Respiratory carriers enzymes

Many of these copper proteins are enzymes, and the copper is a part of their active group (Nos. 1, 10-14 in Table 4), while others have no known enzyme activity (Nos. 2-9). As far as is known, none of these proteins functions as a respiratory carrier, as hemocyanin does in mollusks. It has been suggested, but not proven, that the human liver copper protein of Morell et al. (M32) and the hepatic mitochondrocuprein of Porter et al. (P13, P15) may function as copper storage proteins, similar perhaps to ferritin in the case of iron. 

Flavoprotein, Warburg s yellow enzyme, is a widely distributed natural pigment which acts as a respiratory carrier in the oxidation of hexose phosphates, malate and alcohol. It is a conjugated protein, the prosthetic flavin of which was discovered by Szent-Gybrgyi, who named it cytoflave prior to its indentifica-tion with vitamin Bg. In the absence of the protein component, the flavin has no respiratory activity. 

A. Dehydrogenases, dehydrases, oxido-reductases or hydrogen-transportases are widely distributed in vertebrate, invertebrate and plant tissues. Most of them are highly specific enzymes, and operate in association with respiratory carriers, which are much less specific. As a class, they are inhibited by narcotics, but not by cyanide. Important examples are ... 

In the yellow enzyme, the prosthetic group is the phosphate of ribofiavin, or vitamin Bg (p. 256). The flavoprotein carrier differs from hematin carriers in three respects (i.) it does not contain iron and is not inhibited by CO, HCN and H S (ii.) it requires the presence of an additional carrier, co-enzyme II (iii.) it is capable of being reoxidised by free oxygen without the aid of an oxidase. At the same time, flavoprotein can work in conjunction with cytochrome to form a system containing three successive respiratory carriers. 

NAD+ and NADP+ are coenzymes of dehydrogenases. NADH and NADPH are intermediate carriers of both hydrogen and electrons. Most NAD-dependent enzymes are located in the mitochondria and deliver H2 to the respiratory chain whereas NADP-dependent enzymes take part in cytosolic syntheses (reductive biosyntheses). 

In the third step, 1, -/3-hydroxyacyl-CoA is dehydrogenated to form /3-ketoacyl-CoA, by the action of /3-hydroxyacyl-CoA dehydrogenase NAD+ is the electron acceptor. This enzyme is absolutely specific for the l stereoisomer of hydroxyacyl-CoA The NADH formed in the reaction donates its electrons to NADH dehydrogenase, an electron carrier of the respiratory chain, and ATP is formed from ADP as the electrons pass to 02. The reaction catalyzed by /3-hydroxyacyl-CoA dehydrogenase is closely analogous to the malate dehydrogenase reaction of the citric acid cycle (p. XXX). 

The inner mitochondrial membrane can be disrupted into five sepa rate enzyme complexes, called complexes I, II, III, IV, and V. Complexes I to IV each contain part of the electron transport chain (Figure 6.8), whereas complex V catalyzes ATP synthesis (see p. 78). Each complex accepts or donates electrons to relatively mobile electron carriers, such as coenzyme Q and cytochrome c. Each car rier in the electron transport chain can receive electrons from an electron donor, and can subsequently donate electrons to the next carrier in the chain. The electrons ultimately combine with oxygen and protons to form water. This requirement for oxygen makes the electron transport process the respiratory chain, which accounts for the greatest portion of the body s use of oxygen. 

Warburg and Christian showed that the color of this old yellow enzyme came from a flavin and proposed that its cyclic reduction and reoxidation played a role in cellular oxidation. When NADP+ was isolated the proposal was extended to encompass a respiratory chain. The two hydrogen carriers NADP+ and flavin would work in sequence to link dehydrogenation of glucose to the iron-containing catalyst that interacted with oxygen. While we still do not know the physiological function of the old yellow enzyme,b the concept of respiratory chain was correct. 

Figure 3.2 shows the respiratory chain. It consists of enzymes, which are electron carriers, each of which interacting with one another is capable of reversible oxidation—restoration. 

The energy saved in the electron carriers NADH and FADH2 is transferred into the respiratory chain. The enzyme complexes of the respiratory chain are physically located in the inner membrane (Figure 17.2). The unique characteristic (impermeability to almost all molecules) of the inner membrane is essential for this reaction as described below. The reaction in the respiratory chain is very simple. 

Fig. 1. A schematic illustration of the mitochondrial (A) and Escherichia coli (B) respiratory chains. Respiratory enzymes perform a series of oxidation-reduction reactions by transferring electrons (dashed lines) through mobile electron carriers. Electron transfer is coupled to the pumping of protons (thick black arrows) from the A/-side (negative side) to the P-side (positive side) generating a proton gradient that ultimately drives the conversion of ADP to ATP.

The electron carriers in the respiratory assembly of the inner mitochondrial membrane are quinones, flavins, iron-sulfur complexes, heme groups of cytochromes, and copper ions. Electrons from NADH are transferred to the FMN prosthetic group of NADH-Q oxidoreductase (Complex I), the first of four complexes. This oxidoreductase also contains Fe-S centers. The electrons emerge in QH2, the reduced form of ubiquinone (Q). The citric acid cycle enzyme succinate dehydrogenase is a component of the succinate-Q reductase complex (Complex II), which donates electrons from FADH2 to Q to form QH2.This highly mobile hydrophobic carrier transfers its electrons to Q-cytochrome c oxidoreductase (Complex III), a complex that contains cytochromes h and c j and an Fe-S center. This complex reduces cytochrome c, a water-soluble peripheral membrane protein. Cytochrome c, like Q, is a mobile carrier of electrons, which it then transfers to cytochrome c oxidase (Complex IV). This complex contains cytochromes a and a 3 and three copper ions. A heme iron ion and a copper ion in this oxidase transfer electrons to O2, the ultimate acceptor, to form H2O. 

A variety of in vitro toxicity tests have been developed to model the effects of toxins on living cells or tissues. In these tests, a carrier medium (such as fetal bovine serum) containing given concentrations, or doses, of a particular toxin are added to cell cultures (cell lines). Various indicators of toxicity, cell morphology transformation, or cell prohferation are then measured after specified periods of time. The cell types used in a particular study can be chosen to approximate the types of cells that would be affected during acmal exposure, such as respiratory cells or tissues. Toxicity indicators include, for example, measures of the percent of viable cells remaining at the end of the test (compared to a control line with no added toxin), and the concentrations various cytokines or other cytoplasmic enzymes induced from the cells by the toxin. Uncertainties with the in vitro toxicity tests include how comparable their results are to those of in vivo toxicity tests, and how well they reproduce actual physiological conditions and processes in the human body (Johnson and Mossman, 2001). 

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