Aspartate carbamoyltransferase catalytic subunit (ATCase (catalytic subunit))

Aspartate carbamoyltransferase catalytic subunit is the enzyme responsible for the condensation of carbamoyl phosphate and aspartate to form carbamoyl aspartate and inorganic phosphate, a committed and rate-limiting step in the de novo biosynthesis of pyrimidine nucleotides (UMP, CTP)[2][6][8]. In organisms such as Escherichia coli, the enzyme consists of a large multimeric complex with six catalytic subunits (arranged as two trimers) at its core and three regulatory dimers, for a total molecular weight over 300 kDa[2][5]. The catalytic subunits perform the enzymatic reaction; the regulatory subunits control enzyme activity through allosteric interactions, modulating the enzyme’s state between inactive (“T” or tense) and active (“R” or relaxed) conformations depending on the levels of purine and pyrimidine nucleotides, thus tightly regulating nucleotide synthesis[2][5]. Key allosteric inhibitors include CTP (end product of the pathway), and activators include ATP[2][5]. The bacterial gene encoding the catalytic subunit is called pyrB[6]. Aspartate carbamoyltransferase does not have a direct clinical role in humans but serves as a model for understanding enzyme allostery, regulation, and is of microbial biochemical importance. Inhibitors like PALA are used as biochemical tools for mechanistic and structural studies[1][5].

Other names

Aspartate transcarbamylase catalytic subunitATC catalytic subunitATCase (catalytic subunit)PyrB (gene/protein designation, especially in bacteria)EC 2.1.3.2

Competitive inhibition (e.g., by PALA which mimics the transition state) Allosteric regulation (CTP as negative effector, ATP as positive effector regulates activity through the regulatory subunit, but not directly acting as inhibitor of the catalytic subunit itself)