Most of the carbon that flows through nucleotide synthetic pathways goes into ribonucleotide triphosphates (rNTPs - ATP, CTP, GTP, and UTP). A relatively small fraction is diverted to the synthesis of deoxyribonucleoside triphosphates (dNTPs). rNTPs are synthesized in excess of dNTPs because most cells contain 5-10 times as much RNA as DNA and because rNTPs have multiple metabolic roles, whereas dNTPs are used only to make DNA.

As seen in Figure 22.12, rNDPs ("D" here refers to di-) are all converted to dNDPs by the enzyme ribonucleoside diphosphate reductase (also called ribonucleotide reductase, rNDP reductase, or RNR). The relatively simple routes giving rise to dATP, dGTP, and dCTP, are in contrast to the involved mechanism for dTTP. Note that dUDP is a precursor of dTTP.

Ribonucleotide reductase, the enzyme catalyzing the synthesis of dNDPs from rNDPs, reduces the hydroxyl at carbon 2 to a hydrogen via a free radical mechanism. The following three classes of ribonucleotide reductases are known:

Class I ribonucleotide reductases - The most widely distributed form of ribonucleotide reductase. Class I enzyme acts upon ribonucleoside diphosphates. The enzyme generates a free radical on a tyrosine residue with the aid of a diferric oxygen bridge.

Class II ribonucleotide reductases - Found in cyanobacteria, some bacteria, and Euglena, the Class II enzyme acts on ribonucleoside triphosphate substrates. It uses adenosylcobalamin, a B12 coenzyme to generate a free radical.

Class III ribonucleotide reductases - Found only in facultative or obligate anaerobes, the Class III enzyme acts on ribonucleoside triphosphate substrates. It uses S-adenosylmethionine and an iron-sulfur center to generate the catalytically essential radical on a glycine residue.

The most common form of the enzyme (Class I) is an 22 dimer. The structure of the E. coli enzyme is shown in Figure 22.13. The two subunits form the large subunit of the protein called R1. It contains the active site. The two subunits make up the small subunit of the protein called R2, which contains the free radical. A clue to the mechanism of action of the enzyme (tyrosine free radical) is shown in Figure 22.14. Hydroxyurea, an inhibitor of ribonucleotide reductase, destroys the free radical.

A proposed mechanism of action for ribonucleotide reductase is shown in Figure 22.15. Reduction of the ribonucleotides requires electrons. These ultimately come from NADPH and are delivered to ribonucleotide reductase by either thioredoxin or glutaredoxin, as shown in Figure 22.16. Evidence exists for a possible third electron carrier in E. coli. Some of the interesting biological activities of thioredoxin are listed in Table 22.1.