Protein-bound redox, generation

An alternative approach to the generation of suitable protein-bound redox was also investigated. Nitration of surface Tyr residues in CCP was carried out to generate protein-bound reducing N02 -Tyr radicals in situ (28), and our preliminary results are provided in the section Pulse Radiolysis Studies of CCP/ Finally, the use of flash photolysis and pulse radiolysis techniques in the study of Fe =0 heme systems is compared. 

There may be common themes in the role of protein-coenzyme contacts in these B -dependent enzymatic processes. In particular, these contacts could alter the relative stability of the Co(III)—R, Co(II), and Co(I) states to enhance reactivity. For coenzyme B 12-dependent enzymes, the deoxyadenosyl radical generates a substrate-derived radical, either directly or via a radical chain mechanism through the intermediacy of a protein-side-chain-based radical, such as S of cysteine or O of tyrosine. This protein-bound substrate-derived radical then undergoes rearrangement, possibly assisted by protein contacts. Thus, cofactor-protein contacts are probably very important in the activation of the Co—C bond, in altering the Co redox potentials, and in assisting in the rearrangements. 

Fig. 4. (A) EPR spectra at 13 K of TSF-I subchloroplast particles at different stages of a reductive titration (B) Plot of EPR-signal amplitudes of four prominent lines for redox titrations at pHs 10 and 9. Open circles in the right panel represent the total signal amplitude generated by a combination of chemical reduction at room temperature and subsequent illumination at 77 K. Figure source Ke, Hansen and Beinert (1973) Oxidation-reduction potentiais of bound iron-suifur proteins of photosystem t. Proc Nat Acad Sci, USA 70 2042, 2043.

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