Adenylosuccinate synthetase governs the first committed step in the de novo synthesis of AMP. Mutations of conserved residues in the synthetase fromEscherichia coli reveal significant roles for Val273 and Thr300 in the recognition ofL-aspartate, even though these residues do not or cannot hydrogen bond with the substrate. The mutation of Thr300 to alanine increases the K m forL-aspartate by 30-fold. In contrast, its mutation to valine causes no more than a 4-fold increase in the K m forL-aspartate, while increasing k catby 3-fold. Mutations of Val273 to alanine, threonine, or asparagine increase the K m forL-aspartate from 15- to 40-fold, and concomitantly decrease the K ifor dicarboxylate analogues ofL-aspartate by up to 40-fold. The above perturbations are comparable with those resulting from the elimination of a hydrogen bond between the enzyme and substrate: alanine mutations of Thr128 and Thr129 increase theK m for IMP by up to 30-fold and the alanine mutation of Thr301 abolishes catalysis supported byL-aspartate, but has no effect on catalysis supported by hydroxylamine. Structure-based mechanisms, by which the above residues influence substrate recognition, are presented.