Structure and Mechanisms of Escherichia coli Aspartate Transcarbamoylase

Abstract

Enzymes catalyze a particular reaction in cells, but only a few control the rate of this reaction and the metabolic pathway that follows. One specific mechanism for such enzymatic control of a metabolic pathway involves molecular feedback, whereby a metabolite further down the pathway acts at a unique site on the control enzyme to alter its activity allosterically. This regulation may be positive or negative (or both), depending upon the particular system. Another method of enzymatic control involves the cooperative binding of the substrate, which allows a large change in enzyme activity to emanate from only a small change in substrate concentration. Allosteric regulation and homotropic cooperativity are often known to involve significant conformational changes in the structure of the protein. Escherichia coli aspartate transcarbamoylase (ATCase) is the textbook example of an enzyme that regulates a metabolic pathway, namely, pyrimidine nucleotide biosynthesis, by feedback control and by the cooperative binding of the substrate, l-aspartate. The catalytic and regulatory mechanisms of this enzyme have been extensively studied. A series of X-ray crystal structures of the enzyme in the presence and absence of substrates, products, and analogues have provided details, at the molecular level, of the conformational changes that the enzyme undergoes as it shifts between its low-activity, low-affinity form (T state) to its high-activity, high-affinity form (R state). These structural data provide insights into not only how this enzyme catalyzes the reaction between l-aspartate and carbamoyl phosphate to form N-carbamoyl-l-aspartate and inorganic phosphate, but also how the allosteric effectors modulate this activity. In this Account, we summarize studies on the structure of the enzyme and describe how these structural data provide insights into the catalytic and regulatory mechanisms of the enzyme. The ATCase-catalyzed reaction is regulated by nucleotide binding some 60 Å from the active site, inducing structural alterations that modulate catalytic activity. The delineation of the structure and function in this particular model system will help in understanding the molecular basis of cooperativity and allosteric regulation in other systems as well.

From feedback inhibition to allostery: the enduring example of aspartate transcarbamoylase

“…My review is dated in its perspective, because I have not contributed to research on ATCase and allostery since the early 1970s (see ), just when the techniques of X‐ray crystallography and rapid reaction kinetics were first being applied to ATCase by other groups, although I am happy to realize in retrospect that there has been an enormous amount of work on the allosteric properties of ATCase in the intervening years, e.g. from the laboratories of W. Lipscomb, E. Kantrowitz, H. Schachman, G. Stark, and G. Herve . Furthermore, the reader should know that other reviews are available on the early research on ATCase .…”

Coupling dynamics and evolutionary information with structure to identify protein regulatory and functional binding sites

“…The protein aspartate transcarbamoylase (ATCase) from Sulfolobus acidocaldarius ATCase plays a vital role in the pyrimidine biosynthesis pathway, catalyzing the carbamoylation of the α‐amino group of l ‐aspartate by carbamoyl phosphate and forming N ‐carbamoyl‐ l ‐aspartate and orthophosphate. It is a heteromeric structure comprised of two chains, catalytic and regulatory . While the catalytic chain comprises aspartate and carbamoyl phosphate binding domains, the allosteric chain has the allosteric domain which binds to regulators and zinc binding domains, which makes contact with the catalytic subunits.…”

New Insight into the Transcarbamylase Family: The Structure of Putrescine Transcarbamylase, a Key Catalyst for Fermentative Utilization of Agmatine

“…Perhaps such type of organization could restrict domain movements, known to be important in transcarbamylases including OTC [3], [28], [39]. However, in pfOTC the dodecameric architecture does not appear to hinder the approach of the C-terminal domain to the N-domain that is associated with catalysis [40].…”