Cytochrome 65 reductase, complex with

Figure 6 Cytochromes P-450 reductase complexed with NADP (cyan). Trp677 (orange) is stacked against FAD and must move for hydride transfer from NADPH.

The structure of the UQ-cyt c reductase, also known as the cytochrome bc complex, has been determined by Johann Deisenhofer and his colleagues. (Deisenhofer was a co-recipient of the Nobel Prize in Chemistry for his work on the structure of a photosynthetic reaction center [see Chapter 22]). The complex is a dimer, with each monomer consisting of 11 protein subunits and 2165 amino acid residues (monomer mass, 248 kD). The dimeric structure is pear-shaped and consists of a large domain that extends 75 A into the mito-... 

Complex III (ubiquinol-cytochrome c oxido-reductase or cytochrome bct complex). Mitochondrial complex III is a dimeric complex, each subunit of which contains 11 different subunits with a total molecular mass of 240 kDa per monomer.104-107 However, in many bacteria the complex consists of only three subunits, cytochrome b, cytochrome c , and the high potential ( 0.3 V) Rieske iron-sulfur protein, which is discussed in Chapter 16, Section A,7. These three proteins are present in all bc1 complexes. 

The purification and characterization of individual cytochromes (simple or complex) and other redox centres in electron-transfer chains leads to a study of their interactions with each other, with the ultimate objective of reconstituting parts of the chain. Approaches to this problem will be illustrated724 with reference to the reaction of cytochrome c with ubiquinol cytochrome c reductase (complex III, cytochrome bcx) and cytochrome oxidase (complex IV) as shown in equation (2l). 

Smith, R. J., Capaldi, R. A., Muchmore, D., and Dahlquist, F. (1978) Cross-linking of ubiquinone cytochrome c reductase (complex III) with periodate-cleavable bifunctional reagents. Biochemistry 17, 3719—3723. 

The principal function of cyt. c is to form complexes through a defined interface with protein partners in our cells. This is most established for eukaryotic cytochrome c within the mitochondrial electron transport chain (ETC), a process required for carrying out the oxidative phosphorylation of ATP.4 Formation of a complex with cyt. c reductase (an electron-donor protein from complex III) and cyt. c oxidase (an electron-acceptor protein from complex IV) leads to the transfer of electrons between otherwise separated proteins. More recently cyt. c has been found to play a critical role in the process of apoptosis or programmed cell death This in turn has led to a resurgence of interest in all aspects of cyt. c research.5 Again protein-protein interactions have been shown be essential with mitochrondrial cyt. c binding to such proteins as APAF-1 to form the multi-protein species known as the apoptosome that is now thought to be a requirement for apoptosis.6,7... 

The system depends on an electron transport pathway that transfers electrons from NADPH through a flavoprotein (NADPH cytochrome P-450 reductase) to cytochrome P-450 that is the terminal oxidase of the chain (10). The xenobiotic first forms a complex with the oxidized form o cytochrome P-450 which is reduced by an electron passing down the chain from NADPH. The reduced cytochrome P-450/substrate complex then reacts with and activates molecular oxygen to an electrophilic oxene species (an electron deficient species similar to singlet oxygen) that is transferred to the substrate with the concommitant formation of water. Cytochrome P-450 thus acts primarily as an oxene transferase (2). Substrate binding is a relatively nonspecific, passive process that serves to bring the xenobiotic into close association with the active center and provide the opportunity for the oxene transfer to occur. 

Recently, de novo-synthesized four-helix polypeptides were applied to mimic functions of cytochrome b and to tailor layered cross-linked electrocatalytic electrodes. A four-helix bundle de novo protein (14728 Da) that includes four histidine units in the respective A -helices was assembled on Au electrodes (Figure 22A). Two units of Fe(III)-protoporphyrin IX were reconstituted into the assembly to yield a vectorial electron-transfer cascade [157]. The de novo-synthesized protein assembly forms affinity complexes with the cytochrome-dependent nitrate reductase (NR) and with Co(II) protoporphyrin IX-reconstituted myoglobin [158]. The resulting layered complex of Fe(III) de novo protein-NR or Fe(lll)-de novo protein-Co(II)-reconstituted myoglobin was cross-linked with glutaric dialdehyde to yield electrically contacted electrocatalytic electrodes. The Fe(lll)-de novo protein-NR electrode assembly was applied for the electrocatalyzed reduction NO3 to NOt" and acted as an amperometric sensor (Figure 22B). The Fe(III)-de novo... 

Mitochondrial cytochrome c is the most widely investigated heme protein with respect to its electrochemical properties. It is active in electron transfer pathways such as the respiratory chain in the mitochondria where it transfers electrons between membrane bmmd C3d ochrome reductase complex III and cytochrome c oxidase. The active site is an iron porphyrin (heme) covalently linked to the protein at Cysl4 and Cysl7 through thioether bonds (heme c). The iron itself lies in the plane of the porphyrin ring, the two axial positions... 

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