Redox potentials and quinone reductase activity of L-aspartate oxidase from E. coli

L-Aspartate oxidase (EC 1.4.3.16) is a flavoprotein that catalyzes the first step in the de novo biosynthetic pathway to pyridine nucleotides both under aerobic and under anaerobic conditions. Despite the physiological importance of this biosynthesis particularly in facultative aerobic organisms, such as Escherichia coli, little is known about the electron acceptor of reduced L-aspartate oxidase in the absence of oxygen. In this report, evidence is presented which suggests that in vitro quinones can play such a role. L-Aspartate oxidase binds menadione and 2,3-dimethoxy-5-methyl- p-benzoquinone with K(d) values of 11.5 and 2.4 μM, respectively. A new L- aspartate:quinone oxidoreductase activity is described in the presence and in the absence of phospholipids, and its possible physiological relevance is discussed. Moreover, considering the striking sequence similarity between L- aspartate oxidase and the highly conserved family of succinate-fumarate oxidoreductases, the redox properties of L-aspartate oxidase were investigated in detail. A value of -216 mV was calculated for the midpoint potential of the couple FAD/FADH2 bound to the enzyme. This result perfectly explains why L-aspartate oxidase may be considered as a very particular fumarate reductase unable to use succinate as the electron donor.