BIOCHEMISTRY - L. Stryer - 1984

VOLUME 2

PART III. BIOSYNTHESIS OF MACROMOLECULAR PRECURSORS

CHAPTER 22. BIOSYNTHESIS OF NUCLEOTIDES

Summary

A variety of precursors contribute to the assembly of the purine ring: glutamine, Glycine, aspartate, methenyltetrahydrofolate, N¹⁰-formyltetrahydrofolate, and CO₂. The committed step in de novo purine nucleotide synthesis is The formation of 5-phosphoribosylamine from PRPP and glutamine. During assembly, the purine ring is built on a ribose phosphate platform. Addition of glycine, formylation, amination, and ring closure yield 5-aminoimidazole ribonucleotide. This intermediate contains the complete five-membered ring of the purine Skeleton. Subsequent addition of CO₂, the nitrogen atom of aspartate, and a formyl group, followed by ring closure, yields the purine nucleotide inosinate (IMP). AMP and GMP are formed from IMP. Purine ribonucleotides can also be synthesized by Salvage Pathways, in which preformed bases react directly with PRPP. Feedback inhibition of 5-phosphoribosyl-1-pyrophosphate synthetase by the end

product and of glutamine phosphoribosyl amidotransferase by purine NUCLEOTIDES plays an important role in regulating their Biosynthesis.

The pyrimidine ring is assembled first and only then attached to ribose phosphate to form a pyrimidine nucleotide—in contrast to The sequence of events in de novo purine nucleotide synthesis. PRPP again serves as the donor of the ribose phosphate moiety. Pyrimidine ring synthesis begins with the formation of carbamoyl aspartate from carbamoyl phosphate and aspartate, a reaction catalyzed by aspartate transcarbamoylase. Dehydration, cyclization, and oxidation reactions yield orotate, which reacts with PRPP to form orotidylate. Decarboxylation of this pyrimidine nucleotide yields UMP. Subsequent amination of UTP yields CTP. Pyrimidine biosynthesis in E. coli is regulated by feedback inhibition of aspartate transcarbamoylase, The enzyme catalyzing the committed step. CTP inhibits the enzyme, whereas ATP stimulates it. Aspartate transcarbamoylase consists of regulatory and catalytic subunits, which can be separated.

In mammalian Cells, all three Enzymes catalyzing the first Three Reactions of pyrimidine biosynthesis are contained within a single polypeptide chain. Deoxyribonucleotides, the precursors of DNA, are formed by the reduction of ribonucleoside diphosphates. These conversions are catalyzed by Ribonucleotide reductase. In the active sites of this enzyme, thioredoxin and glutaredoxin transfer electrons from NADPH to sulfhydryl groups. dTMP is formed by

the methylation of dUMP. The donor of the one-carbon group and electrons in this reaction is N1⁵, N¹⁰-methylenetetrahydrofolate, which is converted to dihydrofolate. Tetrahydrofolate, in turn, is regenerated by the reduction of dihydrofolate. Dihydrofolate Reductase, which catalyzes this reaction, is inhibited by folate analogs, such as aminopterin or amethopterin (methotrexate). These compounds are used as anticancer drugs.