Studies on Aspartate Transcarbamylase and Its Allosteric Interaction

From the ¹ From the Departments of Biochemistry and Neurology, Columbia University, College of Physicians and Surgeons, New York, New York 10032

1. The sigmoid kinetics with respect to asparate and the competitive inhibition by cytidine triphosphate has been confirmed for aspartate transcarbamylase purified from Escherichia coli.

2. The pH dependences of native and heated aspartate transcarbamylase have been examined; that of the native enzyme is remarkable in exhibiting two optima, one at pH 8.5 and a much higher one at pH 10.2.

3. The effects of heat, urea, p-hydroxymercuribenzoate, and pH 10.2 on both enzyme activity and CTP inhibition have been investigated. These treatments resulted in the loss of sigmoid kinetics but, in contradiction with previous reports, sensitivity to CTP inhibition was not destroyed. However, the CTP inhibition of these treated enzymes was no longer dependent upon the concentration of aspartate.

4. Sigmoid kinetics and end product inhibition are thus not inseparably linked properties of aspartate transcarbamylase, and the homotropic and heterotropic interactions are not completely interdependent.

5. The behavior of the enzyme on Sephadex has been examined. The behavior of heated enzyme was consistent with a breakdown to smaller units, but the other treatments which destroy the sigmoid kinetics did not appear to produce dissociation to subunits.

6. The changes in enzymic behavior produced by the action of urea or pH 10.2 are completely reversible.

7. The findings have been discussed in terms of the subunit structure of aspartate transcarbamylase. In general it would appear that in our hands these desensitizing treatments have produced a weakening of interaction between subunits rather than an actual separation into catalytic and regulatory subunits.

CiteULike    Complore    Connotea    Del.icio.us    Digg    Reddit    Technorati    What's this?

This article has been cited by other articles:

				J. F. Vickrey, G. Herve, and D. R. Evans Pseudomonas aeruginosa Aspartate Transcarbamoylase. CHARACTERIZATION OF ITS CATALYTIC AND REGULATORY PROPERTIES J. Biol. Chem., June 28, 2002; 277(27): 24490 - 24498. [Abstract] [Full Text] [PDF]