Carbonyl sulfide (COS) has been investigated as a rapid-equilibrium inhibitor of CO oxidation
by the CO dehydrogenase purified from Rhodospirillum rubrum. The kinetic evidence suggests that the
inhibition by COS is largely competitive versus CO (Ki = 2.3 pM) and uncompetitive versus methylviologen
as electron acceptor (Ki = 15.8 pM). The data are compatible with a ping-pong mechanism for CO oxidation
and COS inhibition. Unlike the substrate CO, COS does not reduce the iron-sulfur centers of dye-oxidized
CO dehydrogenase and thus is not an alternative substrate for the enzyme. However, like CO, COS is capable
of protecting CO dehydrogenase from slow-binding inhibition by cyanide. A true binding constant (KD)
of 2.2 pM for COS has been derived on the basis of the saturable nature of COS protection against cyanide
inhibition. The ability of CO, C02, COS, and related CO/CO, analogues to reverse cyanide inhibition
of CO dehydrogenase is also demonstrated. The kinetic results are interpreted in terms of two binding sites
for CO on CO dehydrogenase from R. rubrum.Keywords: Rhodospirillum rubrum, Carbonyl Sulfide inhibitio
