The structure, catalytic role and representatives of Cytochromes

Cytochromes are two-component enzymes. They consist of apoenzyme and coenzyme as heme. Cytochromes are divided into 4 groups – A, B, C, D. each group is subdivided into subgroups, so group A includes Cyta and Cyta3; group B includes Cytb and Cytb5; group C includes Cytc and Cytc1. Cytochromes of group D occur in plants. Cytochromes of each group differ from each other by structure of heme. Cytochromes of subgroups differ from each other by structure of apoenzyme, i.e. they contain the same hemes.

Cytochromes of group B have got a heme which looks like heme of hemoglobin and myoglobin:

1,3,5,8-tetramethyl-2,4-divinyl-6,7-dipropionic acid iron porphin

The heme of cytochromes of group C is: 1,3,5,8-tetramethyl-2,4-diethyl-6,7-dipropionic acid iron porphin

The heme of cytochromes of group A is: 1,3-dimethyl-2-radical-4-vinyl-8-formyl-6,7-dipropionic acid iron porphin

The iron of hemes of cytochromes may be 3+ and 2+:

+e

Cyt (Fe3+) --------------------------------------------à Cyt (Fe2+)

(ferri) ß-------------------------------------------- (ferro)

– e

ferri form of cytochrome is oxidized. Ferro form of cytochrome is reduced one.

The subsequence of cytochromes in BO depend on their oxidative-reductive potential. They are located according increase of their potential: cytbà cytc1àcytcàcytaàcyta3àO2

The structure and catalytic role of Catalase and peroxidases

Catalase and peroxidases consist of apoenzyme and coenzyme as heme. Apoenzyme is represented by 1 polypeptide chain. Heme is the same of heme of hemoglobin and myoglobin but iron has 3 valency. Catalase contains 4 hemes whereas peroxidases contain 1 heme.

Catalase catalyzes the next reaction: 2H2O2 à 2H2O + O2

Peroxidases catalyze the reaction: H2O2 à H2O + [O]

The structure and catalytic role of oxygenases

Oxygenases consist of apoenzyme and coenzyme as vitamin C (ascorbic acid).

MOG oxidize substrates by dint of inclusion of atom of oxygen. This results in formation of hydroxylated product or product like SO: S+O2+NADPH2àSO(H)+H2O+NADP

DOG include molecule of oxygen in substrate. This results in cleavage of double bond:

S+O2àSO2 (e.g. formation of vitamin A from carotene and formation of dioxide of oleinic acid).

BIOENERGETICS

Bioenergetics, or biochemical thermodynamics is the study of the energy changes accompanying biochemical reactions.

Nonbiologic systems may utilize heat energy to perform work, but biologic systems are essentially isothermic and use chemical energy to power the living processes. Suitable fuel is required to provide the energy that enables the animal to carry out its normal processes.

How the organism obtains this energy from its food is basic to the understanding of normal nutrition and metabolism. Death from starvation occurs when available energy reserves are depleted and certain forms of malnutrition are associated with energy imbalance (kwashiorkor, marasmus). The rate of energy release, measured by the metabolic rate, is controlled by the thyroid hormones. Storage of energy results in obesity, one of the most common diseases of occidental society.

All life on the earth depend from the energy of sun. 0.02% of sun’s energy falls on the surface of earth. In the dependent from the ability of living organisms to use of sun’s energy all organisms are divided into 2 groups – autotrophic and heterotrophic.

Inorder to maintain living processes all organisms must obtain supplement of free energy from their environment. Autotrophic organisms couple their metabolism to some simple exergonic process in their surroundings, they are able to absorb the energy of sun light and carry out of the primary synthesis of organic compounds – photosynthesis due to the presence of chlorophill (eg, green plants), some autotrophic bacteria utilize the energy of the reaction Fe2+ à Fe3+ for primary photosynthesis. Reaction of photosynthesis: 6CO2 + 6H2O + 2847kJ ↔ C6H12O6 + 6O2, where 2847kJ is an energy of sun, direct reaction occurs in plants (photosynthesis), reverse reaction occurs in animals and human and oxidation is called.

On the other hand heterotriphic organisms obtain free energy by coupling their metabolism to the breakdown of complex organic molecules in their environment. They must to receive these compounds eating different plants or animals.

The vital processes are a synthetic reactions, muscular contraction, nerve impulse conductions, active transport and cetera. They obtain energy by chemical linkage or coupling to oxidative reactions. Chemically, oxidation is defind as the removal of electrons and reduction as the gain of electrons.

Fe2+ ↔ Fe3+ where direct reaction is called oxidation (remove of electron); reverse reaction is called reduction (addition of electron). It follows that oxidation is always accompanied by reduction of an electron acceptor. This principle of oxidation-reduction applies equally to biochemical systems and is an important concept underlying understanding of the nature of biologic oxidation.

Although certain bacteria (anaerobis) survive in the absence of oxygen, the life of higher animals is absolutely dependent upon a supply of oxygen.

The principal use of oxygen is in respiration which nmay be defind as the process by which cells derive energy in the form of ATP from the controlled reaction of hydrogen with oxygen to form water. The terms exergonic and endergonic are used to indicate that a process is accompanied by loss or gain of free energy. The exergonic reactions are termed catabolism (the breakdown or oxidation of fuel molecules), whereas the synthetic reactions that build up substances are termed anabolism. The total of all is metabolism.

ATP plays a central role in the transfer of free energy from the exergonic to the endergonic processes.

There are 3 major sources of ATP taking part in energy conversation: 1) oxidative decarboxylation. This process is a greatest quantitative source of ATP in aerobic conditions. The free energy to drive this process comes from respiratory chain oxidation within mitochondria. 2) glycolysis is a formation of 2ATP results from the formation of lactate from 1 molecule of glucose. 3) the citric acid cycle yields 1 ATP.

Bioenergetic metabolism has a few stages:

1) the specific cleavage of the main nutrients (that results in formation of acetylCoA)

2) Crebs cycle (where the reduced forms of dehydrogenases are formed)

3) Biologic oxidation (in this process the reduced dehydrogenases are oxidized to give energy and water)

4) Oxidative phosphorylation (formation of ATP with usage of energy of biologic oxidation)

During 1 st stage proteins are hydrolyzed to aminoacids; lipids (mainly trigliceraides) to glycerol and FFA: carbohydrates (mainle glycogen and starch) to glucose. It occurs in gastrointestinal tract under action of hydrolases of salivary glands (alfa-amylase), pancreas (trypsin, chymotrypsin, carboxypeptidase, aminopeptidase, dipeptidases, tripeptidases, lipases, phospholipases, cholesterolesterase etc). then glucose, amino acids, glycerol and FFA are absorbed anf transformed in tissues. Glucose is catabolized to pyruvic acid; glycerol through phosphotrioses to pyruvic acid, amino acids to pyruvic or active acetic acid and FFA to active acetic acid. Pyruvic acid undergoes changes, giving active acetic acid (or acetylCoA). This reaction is named as oxidative decarboxylation of pyruvic acid. To sum up, the 1^st stage is ended by formation of acetylCoA which goes to Crebs cycle (2^nd stage)

Crebs stage occurs in mitochondrions. Its summary is: acetylCoAà 3 NADH2+FPH2+ATP+2CO2.

This process includes the following reactions:

1) AcetylCoA+oxaloacetic acidàcitric acid (citratesynthase)