L-Aspartate oxidase, a newly discovered enzyme of Escherichia coli, is the B protein of quinolinate synthetase

Abstract

In Escherichia coli, quinolinic acid, a precursor of NAD+, is synthesized from L-aspartate and dihydroxyacetone phosphate. This synthesis requires two enzymes, a FAD-containing "B protein" and an "A protein." The B protein has been purified 500-fold from E. coli cells. The enzyme behaves as an L-aspartate oxidase. In the absence of A protein, it converts L-aspartate to oxaloacetate. To our knowledge, no enzyme with this activity has been described previously. The enzyme displays some unusual properties. In its role as B protein in quinolinic acid synthetase, product formation (quinolinic acid) is linear with protein concentration; however, when it functions as an L-aspartate oxidase, product formation (oxaloacetate) is a parabolic function of protein concentration. The L-aspartate oxidase activity also shows marked substrate activation at substrate concentrations above 1.0 mM. The L-aspartate oxidase and B protein activities of the enzyme are inhibited by NAD+, which is competitive with FAD. The immediate reaction product of the enzyme has the same characteristics (rate of decay to oxaloacetate, and condensation with dihydroxyacetone phosphate to form quinolinate) as the unstable reaction product (iminoaspartate) formed from D-aspartate oxidase. A reaction mechanism for the A protein-catalyzed condensation of dihydroxyacetone phosphate and iminoaspartate to form quinolinate is presented.