Ribonucleotide reduction and the possible role of cobalamin in evolution.

The biological pathways of ribonucleotide reduction are briefly reviewed. The hypothesis is presented that reduction of ribonucleoside triphosphates to their deoxynucleotide analogs through the mediation of vitamin B12 or a similar corrinoid preceded and was necessary for the subsequent development of a DNA-type genome. There are two known biological systems for ribonucleotide reduction: (1) The ribonucleoside diphosphate reduction system which utilizes a nonheme iron ribonucleotide reductase enzyme, thioredoxin and its reductase, and NADPH. This enzyme complex is found in most bacteria, some higher organisms, and in all animals. (2) The ribonucleoside triphosphate reduction system which utilizes adenosyl cobalamin, ribonucleotide reductase and either thioredoxin or a disulfhydryl compound. The cobalamin-dependent reductase is restricted to a few species of bacteria and blue-gree algae. This system is considered more primitive than the iron reductase one based on their differences in distribution, components, and products.