Oxidation-reduction reactions cytochrome

The abihty of iron to exist in two stable oxidation states, ie, the ferrous, Fe ", and ferric, Fe ", states in aqueous solutions, is important to the role of iron as a biocatalyst (79) (see Iron compounds). Although the cytochromes of the electron-transport chain contain porphyrins like hemoglobin and myoglobin, the iron ions therein are involved in oxidation—reduction reactions (78). Catalase is a tetramer containing four atoms of iron peroxidase is a monomer having one atom of iron. The iron in these enzymes also undergoes oxidation and reduction (80). 

Chlorophyll, plastoquinone, and cytochrome are complicated molecules, but each has an extended pattern of single bonds alternating with double bonds. Molecules that contain such networks are particularly good at absorbing light and at undergoing reversible oxidation-reduction reactions. These properties are at the heart of photosynthesis. 

Metal ions in the form of organometallic complexes such as the iron atom in heme can undergo one-electron transfers in oxidation-reduction reactions catalyzed by oxidoreductases with associated cytochromes. 

Complex IV Cytochrome Oxidase. Cytochrome oxidase contains two atoms of Cu in addition to the hemes of cytochromes a and a3. The Cu atoms undergo one-electron oxidation-reduction reactions between the cuprous (Cu+) and cupric (Cu2+) states. One of the Cu atoms (CuB) is close to the Fe of cytochrome a3 (fig. 14.12). The other (CuA) is associated with cytochrome a, but not so intimately. Oxidation of cytochrome c takes place on the side of the membrane facing the intermembrane space, whereas the reduction of 02 by cytochrome a3 and CuB occurs on the matrix side. 

Cytochrome c, a small heme protein (mol wt 12,400) is an important member of the mitochondrial respiratory chain. In this chain it assists in the transport of electrons from organic substrates to oxygen. In the course of this electron transport the iron atom of the cytochrome is alternately oxidized and reduced. Oxidation-reduction reactions are thus intimately related to the function of cytochrome c, and its electron transfer reactions have therefore been extensively studied. The reagents used to probe its redox activity range from hydrated electrons (I, 2, 3) and hydrogen atoms (4) to the complicated oxidase (5, 6, 7, 8) and reductase (9, 10, 11) systems. This chapter is concerned with the reactions of cytochrome c with transition metal complexes and metalloproteins and with the electron transfer mechanisms implicated by these studies. 

The pH dependence of cytochrome c oxidation-reduction reactions and the studies of modified cytochrome c thus demonstrate that the coordination environment of the iron and the conformation of the protein are relatively labile and strongly influence the reactivity of the metallo-protein toward oxidation and reduction. The effects seen may originate chiefly from alterations in the thermodynamic barriers to electron transfer, but the conformation changes are expected to affect the intrinsic barriers also. One such conformation change is the opening of the heme crevice referred to above. The anation and Cr(II) reduction studies provide an estimate of 60 sec 1 for this process in Hh(III) at 25°C (59). To date, no evidence has been found for a rapid heme-crevice opening step in ferrocytochrome c. 

The vast majority of flavoenzymes catalyze oxidation-reduction reactions in which one substrate becomes oxidized and a second substrate becomes reduced and the isoalloxazine ring of the flavin prosthetic group (Figure 1) serves as a temporary repository for the substrate-derived electrons. The catalytic reaction can be broken conveniently into two steps, a reductive half reaction (from the viewpoint of the flavin) and an oxidative half reaction. The flavin ring has great utility as a redox cofactor since it has the ability to exist as a stable semiquinone radical. Thus, a flavoenzyme can oxidize an organic substrate such as lactate by removal of two electrons and transfer them as a pair to a 2-electron acceptor such as molecular oxygen, or individually to a 1-electron acceptor such as a cytochrome. 

Fig. 6.3. Scheme for PP - and ATP-induced oxidation reduction reactions in cytochromes in R. rubrum chromatophores. 

Finally, because transition metals have two or more valence states, they can mediate oxidation-reduction reactions. For example, the reversible oxidation of Fe2+ to form Fe3+ is important in the function of cytochrome P450. Cytochrome P/sn is a microsomal enzyme found in animals that processes toxic substances (Chapter 10). 

There are many examples of biological oxidation-reduction reactions. For example, the electron-transport chain of aerobic respiration involves the reversible oxidation and reduction of iron atoms in cytochrome c. 

Before we try to understand the mechanism of oxidative phosphorylation, let s first look at the molecules that carry out this complex process. Embedded within the mitochondrial inner membrane are electron transport systems. These are made up of a series of electron carriers, including coenzymes and cytochromes. All these molecules are located within the membrane in an arrangement that allows them to pass electrons from one to the next. This array of electron carriers is called the respiratory electron transport system (Figure 22.7). As you would expect in such sequential oxidation-reduction reactions, the electrons lose some energy with each transfer. Some of this energy is used to make ATP. 

The cytochrome P-450 isozymes are a family of microsomal enzymes that collectively have the capacity to transform thousands of different molecules. The transformations include hydroxylations and dealkylations, as well as the promotion of oxidation/reduction reactions. These enzymes have an absolute requirement for NADPH and Oj. The various isozymes have different substrate and inhibitor specificities. 

One of the tenets of the chemiosmotic theory is that energy from the oxidation-reduction reactions of the electron transport chain is used to transport protons from the matrix to the intermembrane space. This proton pumping is generally facilitated by the vectorial arrangement of the membrane spanning complexes. Their stracture allows them to pick up electrons and protons on one side of the membrane and release protons on the other side of the membrane as they transfer an electron to the next component of the chain. The direct physical link between proton movement and electron transfer can be illustrated by an examination of the Q cycle for the b-Ci complex (Fig. 21.9). The Q cycle involves a double cycle of CoQ reduction and oxidation. CoQ accepts two protons at the matrix side together with two electrons it then releases protons into the intermembrane space while donating one electron back to another component of the cytochrome b-Ci complex and one to cytochrome c. 

Energy transformation Cytochromes are proteins found in all cells. They extract energy from food molecules by transferring electrons in a series of oxidation-reduction reactions. 

Reflect and Apply Cytochrome oxidase and succinate-CoQ oxido-reductase are isolated from mitochondria and are incubated in the presence of oxygen, along with cytochrome c, coenzyme Q, and succinate. What is the overall oxidation-reduction reaction that can be expected to take place ... 

Reflect and Apply Why do the electron-transfer reactions of the cytochromes differ in standard reduction potential, even though all the reactions involve the same oxidation-reduction reaction of iron ... 

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