Theoretical studies on the activation of the pterin cofactor in the catalytic mechanism of dihydrofolate reductase

作者：Jill E. Gready · 发表于：Biochemistry · 年份：1985 · DOI：10.1021/bi00339a008 · 被引用次数：62 · 研究领域：Porphyrin Metabolism and Disorders、Enzyme Structure and Function、RNA regulation and disease

Two mechanisms for facilitating hydride ion transfer from NADPH involving preprotonation of the pteridine rings of the dihydrofolate reductase substrates folate and dihydrofolate have been investigated by ab initio quantum mechanical methods. Protonation energies and effective solution pKas have been calculated for four protonated forms, three of which are nonpreferred in aqueous solution and therefore not directly accessible to experimental study. The pattern and degree of redistribution of the positive charge over the component rings of the N-heterobicyclic pi-system in these protonated forms have been analyzed in terms of changes in the electron populations of the ring atoms and total ring charges. The effects of such changes in promoting hydride ion transfer to C7 in folate and C6 in dihydrofolate have been evaluated by considering the extent of development of partial carbonium ion character at these carbon atoms and also the degree of electron deficiency in the pyrazine ring as a whole. The results illustrate that perturbations due, for instance, to protonation may be propagated by pi-electron coupling effects over medium-range distances of 4-6 A across the pteridine ring. The two mechanisms have been assessed in terms of the calculated absolute and relative pKas of the protonated species taking into account experimental information regarding possible stabilization of these forms in the enzyme active site and also the effectiveness of the various protonations in assistin...