Molecular Mechanisms of Allosteric Regulation in Aspartate Transcarbamylase

Abstract

Escherichia call aspartate transcarbamylase (ATCase) catalyzes the first reaction of the pyrimidine pathway, i.e., the carbamylation of the amino group of aspartate by carbamylphosphate. This catalysis is feedback inhibited by the end-product CTP and is stimulated by ATP, an antagonism that in the cell tends to balance the production of purines and pyrimidines. ATCase has been extensively studied as a model system for allosteric regulation and its properties thoroughly reviewed (1–3). In terms of structure, this enzyme is composed of two trimers of identical catalytic chains (MW 33,000 daltons each) that are in contact and held together by three dimers of regulatory chains (MW 17,000 daltons each). This association requires the presence of six atoms of zinc (one per regulatory chain), which are linked to the SH groups of cysteine residues clustered in the C-terminal region of the regulatory chains (4). Catalytic trimers (called catalytic subunits) and regulatory dimers (called regulatory subunits) can be easily and reversibly separated under the influence of mercurials (5). The isolated catalytic subunits are fully active but do not exhibit any of the regulatory properties of the native enzyme. The isolated regulatory subunits bear the regulatory sites where the effectors CTP and ATP bind competitively. The amino acid sequences of these two kinds of chain are known (6–9).