Specific modification of a Na+ binding site in NADH:quinone oxidoreductase from Klebsiella pneumoniae with dicyclohexylcarbodiimide

The respiratory NADH:quinone oxidoreductase (complex I) (NDH-1) is a multisubunit enzyme that translocates protons (or in some cases Na+) across energy-conserving membranes from bacteria or mitochondria. We studied the reaction of the Na+-translocating complex I from the enterobacterium Klebsiella pneumoniae with N,N'-dicyclohexylcarbodiimide (DCCD), with the aim of identifying a subunit critical for Na+ binding. At low Na+ concentrations (0.6 mM), DCCD inhibited both quinone reduction and Na+ transport by NDH-1 concurrent with the covalent modification of a 30-kDa polypeptide. In the presence of 50 mM Na+, NDH-1 was protected from inhibition by DCCD, and the modification of the 30-kDa polypeptide with [14C]DCCD was prevented, indicating that Na+ and DCCD competed for the binding to a critical carboxyl group in NDH-1. The 30-kDa polypeptide was assigned to NuoH, the homologue of the ND1 subunit from mitochondrial complex I. It is proposed that Na+ binds to the NuoH subunit during NADH-driven Na+ transport by NDH-1.