Redox coupled conformational change

D. Fourier Transform Infrared Spectroscopic Examination of Redox-Coupled Conformational Change in the Protein Moiety... 

E. Redox-Coupled Conformational Change in Bovine Heart Cytochrome c Oxidase... 

As stated above, for proton transfers inside cytochrome c oxidase, redox-coupled conformational changes are required. As shown in Fig. 18 (see color insert for A), a fairly large conformational change, including even a movement of the peptide backbone in a loop region between helices I and II of subunit I of bovine heart cytochrome c oxidase, was... 

Fig. 18. Redox-coupled conformational change in a loop between helices I and II of subunit I. A stereoview (A, see color insert) and a schematic representation of the hydrogen bond network connecting Asp-51 with the matrix space (B). (A) The molecular surface on the intermembrane side is shown by small dots. Maroon and green sticks represent the structures in the fully oxidized and reduced states. (B) Dotted lines show hydrogen bonds. The rectangle represents a cavity near heme a. The two dotted lines connecting the matrix surface and the cavity represent the water path. The dark balls show the positions of the fixed water molecules.

FIGURE 7. Redox coupled conformational change in the segment from Gly49 to Asn55. The accessible surface on the intermembrane side for the fully oxidized state is indicated by dots. 

The redox-coupled conformational change and the two-electron change from P to P° has led most researchers to view the P cluster as a diode and/or capacitor for electron flow into the FeMoco. This hypothesis explains the ability of the nitrogenase protein to build up sufficient reducing equivalents at one site (the FeMoco) to accomplish the difficult reduction of N2. However, more work is necessary to establish the ways in which the protein controls the rate and direction of electron transfer. " ... 

Upon addition of Ba2+ cations, the 2+/l+ bipyridinium redox couple is shifted anodically by 45 mV and the l+/0 couple is shifted cathodically by 10 mV. K+ and NH4 produce similar effects (Table 15). However, addition of Na+ cations causes a small cathodic shift to the 2+/l+ couple and an anodic perturbation to the l+/0 couple. This is in agreement with the proposed conformational change pathway for coupling the complexation and redox reactions. 

How eould this eatalytie bias be controlled One possibility is that the proton transfer pathway eould eontribute to specifieity (Peters et al., 1998). Another possibility is that differences in midpoint potential of the FeS clusters (or other redox sites) that constitute the intramolecular wire could be tuned to facilitate one of the two directions of the reaction. For example, these redox sites could best match the midpoint potentials of a particular oxidized or reduced electron carrier (Holm and Sander, 1999). Apparently, a conformational change in succinate dehydrogenase, coupled to the reduction of FAD, is responsible for its catalytic bias for fumarate reduction (Hirst et al., 1996). 

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