Enzymes of Nucleotide Modification

Inactivation of the reductase by 2 -azido-2 -deoxyuridine 5 -diphosphate (26, Fig. 56) is accompanied by loss of the tyrosyl radical (Thelander et al., 1976) 

Thymidylate synthase catalyzes the conversion of 2 -deoxyuridine 5 -phosphate to thymidine 5 -phosphate, and it is the sole source of this essential component of DNA. The reaction proceeds by transfer of a one-carbon fragment from the cofactor, 5,10-methylene tetrahydrofolate (CH2H4folate), of the 

The nature of the linkage of the cofactor to the inactive ternary complex has 

A mechanism consistent with this substantial body of evidence is shown in Fig. 57. Addition of cysteine to C-6 followed by attack of the C-5 enolate on the methylene group of CH2H4folate occurs in analogy with the mechanism suggested for the normal methylation reaction. With 5-fluoro-dUMP, however, the synthase appears to become frozen in this covalent ternary complex, resulting in loss of activity. 

AdoHcy hydrolase is inactivated by a variety of other nucleosides, including the cyclopentenyl analog of adenosine, neplanocin A (Borchardt et al., 1984). Complete inactivation is observed with 0.5 equivalents of neplanocin A per active site, yielding 0.5 equivalents each of NADH and adenine (Wolfson et al., 1986). The mechanism of the reaction was proposed to parallel that suggested for 2 -dAdo, although the elimination of adenine would require a cis-elimination, in contrast to the trans-elimination possible with a 2 -deoxy-3 -ketonucleoside. Subsequent studies concurred that NADH was formed in the reaction but demonstrated that the inactivation is reversible by addition of NAD, suggesting that inactivation is simply due to reduction of the coenzyme (Matuszewska and Borchardt, 1987). 

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