CAD: A Multifunctional Protein LeadingDe NovoPyrimidine Biosynthesis

Abstract

AbstractPyrimidines are essential precursors for DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) synthesis, protein glycosylation and lipid synthesis. In resting cells, pyrimidines are largely obtained through salvage pathways, but in proliferating cells, particularly in tumours, the synthesis of pyrimidinesde novois indispensable to fuel the high demand of nucleic acids and other cellular components. In animals, thede novopathway is initiated and controlled by CAD, a ∼240‐kDa multifunctional protein with four different enzymatic domains: glutaminase (GLN), carbamoyl phosphate synthetase (CPS), dihydroorotase (DHO) and aspartate transcarbamoylase (ATC). In contrast, in bacteria, archaeans and plants, GLN, CPS, DHO and ATC are distinct monofunctional proteins. The structures of a number of these enzymes from bacteria and archaea are known, but until recently, there was no structural information about CAD other than that it self‐assembles into ∼1.5‐megaDa hexamers.Key ConceptsDe novosynthesis of pyrimidine nucleotides is essential for cell growth and proliferation.In animals, the multifunctional protein CAD catalyses the first three reactions ofde novopyrimidine synthesis.CAD is a 243‐kDa polypeptide with four enzymatic domains [glutaminase (GLN), carbamoyl phosphate synthetase (CPS), dihydroorotase (DHO) and aspartate transcarbamoylase (ATC)] that oligomerises into 1.5‐megaDa hexamers.In bacteria, GLN, CPS, DHO and ATC are individual proteins for which structural information is available.The crystal structures of the DHO and ATC domains of human CAD were recently reported.The GLN and CPS domains of CAD are expected to be similar to theEscherichia coliCPS and human mitochondrial CPS1 crystal structures.A model of CAD is proposed that sets the DHO and ATC domains as the central framework of the hexameric particles.