Purine synthesis starts with ribose-5-phosphate

De novo synthesis of purines is most active in the liver, which exports the bases and nucleosides to other tissues. Most tissues have a limited capacity for de novo purine synthesis, although they can synthesize the nucleotides from externally supplied bases.

The pathway of purine biosynthesis is shown in Figure 28.2. It starts with ribose-5-phosphate, a product of the pentose phosphate pathway (see Chapter 22). In the reactions of the pathway, all of them cytoplasmic, the purine ring system is built up step by step, with C-1 of ribose-5-phosphate used as a primer.

Figure 28.2 De novo pathway of purine biosynthesis. THF, Tetrahydrofolate.

The first enzyme, 5-phosphoribosyl-1-pyrophosphate (PRPP) synthetase, transfers a pyrophosphate group from ATP to C-1 of ribose-5-phosphate, forming PRPP. PRPP is the activated form of ribose for nucleotide synthesis. The next enzyme, PRPP amidotransferase, replaces the pyrophosphate group of PRPP with a nitrogen from the side chain of glutamine. This reaction is the committed step of purine biosynthesis. In the following reactions, the purine ring is constructed from simple building blocks:

where THF = tetrahydrofolate. The first nucleotide formed in the pathway is inosine monophosphate (IMP), which contains the base hypoxanthine. IMP is a branch point in the synthesis of AMP and GMP (Fig. 28.3). These nucleoside monophosphates are in equilibrium with their corresponding diphosphates and triphosphates through kinase reactions.

Figure 28.3 Synthesis of AMP and GMP from IMP.

As expected, purine synthesis is regulated by feedback inhibition (Fig. 28.4). The first two enzymes of the pathway, PRPP synthetase and PRPP amidotransferase, are inhibited by the purine nucleotides. In addition, the reactions leading from IMP to AMP and GMP are feedback inhibited by the end products.

Figure 28.4 Feedback inhibition of de novo purine biosynthesis by nucleotides. ADP, Adenosine diphosphate; AMP, adenosine monophosphate; ATP, adenosine triphosphate; GDP, guanosine diphosphate; GMP, guanosine monophosphate; GTP, guanosine triphosphate; IMP, Inosine monophosphate; PRPP, 5-phosphoribosyl-1-pyrophosphate.

Purines are degraded to uric acid

The degradation of purine nucleotides starts with the hydrolytic removal of phosphate from the nucleotides. The nucleosides thus formed are then cleaved into free base and ribose-1-phosphate by purine nucleoside phosphorylase. Adenosine is a poor substrate of the nucleoside phosphorylase. Therefore it is deaminated to inosine first (Fig. 28.5).

Figure 28.5 Degradation of purine nucleotides to uric acid, and the salvage of purine bases. , 5′-Nucleotidase; , AMP deaminase; , adenosine deaminase; , purine nucleoside phosphorylase; , guanine deaminase; , xanthine oxidase; , adenine phosphoribosyltransferase; , hypoxanthine-guanine phosphoribosyltransferase. IMP, Inosine monophosphate; PRPP, 5-phosphoribosyl-1-pyrophosphate.

Uric acid is the end product of purine degradation in humans. It is synthesized by xanthine oxidase via hypoxanthine and xanthine (reaction in Figure 28.5). This enzyme contains flavin adenine dinucleotide (FAD), nonheme iron, and molybdenum. Like other nonmitochondrial flavoproteins, it regenerates its FAD by transferring hydrogen from FADH₂ to molecular oxygen, forming hydrogen peroxide.

Free purine bases can be salvaged

As an alternative to degradation, the free bases can be recycled into the nucleotide pool. This requires the PRPP-dependent salvage enzymes hypoxanthine-guanine phosphoribosyltransferase (HGPRT) and adenine phosphoribosyl transferase (APRT):

The salvage reactions are the only source of purine nucleotides for tissues that cannot synthesize the nucleotides de novo. HGPRT is quantitatively by far the more important salvage enzyme because most of the adenine is released as hypoxanthine. HGPRT is competitively inhibited by IMP and GMP, whereas APRT is inhibited by AMP.

Pyrimidines are synthesized from carbamoyl phosphate and aspartate

Most proliferating cells synthesize pyrimidines de novo, whereas quiescent cells synthesize pyrimidine nucleotides from imported bases. Most cancer cells have highly active de novo pyrimidine synthesis.

Unlike the purine ring, the pyrimidine ring is synthesized before the ribose is added (Fig. 28.6). The pathway starts with carbamoyl phosphate and aspartate, and orotic acid is formed as the first pyrimidine. Orotic acid is processed to the uridine nucleotides, which are the precursors of the cytidine nucleotides. The enzymes of the pathway are cytosolic except for dihydroorotate dehydrogenase (reaction in Figure 28.6), which is on the outer surface of the inner mitochondrial membrane.

Figure 28.6 Biosynthesis of pyrimidine nucleotides. , Carbamoyl phosphate synthetase II; , aspartate transcarbamoylase; , dihydroorotase; , dihydroorotate dehydrogenase; , orotate phosphoribosyltransferase; , orotidylate decarboxylase; , CTP synthetase. PRPP, 5-Phosphoribosyl-1-pyrophosphate.

The carbamoyl phosphate for pyrimidine biosynthesis is synthesized by the cytoplasmic carbamoyl phosphate synthetase II. Unlike the mitochondrial enzyme, which makes carbamoyl phosphate for the urea cycle, the cytoplasmic enzyme uses the side chain of glutamine as a source of nitrogen.

The first three enzymes of the pathway, including carbamoyl phosphate synthetase II, aspartate transcarbamoylase, and dihydroorotate dehydrogenase (enzymes 1, 2, and 3 in Fig. 28.6), are formed by different domains of a single large polypeptide. Both this multienzyme complex and the CTP synthetase are feedback inhibited by CTP.

An inhibitor of dihydroorotate dehydrogenase, leflunomide, blocks pyrimidine biosynthesis. It has been used as an immunomodulatory drug for the treatment of rheumatoid arthritis and psoriatic arthritis.

The pyrimidines are degraded to water-soluble products that are either excreted as such or oxidized to carbon dioxide and water (Fig. 28.7).

Figure 28.7 Degradation of pyrimidines.