Structural and mechanistic roles of three consecutive Pro residues of porcine NADH-cytochrome b5 reductase for the binding of β-NADH

Well-conserved three consecutive Pro residues (Pro247–249) in the NADH-binding subdomain of NADH-cytochrome b₅ reductase were proposed to form a basal part of the NADH-binding site. To investigate the structural and mechanistic roles of these residues, we expressed site-directed mutants for a soluble domain of the porcine enzyme where each of the residues was replaced with either Ala or Leu residue, respectively, using a heterologous expression system in Escherichia coli. Six mutants (P247A, P247L, P248A, P248L, P249A, and P249L) were produced as a fusion protein containing a 6×His-tag sequence at the NH₂-terminus and were purified to homogeneity with a stoichiometric amount of bound FAD. Mutations were each confirmed for the purified proteins by MALDI-TOF mass spectrometry. Steady-state kinetic analyses for NADH:ferricyanide reductase and NADH:cytochrome b₅ reductase acitivities were conducted for all the mutants. Substitution of Pro247 with Leu residue was found to significantly decrease k_cat with slight increase in K_m for the physiological electron donor NADH. However, K_m values for the electron acceptors (both cytochrome b₅ and ferricyanide) of P247L were found to be decreased significantly. Such changes were not observed for P247A or other four mutants. These results suggested that Pro247 among the three consecutive Pro residues has the most important role for the formation of a binding site cavity and that only a slight change in the side-chain volume at this residue from Ala to Leu residue affected the electron transfer reaction from NADH and, further, on the recognition of ferricytochrome b₅.