Aspartate transcarbamoylase (ATCase) is an enzyme that plays a crucial role in the biosynthesis of pyrimidines, which are essential components of nucleic acids like DNA and RNA. It catalyzes the reaction between aspartate and carbamoyl phosphate to produce N-carbamoyl aspartate, a key intermediate in the pathway. ATCase is a multi-subunit enzyme, typically composed of regulatory and catalytic subunits. Its activity is regulated by various molecules, including CTP (cytidine triphosphate), which inhibits the enzyme, and ATP (adenosine triphosphate), which activates it. This regulation helps maintain the balance of nucleotide synthesis in cells.