SUBUNIT FUNCTIONAL-STUDIES OF NAD(P)H-QUINONE OXIDOREDUCTASE WITH A HETERODIMER APPROACH

NAD(P)H:quinone oxidoreductase (NQOR; EC 1.6.99.2) is a homodimeric enzyme which catalyzes the reduction of quinones, azo dyes, and other electron accepters by NADPH or NADH, To pursue subunit functional studies, we expressed a wild-type/mutant heterodimer of NQOR in Escherichia coli, The wild-type subunit of the heterodimer was tagged with polyhistidine and the other subunit contained a His-194 --> Ala mutation (H194A), a change known to dramatically increase the K-m for NADPH. This approach enabled us to efficiently purify the heterodimer (H194A/HNQOR) from the homodimers by stepwise elution with imidazole from a nickel nitrilotriacetate column under nondenaturing conditions, The composition of the purified heterodimer was confirmed by SDS and nondenaturing polyacrylamide gel electrophoresis and immunoblot analysis, The enzyme kinetics of the purified heterodimer were studied with two two-electron accepters, 2,6-dichloroindophenol and menadione, and a four-electron acceptor, methyl red, as the substrates. With two-electron accepters, the K-m(NADPH) and K-m(NADH) values of the heterodimer H194A/HNQOR were virtually identical to those of the wild-type homodimer, but the k(cat(NADPH)) and k(cat(NADH)) values were only about 50% those of the wild-type homodimer. With the four-electron acceptor, the K-m and k(cat) values of H194A/HNQOR for NADPH and NADH were similar to those of the low-efficiency mutant homodimer. These results suggest that the subunits of NQOR function independently with two-electron accepters, but dependently with a four-electron acceptor. This heterodimer approach may have general applications for studying the functional and structural relationships of subunits in dimeric or oligomeric proteins.