Preferential utilization of glutamine for amination of xanthosine 5'-phosphate to guanosine 5'-phosphate by purified enzymes from Escherichia coli

GMP synthetase was purified 180-fold from $\text{E. coli}$ B and 18-fold from the derepressed purine auxotroph, $\text{E. coli}$ B-96. The enzymes from both sources show the same preference for glutamine over ammonia as amino donor. Each is dimeric, consisting of subunits of molecular weight about 60,000. Thus the two are apparently identical. The similarities between GMP synthetase and xanthosine 5′-phosphate aminase of $\text{E. coli}$ B-96 (N. Sakamoto, G.W. Hatfield, and H.S. Moyed, J. Biol. Chem. (1972) $\text{247}$, 5880–5887) in respect to structure, state of derepression, and behavior during purification, lead us to the conclusion that the synthetase and the aminase are a single entity. We observe no loss or separation of glutamine-dependent activity upon purification of GMP synthetase and we suggest that such loss, reported by other workers, results artifactually by inactivation of an intrinsic glutamine-binding site. GMP synthetase appears not to contain a glutamine-binding subunit which is separable from the xanthosine 5′-phosphate-aminating component.