Mechanism of the reaction catalyzed by dihydrofolate reductase from Escherichia coli: pH and deuterium isotope effects with NADPH as the variable substrate.
Mechanism of the reaction catalyzed by dihydrofolate reductase from Escherichia coli: pH and deuterium isotope effects with NADPH as the variable substrate.
复制标题
大肠杆菌二氢叶酸还原酶催化反应机理：以 NADPH 为可变底物的 pH 和氘同位素效应。
作者：
J. Morrison;S. Stone
The variations with pH of the kinetic parameters and primary deuterium isotope effects for the reaction of NADPH with dihydrofolate reductase from Escherichia coli have been determined. The aims of the investigations were to elucidate the chemical mechanism of the reaction and to obtain information about the location of the rate-limiting steps. The V and V/KNADPH profiles indicate that a single ionizing group at the active center of the enzyme must be protonated for catalysis, whereas the Ki profiles show that the binding of NADPH to the free enzyme and of ATP-ribose to the enzyme-dihydrofolate complex is pH independent. From the results of deuterium isotope effects on V/KNADPH, it is concluded that NADPH behaves as a sticky substrate. It is this stickiness that raises artificially the intrinsic pK value of 6.4 for the Asp-27 residue of the enzyme-dihydrofolate complex [Howell, E. E., Villafranca, J. E., Warren, M. S., Oatley, S. J., & Kraut, J. (1986) Science (Washington, D.C.) 231, 1123] to an observed value of 8.9. Thus, the binary enzyme complex is largely protonated at neutral pH. The elevation of the intrinsic pK value of 6.4 for the ternary enzyme-NADPH-dihydrofolate complex to 8.5 is not due to the kinetic effects of substrates. Rather, it is the consequence of the lower, pH-independent rate of product release and the faster pH-dependent catalytic step. At neutral pH, the proportion of enzyme present as a protonated ternary enzyme-substrate complex is sufficient to keep catalysis faster than product release.(ABSTRACT TRUNCATED AT 250 WORDS)
DOI：
10.2210/pdb3dfr/pdb
发表时间：
1982-11
期刊：
The Journal of biological chemistry
影响因子：
--
作者：
J. Bolin;D. Filman;D. Matthews;R. Hamlin;J. Kraut
通讯作者：
J. Bolin;D. Filman;D. Matthews;R. Hamlin;J. Kraut
影响因子：
56.9
作者：
HOWELL, EE;VILLAFRANCA, JE;KRAUT, J
通讯作者：
KRAUT, J