Ribonucleotide reductase

Reaction mechanism

The primary biochemical function of the enzyme is to prepare precursors for DNA chain elongation. During the reaction, ribonucleotide reductase reduces ribonucleotides. The essence of this process lies in the modification of the carbohydrate component of the molecule: a targeted reduction of the hydroxyl group located precisely at the C2 position of the ribose ring takes place. As a result of this chemical transformation, ribonucleotides are converted into deoxyribonucleotides.

Composition of the ribonucleotide reductase complex

For the reaction to proceed efficiently within the cell, a coordinated enzymatic assembly known as the ribonucleotide reductase complex functions. It includes several critically important components:

• Ribonucleotide reductase (RNR) — acts as the main catalyst and the key enzyme of the process.
• Thioredoxin — a specialized protein cofactor. It acts as the direct reducing agent for nucleoside diphosphates (NDPs). The chemical structure of thioredoxin contains reactive sulfhydryl groups (-SH). By donating their hydrogen atoms to reduce ribose, these -SH groups become oxidized.
• Regeneration system — a set of specific molecules that return the oxidized cofactor back to its active state.

Thioredoxin regeneration system

After thioredoxin has fulfilled its role and transferred hydrogen atoms to the NDP molecule, it converts into an oxidized, inactive form. To prevent the DNA synthesis cycle from halting, the cell must continuously reduce this protein cofactor.

A dedicated reduction system is responsible for this process:

1. Thioredoxin reductase — a specialized enzyme that catalyzes the reverse reduction of the oxidized form of thioredoxin.
2. Coenzyme NADPH — serves as the global cellular donor of hydrogen atoms for this reaction. Specifically, NADPH provides the necessary hydrogen to fully restore the sulfhydryl groups of thioredoxin to their initial active state.

Allosteric regulation of RNR

Since DNA synthesis is a critical process, the activity of ribonucleotide reductase is under strict control. The enzyme is allosteric, meaning its catalytic activity is finely modulated depending on the current intracellular concentration of deoxynucleoside triphosphate (dNTP):

• If an excess of dATP accumulates in the cell, it serves as a chemical signal that «building blocks» for DNA replication are already sufficient. In response to this signal, the dATP molecule acts as a potent inhibitor, completely suppressing the reduction of all types of ribonucleotides.
• The dGTP molecule acts more selectively: its accumulation in the medium leads to the specific inhibition of the reduction of pyrimidine NDPs exclusively.

Class and exam questions

What else to study on this topic

• Mechanism of oxidation of thioredoxin sulfhydryl groups
• Structure of the active and allosteric centers of RNR
• Role and sources of coenzyme NADPH in the cell
• Differences in the regulation of purine and pyrimidine nucleotide synthesis

Other topics in the "Nucleotide Metabolism" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.