Deciphering CAD: Structure and function of a mega‐enzymatic pyrimidine factory in health and disease

Abstract

CAD is a 1.5 MDa particle formed by hexameric association of a 250 kDa protein divided into different enzymatic domains, each catalyzing one of the initial reactions for de novo biosynthesis of pyrimidine nucleotides: glutaminasedependent Carbamoyl phosphate synthetase, Aspartate transcarbamoylase, and Dihydroorotase. The pathway for de novo pyrimidine synthesis is essential for cell proliferation and is conserved in all living organisms, but the covalent linkage of the first enzymatic activities into a multienzymatic CAD particle is unique to animals. In other organisms, these enzymatic activities are encoded as monofunctional proteins for which there is abundant structural and biochemical information. However, the knowledge about CAD is scarce and fragmented. Understanding CAD requires not only to determine the threedimensional structures and define the catalytic and regulatory mechanisms of the different enzymatic domains, but also to comprehend how these domains entangle and work in a coordinated and regulated manner. This review summarizes significant progress over the past 10 years toward the characterization of CAD's architecture, function, regulatory mechanisms, and cellular compartmentalization, as well as the recent finding of a new and rare neurometabolic disorder caused by defects in CAD activities. K E Y W O R D Saspartate transcarbamoylase, carbamoyl phosphate synthetase, de novo pyrimidine biosynthesis, dihydroorotase, multienzymatic protein, nucleotide metabolism, rare diseases | INTRODUCTIONPyrimidine nucleotides are building blocks of DNA and RNA and key activators of intermediate metabolites in a number of cellular processes, including glycosylation and carbohydrate and phospholipid synthesis. 1,2 The cells obtain the supply of pyrimidines through two different metabolic pathways (Figure 1). Slowly dividing or differentiated cells use preferentially salvage pathways to uptake preformed nucleosides (e.g., uridine) from the diet or from the breakdown of nucleic acids, whereas, proliferating cells, whether normal (e.g., activated lymphocytes) or tumoral, depend on de novo ("of new") synthesis of pyrimidine nucleotides. The de novo pathway consists of six enzymatic steps yielding uridine 5-monophosphate (UMP), from which all other pyrimidine nucleotides are made (Figure 1). 3 In a first step, carbamoyl phosphate synthetase (CPS; EC 6.3.5.5) makes

Structural Analysis of Saccharomyces cerevisiae Dihydroorotase Reveals Molecular Insights into the Tetramerization Mechanism

“…Aquifex aeolicus DHOase (AaDHOase) is active only when complexed with AaATCase [ 5 ]. Given that CAD is a key enzyme for the cell’s progression through the S phase of the cell cycle and controls the cell proliferation in mammalian cells [ 6 , 7 , 8 ], these distinct differences among species may indicate that DHOase is a promising target [ 9 , 10 , 11 ] for potential antimalarial, anticancer, and antipathogen chemotherapy [ 12 , 13 ].…”

Anticancer and Antioxidant Activities of the Root Extract of the Carnivorous Pitcher Plant Sarracenia purpurea

“…DHOase is the third enzyme in the de novo biosynthesis pathway of pyrimidine nucleotides (Figure 7A) and is considered an attractive target for potential antimalarial, anticancer, and antipathogen chemotherapy [17,[39][40][41]43,[54][55][56][57][58][59][60][61]. This enzyme contains a binuclear metal center (Znα/Znβ) and a residue Asp1686 (Figure 7B) crucial for the catalysis [42,[62][63][64][65]. Asp1686 initiates the reaction by abstracting a proton from the substrate [66,67].…”

Pyrimidine Biosynthetic Enzyme CAD: Its Function, Regulation, and Diagnostic Potential

“…CAD encodes a multifunctional enzyme complex and plays pivotal roles in a variety of biological functions. Thereby, partial or total loss of function of CAD may cause severe congenital metabolic disorders [ 105 ]. Abnormal changes of CAD, such as genetic mutations, result in severe metabolic disease and even have fatal outcomes.…”