CAD, A Multienzymatic Protein at the Head of de Novo Pyrimidine Biosynthesis

CAD is a 1.5 MDa particle formed by hexameric association of a 250 kDa proteinProtein that carries the enzymatic activities for the first three steps in the de novo biosynthesis of pyrimidine nucleotides: glutamine-dependent Carbamoyl phosphate synthetaseCarbamoyl phosphate synthetase, Aspartate transcarbamoylaseAspartate transcarbamoylase and DihydroorotaseDihydroorotase. This metabolic pathway is essential for cell growth and proliferation and is conserved in all living organisms. However, the fusion of the first three enzymatic activities of the pathway into a single multienzymatic proteinProtein only occurs in animals. In prokaryotes, by contrast, these activities are encoded as distinct monofunctional enzymes that functionFunction independently or by forming more or less transient complexes. Whereas the structural information about these enzymes in bacteria is abundant, the large size and instability of CAD has only allowed a fragmented characterization of its structureStructure. Here we retrace some of the most significant efforts to decipher the architecture of CAD and to understand its catalytic and regulatory mechanisms.