Ribonucleotide reduction, general

Many similarities, and strong dissimilarities of the known enzymes have been mentioned above. They are all identical in that a nucleotide-binding protein, with redox-active cysteine/cystine residues, combines with another metal-containing polypeptide or a metal coenzyme in which radical intermediates can be generated and stabilized. The binding protein carries several nucleotide sites so that reaction rates are influenced by allosteric effects, with the same specificity pattern everywhere. On closer inspection it becomes apparent that the considerable individual differences in subunit composition and in the nature of the second, catalytic component can indeed be integrated into a general concept of ribonucleotide reduction. 

In animal cells, the product of ribonucleotide reduction, dUDP, is converted to the two other levels of phosphorylation by reversible, ATP-dependent transphosphorylations catalyzed by thymidylate kinase (see below) and the general nucleoside diphosphate kinase. 

SH)2 (with opening and relaxation of the 14-membered polypeptide ring.) As a reductant, thioredoxin functions as an electron transport protein between NADPH and the reduction of ribonucleotides to deoxyribonucleotides, as a general catalyst for the reduction of protein disulphides, in other reductive processes and in the regulation of the hormonal response . ... 

In a general sense, reduced thioredoxins appear to be capable of serving as reductants to heterologous ribonucleotide reductases. For example, the E. coli thioredoxin will function as a hydrogen donor for the ribonucleotide reductase of the Novikoff hepatoma, and yeast thioredoxin will serve the E. coli reductase. On the other hand, reduction of any particular thioredoxin is catalyzed only by the homologous thioredoxin reductase, that is, both must be obtained from the same organism. 

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