Ribonucleotide reductases substrate analogs

The mechanism proposed in Scheme 5 makes experimentally verifiable predictions concerning the mechanism of the ribonucleotide reductase reaction with normal substrates and substrate analogs. Two crucial predictions are the cleavage of the 3 -carbon-hydrogen bond and the return of (possibly the same) hydrogen to the 3 -position on the same face from which the original atom was abstracted. 

The proposed mechanisms for the utilization of substrates and substrate analogs by the ribonucleotide reductases. Schemes 5 and 6, predict the formation of various radical intermediates. To search for these intermediates and their direct participation in these reactions, EPR spectroscopy has been employed (60). However, these attempts have not revealed the generation of new radical signals. 

The activation and inhibition patterns established for calf thymus and human ribonucleotide reductases resemble that of the E. coli enzyme in many ways (Table 6). Two different classes of effector sites are not only likely from the analogous stimulatory and inhibitory effects of deoxyribonucleotides and ATP on substrate reduction. In certain mouse cell lines two regulatory protein domains on reductase subunit M1 have indeed been identified which are mutated independent of each other On the other hand phage T4 enzyme, with closely comparable protein structure, behaves different in that the inhibitory effect of dATP is missing, indicating alteration of the activity sites. 

As with the bacterial reductases, a complex pattern of activation and inhibition by nucleoside triphosphates has been demonstrated for the tumor reductase dATP inhibits reduction of all four substrates (S2). A similar pattern of nucleotide regulatory effects was found with partly purified reductase from rat embryo extracts (S5). Thus, presuming that analogy with the bacterial ribonucleotide reductases is valid, it would appear that the animal reductases are allosteric enzymes subject to a complicated regulation by nucleotides again, the function of such regulation would seem to be that of ensuring a balanced supply of deoxyribonucleo-tides for DNA synthesis. 

Leishmania adenylosuccinate synthetase has a narrow substrate specificity but accepts several IMP analogs which include allopurinol ribonucleotide (34). The GMP reductase from L. donovani is quite different from the human GMP reductase (35) and IMP analogs are more potent inhibitors for it. Other leishmanial enzymes that have been investigated include IMP dehydrogenase (36), nucleoside hydrolase and phos-phorylase activities (37,38), adenosine kinase (39), nucleotidases (40) and the adenylosuccinate lyase (34). 

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