1H NMR studies on the catalytic subunit of aspartate transcarbamoylase

The 1H NMR spectrum of the catalytic subunit of Escherichia coli aspartate transcarbamoylase (EC 2.1.3.2) was simplified by using strains auxotrophic for the aromatic amino acids and a growth medium containing fully deuterated Trp, Phe, and His and partially deuterated Tyr. 1H resonances for Tyr in the catalytic trimer (M(r) = 10(5)) were partially resolved into five peaks at 27 degrees C, which above 50 degrees C were further resolved to give a distinct resonance for each of the eight Tyr residues in the polypeptide chain. Experiments on chemically modified catalytic subunits and on a mutant form in which Tyr-165 was converted to Ser-165 led to the assignment of resonances for Tyr-165, Tyr-240, and Tyr-185. Binding of the substrate, carbamoyl phosphate, caused shifts of two of the unassigned resonances, and the subsequent binding of the aspartate analog succinate perturbed the resonances corresponding to Tyr-165 and Tyr-240. The bisubstrate analog N-(phosphonacetyl)-L-aspartate produced a spectrum differing considerably from that caused by the combination of carbamoyl phosphate and succinate. The NMR spectrum for the Tyr-165-->Ser mutant trimer showed clearly that the single amino acid substitution caused conformational changes affecting the environment of residues remote from the position of the replacement. In contrast, the inactive mutant subunit in which Gly-128 was replaced by Asp exhibited a spectrum virtually identical to that of the wild-type protein. However, addition of the substrate carbamoyl phosphate caused a marked change in the spectrum of the mutant enzyme, whereas that of the wild-type trimer was altered only slightly, showing that the effect of the amino acid substitution was manifested in the NMR spectrum only with the liganded enzyme.