Electron-paramagnetic-resonance studies on nitrogenase of Klebsiella pneumoniae. Evidence for acetylene- and ethylene-nitrogenase transient complexes

Abstract

K. pneumoniae nitrogenase exhibited 4 new EPR signals during turnover at 10.degree. C, pH 7.4, which were assigned to intermediates present in low concentrations in the steady state. 57Fe-substituted Mo-Fe protein showed that they arose from Fe-S clusters in the Mo-Fe protein of nitrogenase. The new signals are designated: Ic, g values at 4.67, 3.37 and approximately 2.0; VI, g values at 2.125, 2.000 and 2.000; VII, g values at 5.7 and 5.4; VIII, g values at 2.092, 1.974 and 1.933. The sharp axial signal VI arises from a Fe4S4 cluster at the -1 oxidation level. This signal was only detected in the presence of ethylene and provides the 1st evidence of an enzyme-product complex for nitrogenase. [13C]Acetylene and [13C]ethylene provided no evidence for direct binding of this substrate and product to the Fe-S clusters giving rise to these signals. The dependence of signal intensities on acetylene concentration indicated 2 types of binding site, with apparent dissociation constants K < 16 .mu.M and K .apprx. 13 mM. A single binding site for ethylene (K = 1.5 mM) was detected. A scheme is proposed for the mechanism of reduction of acetylene to ethylene and inhibition of this reaction by CO.