A Chronoamperometric Kinetic Study of the Reduction of the Double Fe\u3csub\u3e4\u3c/sub\u3eS\u3csub\u3e4\u3c/sub\u3e Clustered Ferredoxin of Clostridium Pasteurianum by Methylviologen

Abstract

In advancing our earlier work on single cluster redox proteins, we report here a chronoamperometric kinetic study of the homogeneous reduction of Clostridium pasteurianum ferredoxin (n = 2, U°7.6 = −395 mV versus n.h.e.) by methylviologen (U°7.0 = −420 mV). This ferredoxin has two Fe4S4* (S* = inorganic sulfide) clusters each of which has n = 1 and can be formally represented as 8 Fe0,0, 8 Fe0,r and 8 Fer,r′ This ferredoxin is similar to P. aerogenes ferredoxin for which the X-ray crystallographic structure shows that the two clusters are about 1.2 nm apart. Rates were determined at two temperatures. Using computer simulation of the catalytic mechanism with two homogeneous electron transfer steps, it was found that both electrons are transferred at equal rates, with k25°C = 4.7×105 M−1 s−1 and k15°C = 1.5 × 105M−1 s−1. Since k1 = k2 at both temperatures it is concluded that under these reducing conditions, where 8 Fe0,r does not come to equilibrium with other species during the reduction process, the two clusters operate independently of each other