Farver O, Kroneck PMH, Zumft WG, Pecht I
Intramolecular electron transfer in cytochrome cd1 nitrite reductase from Pseudomonas
stutzeri; kinetics and thermodynamics.
Biophys Chem 98:27-34 (2002)
Cytochrome cd1 nitrite reductase from Pseudomonas
stutzeri catalyzes the one electron reduction of nitrite to nitric oxide. It
is a homodimer, each monomer containing one heme-c and one heme-d1, the former being the electron
uptake site while the latter is the nitrite reduction site. Hence, internal
electron transfer between these sites is an inherent element in the catalytic
cycle of this enzyme. We have investigated the internal electron transfer
reaction employing pulse radiolytically produced N-methyl nicotinamide radicals
as reductant which reacts solely with the heme-c in an essentially diffusion
controlled process. Following this initial step, the reduction equivalent is
equilibrating between the c and d1 heme sites in a unimolecular process (k=23 s-1, 298 K, pH 7.0) and an equilibrium
constant of 1.0. The temperature dependence of this internal electron transfer
process has been determined over a 277-313 K temperature range and yielded both
equilibrium standard enthalpy and entropy changes as well as activation
parameters of the specific rate constants. The significance of these parameters
obtained at low degree of reduction of the enzyme is discussed and compared
with earlier studies on cd1 nitrite reductases from other sources.