Dealkylation of quaternary ammonium salts by thiolate anions: A model of the cobalamin-independent methionine synthase reaction.
✍ Scribed by Ellen Hilhorst; Tjoe B.R.A. Chen; Atef S. Iskander; Upendra K. Pandit
- Publisher
- Elsevier Science
- Year
- 1994
- Tongue
- French
- Weight
- 970 KB
- Volume
- 50
- Category
- Article
- ISSN
- 0040-4020
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✦ Synopsis
The reactions of thiolate ions derived from thiophenol and homocysteioe with substituted quatemary ammoniom salts result in alkyl tmnsfer from nitrogen to sulfur. A radical mechanism for this tmnsakylation, accounts for the reactivity pattern of the substrate salts. In a model study of the cobalamin-independent methionine synthase reaction, 5,5.6,7-tetramerhyl-5.6;7,8-tetrahydroptcridiniumsalt(25),whichcanbcconsidaedasamodelf~thenaMal
UxxEymc 5-CH$-I4-folate (1). was allowed to react with the thiilate of homocysteiae, whereupon the won of m&ion& was observed in good yield. 'These results suggest that in the enzymatic prcnxss the N(S)-CH3 bond may be activated for the methyl transfer step, by uxndination of the N( ) with an electmphde or a proton at the active site.
The methionine synthase catalyzed reaction2 involves the overall methyl transfer from the coenzyme 5-methyltetrahydrofolate
(1) to the thiol group of the substrate homocysteine (2), generating methionine (3) and tetrahydrofolate (4) (Scheme 1).
Too-
iH3+ 1 (5methyltetrahydrofolate) 2 (homocysteine) yoo-HF-(CHd2 SCH3 NHs+ 4 (tetrahydrofolate) 3 (methionine) Scheme 1
Two types of enzymes have been distinguished2 for the catalysis of the conversion of 2 to 3. These are: (i) the cobalamin-independent and (ii) the cobalamin-dependent methionine synthases. The cobalamin-