[Investigation of essential SH-groups of bacterial formate dehydrogenase]

Modification of two SH-groups in the molecule of formate dehydrogenase by dithiobisnitrobenzoate or to dacetamide results in the enzyme inactivation. Coenzymes, but not the substrate, protect the enzyme against the inactivation. NAD in the presence of potassium azide completely preserves the enzyme activity. Two SH-groups per enzyme molecule are protected from modification. The Km values for partially inactivated formate dehydrogenase remain constant for both substrates. The enzyme with modified SH-groups does not bind conezymes. The pH-dependence of the inactivation rate reveals the ionizable group with pK 9.6 (25 degrees C). The involvement of essential SH-groups in coenzyme binding is discussed.