1. Purine Biosynthesis and Regulation will be available on

T1 - Pyrimidine biosynthesis links mitochondrial respiration to the p53 pathway

PYRIMIDINE AND PURINE BIOSYNTHESIS AND DEGRADATION IN ..

N2 - The first 3 reaction steps of the de novo pyrimidine biosynthetic pathway are catalyzed by carbamoyl-phosphate synthetase II (CPSII), aspartate transcarbamoylase (ATC), and dihydroorotase (DHO), respectively. In eukaryotes, these enzymes are structurally classified into 2 types: (1) a CPSII-DHO-ATC fusion enzyme (CAD) found in animals, fungi, and amoebozoa, and (2) stand-alone enzymes found in plants and the protist groups. In the present study, we demonstrate direct intermolecular interactions between CPSII, ATC, and DHO of the parasitic protist Trypanosoma cruzi, which is the causative agent of Chagas disease. The 3 enzymes were expressed in a bacterial expression system and their interactions were examined. Immunoprecipitation using an antibody specific for each enzyme coupled with Western blotting-based detection using antibodies for the counterpart enzymes showed co-precipitation of all 3 enzymes. From an evolutionary viewpoint, the formation of a functional tri-enzyme complex may have preceded-and led to-gene fusion to produce the CAD protein. This is the first report to demonstrate the structural basis of these 3 enzymes as a model of CAD. Moreover, in conjunction with the essentiality of de novo pyrimidine biosynthesis in the parasite, our findings provide a rationale for new strategies for developing drugs for Chagas disease, which target the intermolecular interactions of these 3 enzymes.

Biosynthesis of the pyrimidine ring. Precursors of the ring and numbering of the ring atoms.

PYRIMIDINE AND PURINE BIOSYNTHESIS AND DEGRADATION IN PLANTS

The synthesis of the pyrimidines CTP and UTP occurs in the cytoplasm and starts from the condensation of aspartate with carbamoyl-phosphate to form orotic acid.

T1 - Effects of acivicin and pala, singly and in combination, on de novo pyrimidine biosynthesis

Nucleotides play a variety of important roles in all cells. They are the activated precursors of DNA and RNA. ATP, an adenine nucleotide, is a universal currency of energy in biological systems. GTP is an essential carrier of chemical energy. Adenine nucleotides are components of the coenzymes NAD+, NADP+, FMN, FAD and Coenzyme A. IMP is synthesized from ribose 5-phosphate. There are 11 reactions in the formation of IMP. Nucleoside monophosphates are converted to nucleoside diphosphates by base specific monophosphate kinases. Purine nucleotide synthesis is regulated by feedback inhibitor – AMP, GMP and IMP. Recycling of purines formed by the degradation of nucleotides is possible. Pyrimidine ring is synthesized as free pyrimidine and then it is incorporated into the nucleotide. Nucleotides of a cell undergo continuous turnover. Uric acid is the breakdown product of purine nucleotide. Gout is a disease characterized by elevated levels of uric acid in body fluids. Pyrimidines on degradation give rise to carbon dioxide, ammonia, β-alanine and β-amino isobutyrate.

T1 - Molecular interaction of the first 3 enzymes of the de novo pyrimidine biosynthetic pathway of Trypanosoma cruzi


Purine and pyrimidine metabolism

Degradation of nucleotides
Nucleotides of a cell undergo continuous turnover. Purines are catabolized and the end product is uric acid. Gout is a disease characterized by elevated levels of uric acid in body fluids. Sodium urate crystals are precipitated in the joints and soft tissues to cause painful arthritis. In Lesch-Nyhan syndrome, HGPRT deficiency occurs, leading to excessive uric acid production through PRPP accumulation. Gout is treated by allopurinol administration. Animal cells degrade pyrimidine nucleotides to their component bases by dephosphorylation, deamination and glycosidic bond cleavages to give rise to carbon dioxide, ammonia, β-alanine and β-amino isobutyrate.

PYRIMIDINE AND PURINE BIOSYNTHESIS AND DEGRADATION …

Degradation of nucleotides
Nucleotides of a cell undergo continuous turnover. Purines are catabolized and the end product is uric acid. Gout is a disease characterized by elevated levels of uric acid in body fluids. Sodium urate crystals are precipitated in the joints and soft tissues to cause painful arthritis. In Lesch-Nyhan syndrome, HGPRT deficiency occurs, leading to excessive uric acid production through PRPP accumulation. Gout is treated by allopurinol administration. Animal cells degrade pyrimidine nucleotides to their component bases by dephosphorylation, deamination and glycosidic bond cleavages to give rise to carbon dioxide, ammonia, β-alanine and β-amino isobutyrate.

Pyrimidine and purine biosynthesis and degradation in plants

AB - The inhibitory activities of two oncolytic amino acid analogs, acivicin and n-(phosphonacetyl)-l-aspartate, on pyrimidine biosynthesis have been examined in a murine tumor line, the Lewis lung carcinoma. Acivicin, an antimetabolite elaborated by Streptomyces sviceus, inhibits a spectrum of l-glutamine utilizing enzymes including carbamoyl phosphate synthetase II, the inaugurating enzyme of de novo pyrimidine biosynthesis. Profound inhibition of carbamoyl phosphate synthetase II activity by acivicin is demonstrated in vitro as well as in vivo. n-(Phosphonacetyl)-l-aspartate, a rationally-designed transition-state analog of the reaction catalyzed by l-aspartate transcarbamylase, the second enzyme of the pathway, is a potent and specific inhibitor of l-aspartate transcarbamylase. Both agents, at therapeutic doses, exert marked inhibitions of their respective target enzymes and impede flux through the pathway as monitored by inhibition of pyrazofurin-provoked accumulation of orotate and orotidine. Additionally, synergistic effects are observed when acivicin and n-(phosphonacetyl)-l-aspartate are used in combination, both in terms of biochemical and therapeutic endpoints. The salient features of the actions of these drugs on pyrimidine biosynthesis in the Lewis lung carcinoma are summarized in Table 6. Comparison of the effects of acivicin with those of n-(phosphonacetyl)-l-aspartate suggest divergent actions on nucleotide biosynthesis. In spite of its pronounced sensitivity to acivicin, carbamoyl phosphate synthetase II appears not to be a critical target for the antineoplastic activity of this drug.