Thymidylate synthetase catalyzes the reductive methylation of 2'-deoxyuridylate (dUMP) to thymidylate (TMP) with the concomitant conversion of 5, 10-methylene tetrahydrofolate (CH 2 FAH 4 ) to 7, 8-dihydrofolate (FAH 2 ) as depicted in Fi gure 1 (for a recent review, see reference 1). In this process the hydrogen at C-6 of FAH 4 is directly transferred to the methyl group of TMP (2).One of our objectives over the past few years has been to establish the mechanism of catalysis of thymidylate synthetase. Extensive investigations of chemical counterparts (3-6) have in dicated that the reaction is initiated by attack of a nucleophile at the 6-position of dUMP and that many, if not all, reactions along the pathway are facilitated by analogous nucleophilic catalysis.The proposed mechanism of this enzyme, as derived from investigations of chemical models is illustrated in Figure 2. It is proposed that the reaction is initiated by attack of a nucleophilic group of the enzyme to the 6-position of dUMP. In this manner, the 5-position of dUMP could be made sufficient ly nucleophilic (viz I, Figure 2) to react with CH 2 FAH 4 or an equivalent reactive species of formaldehyde. Thus, the initial condensation product between dUMP and CH 2 FAH 4 is now generally accepted (1, 7) to be one which is covalently bound to the enzyme and saturated across the 5, 6-double bond of dUMP (II). Proton abstraction from II would give the intermediate enolate III. As with the chemical models, III should readily undergo a β-elimination to produce the highly reactive exocyclic methylene intermediate IV and FAH 4 , bound to the enzyme in close proximity. Intermolecular hydride transfer from FAH 4 to IV would yield dTMP, FAH 2 , and the native enzyme. It should be emphasized that all of the aforementioned reactions and inter mediates have direct chemical counterparts, and are in complete accord with all available biochemical data.With the availability of a stable enzyme from an amethop terin resistant strain of Lactobacillus casei (8, 9) and facile methods for its purification (9-11), we undertook studies which 57 Downloaded by GEORGETOWN UNIV on August 25, 2015 |