Pyrimidine nucleotides synthesis, PRPP precursor

The purine ring is assembled from a variety of precursors glutamine, glycine, aspartate, N formyltetrahydrofolate, and (XT. The committed step in the de novo synthesis of purine nucleotides is the formation of 5-phosphoribosyIamine from PRPP and glutamine. The purine ring is assembled on ribose phosphate, in contrast with the de novo synthesis of pyrimidine nucleotides. The addition of glycine,... 

However, since the fate of the PRPP can be to form purine nucleotides, pyrimidine nucleotides, or coenzymes, the control must be complex and not limited to only purine nucleotides, pyrimidine nucleotides, or coenzymes. The next enzyme under control by purine nucleotides is PRPP transamidinase, catalyzing the formation of the phosphoribosyl amide (PRPP + glutamine PRNH2 + glutamate + pyrophosphate), the first unique precursor of purines. Since the synthesis ofPRPP is slow, there must be controls to allow its utilization in other processes when purine nucleotide pools are satisfied. The control of this system is interesting and shows the phenomena of synergism. That is, AMP, the product of one branch of the pathway, can inhibit the enzyme activity approximately 10% GMP, the product of the other branch of the pathway, can also cause approximately a 10% inhibition. 

The first three reactions are catalyzed by a trifunctional protein which contains carbamoyl-phosphate synthetase II, aspartate carbamoyltransferase and dihydro-orotase. This set of reactions begins with the synthesis of carbamoyl phosphate followed by its condensation with aspartic acid. The third step involves the closure of the ring through the removal of water by the action of dihydro-orotase to yield dihydro-orotate. The fourth enzyme, dihydro-orotate oxidase, oxidizes dihydro-orotate to orotate and is a mitochondrial flavoprotein enzyme located on the outer surface of the inner membrane and utilizes NAD" " as the electron acceptor. The synthesis of UMP from orotate is catalyzed by a bifunctional protein which comprises orotate PRTase and orotidine 5 -phosphate (OMP) decarboxylase. The former phosphoribosylates orotate to give OMP the latter decarboxylates OMP to UMP, the immediate precursor for the other pyrimidine nucleotides. It is interesting to note that whereas five molecules of ATP (including the ATP used in the synthesis of PRPP) are used in the de novo synthesis of IMP, no net ATP is used in the de novo synthesis of UMP. In de novo pyrimidine synthesis, two ATP molecules are used to synthesize carbamoyl phosphate and one ATP is needed to synthesize the PRPP used by orotate PRTase but 3 ATPs... 

The first step in de novo pyrimidine biosynthesis is the synthesis of carbamoyl phosphate from bicarbonate and ammonia in a multistep process, requiring the cleavage of two molecules of ATP. This reaction is catalyzed by carbamoyl phosphate synthetase (CPS), and the bicarbonate is phosphorylated by ATP to form carboxyphosphate and ADP (adenine dinucleotide phosphate). Ammonia then reacts with carboxyphosphate to form carbamic acid. The latter is phosphorylated by another molecule of ATP with the mediation of CPS to form carbamoyl phosphate, which reacts with aspartate by aspartate transcarbamoy-lase to form A-carbamoylaspartate. The latter cyclizes to form dihydroorotate, which is then oxidized by NAD-1- to generate orotate. Reaction of orotate with 5-phosphoribosyl-l-pyrophosphate (PRPP), catalyzed by pyrimidine PT, forms the pyrimidine nucleotide orotidylate. This reaction is driven by the hydrolysis of pyrophosphate. Decarboxylatin of orotidylate, catalyzed by orotidylate decarboxylase, forms uridylate (uridine-5 -monophosphate, UMP), a major pyrimidine nucleotide that is a precursor of RNA (Figure 6.53). 

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