Why do cells maintain salvage pathways if they can make nucleotides de novo?

Salvage pathways reuse preformed bases at far lower energetic cost than building rings from scratch, which is advantageous in tissues with high demand or limited synthetic capacity.

Where do the deoxyribonucleotides for DNA come from?

They are made from the corresponding ribonucleotides by ribonucleotide reductase, which removes the 2'-hydroxyl group through a radical mechanism.

Nucleotide Metabolism

Nucleotide metabolism comprises the pathways that build nucleotides from simple precursors and recycle or degrade them, supplying the building blocks for nucleic acids and energy carriers.

Definition

Nucleotide metabolism is the set of biosynthetic and catabolic pathways that produce purine and pyrimidine nucleotides by de novo and salvage routes and that break them down to disposal products.

Scope

This topic covers de novo synthesis of purine and pyrimidine nucleotides, the energetically economical salvage pathways that reuse preformed bases, the conversion of ribonucleotides to deoxyribonucleotides by ribonucleotide reductase, and the degradation of nucleotides to excretory end products.

Core questions

How do de novo and salvage pathways differ in cost and strategy?
How are purine and pyrimidine rings assembled?
How are deoxyribonucleotides produced from ribonucleotides?
What are the end products of nucleotide degradation?

Key theories

Ribonucleotide reduction: Ribonucleotide reductase converts ribonucleotides to deoxyribonucleotides by a radical-based mechanism, the sole route to the deoxyribonucleotide precursors of DNA and a key control point for balanced supply.

Mechanisms

Purine nucleotides are built de novo by assembling the ring stepwise on a ribose-phosphate scaffold, whereas pyrimidine rings are made first and then attached to ribose-phosphate. Salvage pathways reattach preformed bases to ribose-phosphate at much lower energetic cost. Ribonucleotide reductase then supplies deoxyribonucleotides for DNA. Degradation funnels purines to uric acid or further products and pyrimidines to soluble small molecules, with feedback regulation balancing the nucleotide pools.

Clinical relevance

Nucleotide pathways are classic examples of regulated biosynthesis and are targets for many enzyme inhibitors studied in medicinal chemistry. The treatment is descriptive and non-prescriptive.

History

Isotopic labeling studies in the mid-twentieth century, notably by Buchanan, traced the origins of the atoms in the purine ring, while Reichard and others elucidated ribonucleotide reductase, establishing the chemistry of nucleotide biosynthesis.

Key figures

John Buchanan
Arthur Kornberg
Peter Reichard

Seminal works

nelson2021
berg2019

Frequently asked questions

Why do cells maintain salvage pathways if they can make nucleotides de novo?: Salvage pathways reuse preformed bases at far lower energetic cost than building rings from scratch, which is advantageous in tissues with high demand or limited synthetic capacity.
Where do the deoxyribonucleotides for DNA come from?: They are made from the corresponding ribonucleotides by ribonucleotide reductase, which removes the 2'-hydroxyl group through a radical mechanism.