tailieunhanh - Báo cáo khoa học: Conformation-dependent hydride transfer in neuronal nitric oxide synthase reductase domain

Calmodulin (CaM) activates the constitutive isoforms of mammalian nitric oxide synthase by triggering electron transfer from the reductase domain FMN to the heme. This enables the enzymes to be regulated by Ca 2+ con-centration. CaM exerts most of its effects on the reductase domain; these include activation of electron transfer to electron acceptors, and an increase in the apparent rate of flavin reduction by the substrate NADPH. | ỊFEBS Journal Conformation-dependent hydride transfer in neuronal nitric oxide synthase reductase domain Andrew Welland and Simon Daff Schoolof Chemistry University of Edinburgh UK Keywords electron transfer isotope effect kinetics nitric oxide nitric oxide synthase NOS Correspondence Simon Daff Schoolof Chemistry University of Edinburgh West Mains Road Edinburgh EH9 3JJ Uk Tel 44 131 650 7378 E-mail Received 4 June 2010 revised 15 July 2010 accepted 22 July 2010 doi Calmodulin CaM activates the constitutive isoforms of mammalian nitric oxide synthase by triggering electron transfer from the reductase domain FMN to the heme. This enables the enzymes to be regulated by Ca2 concentration. CaM exerts most of its effects on the reductase domain these include activation of electron transfer to electron acceptors and an increase in the apparent rate of flavin reduction by the substrate NADPH. It has been shown that the former is caused by a transition from a conformation-ally locked form of the enzyme to an open form as a result of CaM binding improving FMN accessibility but the latter effect has not been explained satisfactorily. Here we report the effect of ionic strength and isotopic substitution on flavin reduction. We found a remarkable correlation between the rate of steady-state turnover of the reductase domain and the rate of flavin reduction over a range of different ionic strengths. The reduction of the enzyme by NADPH was biphasic and the amplitudes of the phases determined through global analysis of stopped-flow data correlated with the proportions of enzyme known to exist in the open and closed conformations. The different conformations of the enzyme molecule appeared to have different rates of reaction with NADPH. Thus proximity of FMN inhibits hydride transfer to the FAD. In the CaM-free enzyme slow conformational motion opening and closing limits turnover. It is now clear that this motion also controls