, the course of redox reactions is disrupted and the synthesis of mycolic acid is suppressed, structural element cell wall of Mycobacterium tuberculosis.

Enzymes, like proteins, are divided into 2 groups: simple And complex. Simple ones consist entirely of amino acids and, upon hydrolysis, form exclusively amino acids. Their spatial organization is limited by the tertiary structure. These are mainly gastrointestinal enzymes: pepsin, trypsin, lysacym, phosphatase. Complex enzymes, in addition to the protein part, also contain non-protein components. These non-protein components differ in the strength of binding to the protein part (alloenzyme). If the dissociation constant of a complex enzyme is so small that in solution all polypeptide chains are associated with their non-protein components and are not separated during isolation and purification, then the non-protein component is called prosthetic group and is considered as an integral part of the enzyme molecule.

Under coenzyme understand an additional group that is easily separated from the alloenzyme upon dissociation. Between the alloenzyme and the simplest group There is a covalent bond, quite complex. There is a non-covalent bond (hydrogen or electrostatic interactions) between the alloenzyme and the coenzyme. Typical representatives coenzymes are:

B 1 - thiamine; pyrophosphate (it contains B)

B 2 - riboflavin; FAD, FNK

PP - NAD, NADP

H – biotin; biositine

B 6 - pyridoxine; pyridoxal phosphate

Pantothenic acid: coenzyme A

Many divalent metals (Cu, Fe, Mn, Mg) also act as cofactors, although they are neither coenzymes nor prosthetic groups. Metals are part of the active center or stabilize best option structure of the active center.

METALSENZYMES

Fe, Fehemoglobin, catalase, peroxidase

Cu,Cu cytochrome oxidase

ZnDNA – polymerase, dehydrogenase

Mghexokinase

Mnarginase

Seglutathione reductase

ATP, lactic acid, and tRNA can also perform a cofactor function. One thing to note distinctive feature two-component enzymes, which consists in the fact that neither the cofactor (coenzyme or prosthetic group) nor the alloenzyme individually exhibit catalytic activity, and only their combination into a single whole, proceeding in accordance with the program of their three-dimensional organization, ensures the rapid occurrence of chemical reactions.

Structure of NAD and NADP.

NAD and NADP are coenzymes of pyridine-dependent dehydrogenases.

NICOTINAMIDE ADNINE DINE NUCLEOTIDE.

NICOTINAMIDE ADNINE DINE NUCLEOAMIDE PHOSPHATE (NADP)

The ability of NAD and NADP to play the role of an accurate hydrogen carrier is associated with the presence in their structure -

nicotinic acid reamide.

In cells, NAD-dependent dehydrogenases are involved

in the processes of electron transfer from the substrate to O.

NADP-dependent dehydrogenases play a role in the process -

sah biosynthesis. Therefore, the coenzymes NAD and NADP

differ in intracellular localization: NAD

concentrated in mitochondria, and most of the NADP

is located in the cytoplasm.

Structure of FAD and FMN.

FAD and FMN are prosthetic groups of flavin enzymes. They are very firmly attached to the alloenzyme, unlike NAD and NADP.

FLAVIN MONONUCLEOTIDE (FMN).

FLAVINACETYLDINUCLEOTIDE.

The active part of the FAD and FMN molecule is the isoalloxadine ring riboflavin, to the nitrogen atoms of which two hydrogen atoms can be attached.

Adenosine triphosphoric acid (ATP) is a universal source and main energy accumulator in living cells. ATP is found in all plant and animal cells. The amount of ATP is on average 0.04% (of the wet weight of the cell), greatest number ATP (0.2-0.5%) is contained in skeletal muscles. In a cell, an ATP molecule is used up within one minute of its formation. In humans, an amount of ATP equal to body weight is produced and destroyed every 24 hours.

ATP is a mononucleotide consisting of nitrogenous base residues (adenine), ribose and three residues phosphoric acid. Since ATP contains not one, but three phosphoric acid residues, it belongs to ribonucleoside triphosphates.

Most of the work that happens in cells uses the energy of ATP hydrolysis. In this case, when the terminal residue of phosphoric acid is eliminated, ATP transforms into ADP (adenosine diphosphoric acid), and when the second phosphoric acid residue is eliminated, it turns into AMP (adenosine monophosphoric acid). The free energy yield upon elimination of both the terminal and second residues of phosphoric acid is about 30.6 kJ/mol. The elimination of the third phosphate group is accompanied by the release of only 13.8 kJ/mol. The bonds between the terminal and second, second and first phosphoric acid residues are called macroergic(high energy).

ATP reserves are constantly replenished. In the cells of all organisms, ATP synthesis occurs in the process phosphorylation, i.e. addition of phosphoric acid to ADF. Phosphorylation occurs with varying intensity during respiration (mitochondria), glycolysis (cytoplasm), and photosynthesis (chloroplasts).

ATP is the main link between processes accompanied by the release and accumulation of energy, and processes occurring with energy expenditure. In addition, ATP, along with other ribonucleoside triphosphates (GTP, CTP, UTP), is a substrate for RNA synthesis.

In addition to ATP, there are other molecules with macroergic bonds - UTP (uridine triphosphoric acid), GTP (guanosine triphosphoric acid), CTP (cytidine triphosphoric acid), the energy of which is used for the biosynthesis of protein (GTP), polysaccharides (UTP), phospholipids (CTP). But all of them are formed due to the energy of ATP.

In addition to mononucleotides, important role Dinucleotides (NAD +, NADP +, FAD) belonging to the group of coenzymes (organic molecules that retain contact with the enzyme only during the reaction) play in metabolic reactions. NAD + (nicotinamide adenine dinucleotide), NADP + (nicotinamide adenine dinucleotide phosphate) are dinucleotides containing two nitrogenous bases - adenine and nicotinic acid amide - a derivative of vitamin PP), two ribose residues and two phosphoric acid residues (Fig. .). If ATP is a universal source of energy, then NAD + and NADP + are universal acceptors, and their restored forms are NADH And NADPH – universal donors reduction equivalents (two electrons and one proton). The nitrogen atom included in the nicotinic acid amide residue is tetravalent and carries a positive charge ( NAD +). This nitrogenous base easily attaches two electrons and one proton (i.e., it is reduced) in those reactions in which, with the participation of dehydrogenase enzymes, two hydrogen atoms are removed from the substrate (the second proton goes into solution):

Substrate-H 2 + NAD + substrate + NADH + H +

IN reverse reactions enzymes, oxidizing NADH or NADPH, reduce substrates by adding hydrogen atoms to them (the second proton comes from the solution).

FAD – flavin adenine dinucleotide– a derivative of vitamin B 2 (riboflavin) is also a cofactor of dehydrogenases, but FAD adds two protons and two electrons, reducing to FADN 2.