Determination of the number of regulatory and catalytic sites on aspartate transcarbamylase

Allostery and cooperativity in Escherichia coli aspartate transcarbamoylase

The allosteric enzyme aspartate transcarbamoylase (ATCase) from Escherichia coli has been the subject of investigations for approximately 50 years. This enzyme controls the rate of pyrimidine nucleotide biosynthesis by feedback inhibition, and helps to balance the pyrimidine and purine pools by competitive allosteric activation by ATP. The catalytic and regulatory components of the dodecameric enzyme can be separated and studied independently. Many of the properties of the enzyme follow the Monod, Wyman Changeux model of allosteric control thus E. coli ATCase has become the textbook example. This review will highlight kinetic, biophysical, and structural studies which have provided a molecular level understanding of how the allosteric nature of this enzyme regulates pyrimidine nucleotide biosynthesis.

Characterization of Coxiella burnetii pyrB

The C. burnetii pyrB gene was cloned on a 7-kbp EcoR I fragment. DNA sequence analysis, enzyme assays, and amino acid homologies with E. coli and B. subtilis pyrB gene products suggest that (i) C. burnetii ATCase exists as a trimer, (ii) the microorganism may not synthesize a regulatory polypeptide, and (iii) pyrB may be part of an operon whose expression is under the control of an upstream promoter. The high degree of homology of the active site further suggests that a common mechanism of catalysis for ATCase exists between such diverse organisms as C. burnetii, E. coli, and B. subtilis.

Heterogeneity of sites in isolated catalytic subunits of aspartate transcarbamoylase

Carbamoyl phosphate, a substrate of aspartate transcarbamoylase from Escherichia coli, binds with different modes of association in 3 sites in the unmodified catalytic subunits. Over a narrow pH range (6.6--8.0), positive, negative or no interactions are observed. Several substrate analogues also bind to 3 sites in the catalytic trimer. The association of pyridoxal phosphate, CTP and ATP, all competitive inhibitors of carbamoyl phosphate, exhibit negative interactions. Binding of succinate, an analogue of the second substrate, aspartate, is also characterized by heterogeneity. Dissociation constants to high and low-affinity sites differ by factors of 10-100. These observations clearly indicate that, although not observed kinetically, the active sites in the catalytic subunits of aspartate transcarbamoylase are heterogenous.

Localization of the zinc binding site of aspartate transcarbamoylase in the regulatory subunit

Aspartate transcarbamoylase (EC 2.1.3.2) from Escherichia coli contains six zinc ions per molecule. Renaturation studies of this allosteric enzyme and its isolated subunits show that the metal binding site is in the regulatory polypeptide chain. Ultraviolet difference spectra of the cadmium and zinc derivatives have been used to show that the sites in the isolated subunit and the full enzyme are similar. Results with an apo derivative of the regulatory subunit suggest that the metal is not required for the binding of the feedback inhibitor, but that it is involved in the structural stability of the protein. A close relationship between the reactivity of the cysteine residues in the regulatory subunit and the metal ion has been found.