L-alanosine: a noncooperative substrate for Escherichia coli aspartate transcarbamylase.

L-Alanosine, an antibiotic produced by Streptomyces alanosinicus, can be used by Escherichia coli aspartate transcarbamylase as a substrate instead of L-aspartate. The Michaelis constant of the catalytic subunit for this analogue is about 10 times higher than that for the physiological substrate, and the catalytic constant is about 30 times lower. The saturation curve of the native enzyme for L-alanosine indicates the lack of homotropic cooperative interactions between the catalytic sites for the utilization of this compound. It appears therefore that L-alanosine is unable to promote the allosteric transition. However, N-(phosphonoacetyl)-L-aspartate, a "bisubstrate analogue" of the physiological substrates, stimulates the reaction. This phenomenon is very similar to that reported by Foote and Lipscomb [Foote, J., & Lipscomb, W. N. (1981) J. Biol. Chem. 256, 11428-11433] concerning the reverse reaction using carbamylaspartate. The reaction is normally sensitive to the physiological effectors ATP and CTP. The significance of these results for the mechanism of the allosteric regulation is discussed.