METALLOPROTEIN-COBALT CAGE ELECTRON-TRANSFER AND THE STEREOSELECTIVE REDUCTION OF SPINACH PLASTOCYANIN BY AMBDA-[CO((N(CH3)3)2-SAR]4-[CO((N(CH3)3)2-SAR)]4+( AND DELTA)

Electron transfer kinetics for the reduction of spinach plastocyanin a nd horse heart cytochrome c by several cobalt-cage complexes have been determined. The reduction of plastocyanin by [Co((N(CH3)3)2-sar)]4+ s hows saturation kinetics with K(Co(II)) = (1.5 +/- 0.2) X 10(3) M-1, a nd k'(et) = 4.0 +/- 0.2 s-1. This reaction is stereoselective (k(LAMBD A)/k(DELTA) = 1.7) and the stereoselectivity is independent of ionic s trength. The reaction is observed to depend upon pH, pK(a) = 5.3 +/- 0 .1, with the protonated protein being much less reactive than its conj ugate base. The reaction is inhibited by added [Co((N(CH3)3)2-sar)]5+, K(Co(III)) = (3.9 +/- 10.1) X 10(4) M-1 and the protein-inhibitor com plex has about 7% of the redox reactivity of the free protein, k''(et) = (4.4 +/- 0.2) X 10(2) M-1 s-1. The reduction of plastocyanin by [Co ((NH2)2-sar)]2+ occurs with a second-order rate constant of (1.13 +/- 0.06) X 10(6) M-1 s-1 and without stereoselectivity. Both reactions we re studied at pH 7.5, I = 0.050 M, and 25-degrees-C. Reduction of hors e cytochrome c by [Co-((NH2)2-sar)]2+ and [Co(NO2-capten)]2+ are obser ved to occur with simple second-order kinetics with rate constants of (4.06 +/- 0.05) X 10(4) M-1 s-1 and (2.02 +/- 0.06) X 10(5) M-1 s-1, r espectively, at pH 7.0, I = 0.050 M, and 25-degrees-C. The relevance o f these results to the mechanisms of metalloprotein electron transfer and the suitability of cage complexes as metalloprotein redox mediator s is discussed.