26. METABOLISM OF PURINE NUCLEOTIDES

Introduction:

Purine nucleotides play an essential role in cell growth, division, and the synthesis of DNA and RNA. Deoxyribonucleotides are used for DNA synthesis, while ribonucleotides are used for RNA synthesis. These nucleotides are crucial for normal cell function, and their deficiency or overproduction can lead to metabolic disorders.

The biosynthesis of purine and pyrimidine nucleotides is vital for DNA replication and cell proliferation in all organisms. If this process is blocked, it halts cell division and growth, making it a major target for anti-cancer, anti-bacterial, and anti-viral drugs.

Although nucleotides are present in the diet, they are considered non-essential nutrients because the human body can synthesize them de novo (from simple metabolic intermediates).

Biosynthesis of Purine Nucleotides

The two major purine nucleotides found in nucleic acids are:

Adenosine monophosphate (AMP)
Guanosine monophosphate (GMP)

Purine nucleotides can be synthesized by two pathways:

De novo pathway: New synthesis from basic metabolic intermediates.
Salvage pathway: Recycling of free purine bases into nucleotides.

1) De Novo Biosynthesis of Purine Nucleotides

In the de novo pathway, the purine ring is built step by step directly on a ribose-5-phosphate molecule. This ribose is supplied by the pentose phosphate pathway.

Precursors for the Purine Ring:

Glycine provides C4, C5, and N7.
Aspartate provides N1.
Glutamine provides N3 and N9.
Tetrahydrofolate derivatives supply C2 and C8.
Carbon dioxide (CO₂) provides C6.

Major Steps in De Novo Purine Synthesis:

Formation of PRPP: Ribose-5-phosphate is converted to phosphoribosyl pyrophosphate (PRPP) by PRPP synthetase using ATP.
Formation of Phosphoribosylamine: PRPP is aminated by glutamine via PRPP amidotransferase to form 5-phosphoribosylamine. This is the first committed (rate-limiting) step in purine synthesis.
Addition of Glycine: Glycine combines with 5-phosphoribosylamine using ATP to form glycinamide ribotide.
Formylation: A one-carbon group from N⁵,N¹⁰-methenyl tetrahydrofolate is added to form formylglycinamide ribotide.
Second Amination: Glutamine donates another amino group to form formylglycinamidine ribotide.
Ring Closure: Cyclization forms aminoimidazole ribotide.
Carboxylation: Carbon dioxide adds to form carboxyaminoimidazole ribotide.
Addition of Aspartate: Aspartate condenses to form succinyl aminoimidazole carboxamide ribotide.
Release of Fumarate: The succinyl group is removed as fumarate, leaving aminoimidazole carboxamide ribotide.
Second Formylation: Another one-carbon group is added from tetrahydrofolate to form formiminoimidazole carboxamide ribotide.
Ring Closure: The ring closes to form inosine monophosphate (IMP), the first complete purine nucleotide.

Formation of AMP and GMP from IMP:

AMP formation: IMP reacts with aspartate (catalyzed by adenylosuccinate synthetase using GTP), followed by the release of fumarate to form AMP.
GMP formation: IMP is first oxidized to xanthosine monophosphate (XMP) by IMP dehydrogenase, then an amino group from glutamine is added (via GMP synthetase) using ATP to form GMP.

Regulation of De Novo Purine Synthesis

The pathway is tightly controlled by feedback mechanisms to maintain balance between AMP and GMP production.

PRPP Synthetase: Inhibited by AMP and GMP (end products).
PRPP Amidotransferase: The rate-limiting enzyme inhibited by both AMP and GMP.
IMP Conversion: AMP regulates adenylosuccinate synthetase, while GMP regulates IMP dehydrogenase, ensuring equal production of both nucleotides.

Additionally, a high concentration of PRPP stimulates purine synthesis, whereas a low concentration inhibits it.

2) Salvage Pathway for Purine Nucleotide Synthesis

The salvage pathway recycles free purine bases released during the breakdown of nucleic acids. This process is energy-efficient compared to de novo synthesis and occurs mainly in tissues like the brain and bone marrow.

In this pathway, the purine bases (adenine, guanine, and hypoxanthine) react with PRPP to form nucleotides:

Adenine phosphoribosyltransferase (APRT): Converts adenine to AMP.
Hypoxanthine-guanine phosphoribosyltransferase (HGPRT): Converts hypoxanthine to IMP and guanine to GMP.

Deficiency of HGPRT leads to Lesch-Nyhan syndrome, characterized by excessive uric acid production and neurological symptoms.

Catabolism of Purine Nucleotides

Purine nucleotides are broken down into their base components, ultimately producing uric acid — a sparingly soluble compound that is excreted in urine.

Steps in Purine Catabolism:

AMP and GMP are dephosphorylated to their respective nucleosides (adenosine and guanosine).
Adenosine is deaminated to inosine by adenosine deaminase.
Inosine is then cleaved by purine nucleoside phosphorylase to yield hypoxanthine.
Hypoxanthine is oxidized by xanthine oxidase to xanthine and then further to uric acid.
Guanosine is cleaved to guanine, which is deaminated by guanase to form xanthine.