化学
烟酰胺单核苷酸
烟酰胺磷酸核糖转移酶
焦磷酸盐
立体化学
NAD+激酶
部分
磷酸核糖转移酶
自由能微扰
核糖
酶
分子
次黄嘌呤鸟嘌呤磷酸核糖转移酶
生物化学
烟酰胺腺嘌呤二核苷酸
有机化学
突变体
基因
作者
Ryōsuke Takahashi,Shota Nakamura,Takashi Nakazawa,Katsuhiko Minoura,Takuya Yoshida,Yoshinori Nishi,Yuji Kobayashi,Tadayasu Ohkubo
摘要
Nicotinamide (NM) phosphoribosyltransferase (NMPRTase) catalyzes the reaction of NM and 5′-phosphoribosyl-1′-pyrophosphate (PRPP) to form NM mononucleotide (NMN) and pyrophosphate (PPi) in the pathway of NAD-biosynthesis. Monitoring the 1H and 31P NMR spectra of the reaction mixture, we found that this reaction is reversible as dictated by the equilibrium constant K = [NMN][PPi]/([NM][PRPP]) = 0.14, which agreed well with the ratio of second-order rate constants for forward and backward reactions, K = 0.16. The crystal structures of this enzyme in the free form and bound to NM and PRPP at the resolution of 2.0–2.2 Å were essentially identical to that of the complex with NMN, except for some variations that could facilitate the substitution reaction by fixing the nucleophile and the leaving group for the requisite inversion of configuration at the C1’ carbon of the ribose ring. In the active site near the C1’ atom of the bound PRPP or NMN, there was neither negatively charged group nor waterproof environment necessary to support the feasibility of a ribo-oxocarbocation intermediate inherent in the SN1 mechanism. The structures and catalytic mechanism thus revealed are also discussed in connection with the multiple biological functions of NMPRTase.
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