Intramolecular Electron Transfer from c Heme to d1 Heme in Bacterial Cytochrome cd1 Nitrite Reductase Occurs over the Same Distances at Very Different Rates Depending on the Source of the Enzyme

Intramolecular electron transfer over 12 A from heme c to heme d(1) was investigated in cytochrome cd(1) nitrite reductase from Pseudomonas aeruginosa, following reduction of the c heme by pulse radiolysis. The rate constant for the transfer is relatively slow, k = 3 s(-1). The present observations contrast with a corresponding rate of electron transfer, 1.4 x 10(3) s(-1), measured for cytochrome cd(1) from Paracoccus pantotrophus, though the relative positions of the two heme groups are the same in both enzymes. The rate of intramolecular electron transfer within the enzyme from P. aeruginosa was accelerated 10(4)-fold (1.4 x 10(4) s(-1)) by the binding of cyanide to the d(1) heme. A coordination change at the d(1) heme upon its reduction is suggested to be a major factor in determining the slow rate of electron transfer in the P. aeruginosa enzyme in the absence of cyanide.