Use of structural comparisons to select mutagenic targets in aspartate-beta-semialdehyde dehydrogenase.
L-Aspartate-beta-semialdehyde dehydrogenase (ASA DH) from Escherichia coli has been probed by site-directed mutagenesis to identify residues that play an important function in the catalytic activity of the enzyme. Sequence homology searching among ASA DHs that have been isolated from other species and comparisons with the structures of functionally similar D-glyceraldehyde-3-phosphate dehydrogenases (GAPDH) that have been solved from several species have been utilized to select appropriate targets for mutagenesis. A highly conserved active site glutamine has been identified in the E. coli ASA DH that enhances the reactivity of the enzyme. Alteration of this residue leads to an enzyme with reduced catalytic efficiency, yet with an unchanged binding affinity for substrates and coenzyme. Replacement of an arginine residue that is conserved throughout the ASA DH and GAPDH enzyme families leads to a significant decrease in catalytic turnover and is the only mutation examined that also results in a decreased affinity for the substrates of the reaction. This residue is assigned a role in the binding of the substrate aspartate-beta-semialdehyde. Sequence alignment of ASA DH with other NADP- and NAD-dependent enzymes has resulted in the identification of a putative pyridine nucleotide binding region. Substitution of two amino acids in this region with neutral or positively charged side chains has resulted in a change in enzyme specificity. For wild-type ASA DH, NADP is strongly favored as the coenzyme, while in this mutated enzyme the selectivity has been lowered by a factor of 60, and this enzyme has comparable affinities for either pyridine nucleotide.