The Crystal Structure of the Pseudomonas dacunhae Aspartate-β-Decarboxylase Dodecamer Reveals an Unknown Oligomeric Assembly for a Pyridoxal-5′-Phosphate-Dependent Enzyme

The Pseudomonas dacunhael-aspartate-β-decarboxylase (ABDC, aspartate 4-decarboxylase, aspartate 4-carboxylyase, E.C. 4.1.1.12) is a pyridoxal-5′-phosphate (PLP)-dependent enzyme that catalyzes the β-decarboxylation of l-aspartate to produce l-alanine and CO2. This catalytically versatile enzyme is known to form functional dodecamers at its optimal pH and is thought to work in conjunction with an l-Asp/l-Ala antiporter to establish a proton gradient across the membrane that can be used for ATP biosynthesis. We have solved the atomic structure of ABDC to 2.35 Å resolution using single-wavelength anomalous dispersion phasing. The structure reveals that ABDC oligomerizes as a homododecamer in an unknown mode among PLP-dependent enzymes and has highest structural homology with members of the PLP-dependent aspartate aminotransferase subfamily. The structure shows that the ABDC active site is very similar to that of aspartate aminotransferase. However, an additional arginine side chain (Arg37) was observed flanking the re-side of the PLP ring in the ABDC active site. The mutagenesis results show that although Arg37 is not required for activity, it appears to be involved in the ABDC catalytic cycle.