Biochemistry

Further electron transfer from the reduced form of ubiquinone to oxygen is carried out by a system of cytochromes. Cytochromes. Cytochromes (from the Greek. Cytos - cell, chroma - color) - pigments colored in red due to the presence of iron in their molecule . It has been established that in the oxidation chain the cytochrome link is included in a certain sequence between ubiquinone and oxygen. cit В — » cit Ci — ♦ cit C -* cit A' — > cit A 3 Moreover, cytochromes В, C i , C, play the role of intermediate electron carriers, and cytochrome Л3 (cytochrome oxidase) is the final respiratory enzyme that comes into direct contact with oxygen. All cytochromes have a .prosthetic hemin group identical to the hemoglobin heme. Cytochromes differ from each other not only in prosthetic groups. Differences in the properties of their individual species are due to the structure of the protein part. So, in cytochromes В and C, heme is covered by a “ screen ” of peptides, which prevents their direct reaction with oxygen. During the catalytic process, the valency of the iron contained in the cytochromes reversibly changes (Fe 2+ — ♦ Fe 3+ ). The oxidized form of cytochrome oxidase (Fe 3+ ) receives electrons from the reduced cytochrome C, passing into the reduced form (Fe 2+ ), which is then re ­ oxidized to Fe 3+ by molecular oxygen at the stage of cytochrome A3. In this case, active oxygen is formed, which attaches two protons from the environment, as a result of which a water molecule appears. Thus, cytochromes complete the transport of electrons from the oxidizing substrate to oxygen. 4.3 ELECTRON AND PROTON TRANSPORT AT BIOLOGICAL OXIDATION The entire system of reactions and enzymes of tissue respiration is called the respiratory chain. It consists of three main stages. The first stage is the processes of substrate dehydrogenation, catalyzed mainly by nicotinamide enzymes (NAD and NADP). The second stage is the oxidation processes of reduced forms of nicotinamide enzymes (NADH 2 and NADPH 2 ), catalyzed mainly by flavin enzymes. The third stage is the oxidation processes of reduced forms of flavin enzymes catalyzed by a system of cytochromes. Numerous intermediate proton and electron carriers are involved in the electron transport chain. These include, for example, cysteine, which turns into cystine during oxidation, vitamin C, which forms a reversibly oxidized form 109