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Methylene-tetrahydromethanopterin Dehydrogenase

Re-face Stereospecificity of Methylenetetrahydromethanopterin Dehydrogenases and Methylenetetrahydrofolate Dehydrogenases Predetermined by Intrinsic Properties of the Substrate

S. Bartoscheck, G. Buurman, R. K. Thauer, B. H. Geierstanger, J. P. Weyrauch, C. Griesinger, M. Nilges, M. C. Hutter, V. Helms
Chem. Bio. Chem. 2 (2001) 530-541.

Abstract:

Four different dehydrogenases are known that catalyze the reversible dehydrogenation of N5,N10-methylenetetrahydro methanopterin (methylene-H4MPT) or N5,N10-methylenetetrahydrofolate (methylene-H4F) to the respective N5,N10-methenyl compounds. Sequence comparisons indicates that the four enzymes are phylogenetically unrelated. They all catalyze the Re-face stereo-specific removal of the pro-R hydrogen atom of conenzyme's methylene group. The Re-face stereospecificity is in contrast to the finding that in solution the pro-S hydrogen atom of methylene-H4MPT and of methylene-H4F is more reactive to heterolytic cleavage. For a better understanding we determined the conformations of methylene-H4MPT in solution and when enzyme- bound by using NMR spectroscopy and semiempirical quantum mechanical calculations. For the conformation free in solution we find an envelope conformation for the imidazoline ring, with the flat at N10. The methylene pro-S C-H bond is anticlinal and the methylene pro-R C-H bond is synclinal to the lone pair of N10. Semiempirical quantum mechanical calculations of heats of formation of methylene-H4MPT and methylene-H4F indicate that changing this conformation into an activated one in which the pro-S C-H bond is antiperiplanar, resulting in the preformation of the leaving hydride, would require a ΔΔHfo of 53 kJ mol-1 for methylene-H4MPT and of 51 kJ mol-1 for methylene-H4F. This is almost twice the energy required to force the imidazoline ring in the enzyme-bound conformation of methylene-H4MPT (+29 kJ mol-1) or of methylene-H4F (+35 kJ mol-1) into an activated conformation in which the pro-R hydrogen is antiperiplanar to the lone electron pair of N10. The much lower energy for pro-R hydrogen activation thus probably predetermines the Re-face stereospecificity of the foue dehydrogenases. Results are also presented explaning why the chemical reduction of methenyl-H4MPT and methenyl-H4F with NaBD4 proceeds Si-face-specific, in contrast to the enzyme-catalyzed reaction.

Keywords:conformation analysis; dehydrogenases; enzyme catalysis; oxidoreductases; stereospecificity

M. Hutter July, 30th 2001


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