Electron inventory, kinetic assignment (En), structure, end bonding of nitrogenase turnover intermediates with C2H2 and CO
Electron inventory, kinetic assignment (En), structure, end bonding of nitrogenase turnover intermediates with C2H2 and CO
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DOI：
10.1021/ja054078x
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发表时间：
2005-11-16
影响因子：
15
通讯作者：
Hoffman, BM
中科院分区：
文献类型：
--
作者：
Lee, HI;Sorlie, M;Hoffman, BM
在(H2O)- h -1,2缓冲液中与(C2H2)- h -1,2形成的氮化酶α -195(Gln) MoFe蛋白转换过程中形成的S-EPR1中间体改进的H-1 ENDOR数据，结合最近对丙炔醇形成的中间体的研究，表明SEPR1是一个产物复合物，可能与C2H4作为铁环结合到femo辅助因子活性位点的单个Fe上。富Fe-57的SEPR1辅因子的35 GHz CW和Mims脉冲Fe-57 ENDOR表明，它与CO， [Mo4+, Fe3+, Fe-6(2+), S-9(2-)(d(43))]（+1）转换过程中形成的lo-CO中间体的CO结合辅因子具有相同的价，相对于静息态辅因子减少了m = 2个电子。考虑到Fe-57在SEPR1和lo-CO中的超精细耦合，可以得出CO桥接两个lo-CO的Fe，而SEPR1的C2H4结合其中一个Fe的图像。为了将这些中间体和其他中间体与底物还原的low - thorneley （LT）动力学方案联系起来，我们引入了“电子库存”的概念。它将MoFe蛋白中间体从Fe蛋白接受的电子数(n)划分为传递给底物的电子数(s)、留在中间辅因子上的电子数(m)以及从P簇传递给辅因子的额外电子数(P): n = m + s - P（这里P = 0）。lo-CO和SEPR1的辅因子都被m = 2个电子还原，但中间产物不处于相同的LT还原阶段(E): lo-CO的（n = 2; m = 2, s = 0）；（n = 4; s = 2, m = 2）。这是首次提出的lte动力学状态与酶的明确化学状态的相关性。
Improved H-1 ENDOR data from the S-EPR1 intermediate formed during turnover of the nitrogenase alpha-195(Gln) MoFe protein with (C2H2)-H-1,2 in (H2O)-H-1,2 buffers, taken in context with the recent study of the intermediate formed from propargyl alcohol, indicate that SEPR1 is a product complex, likely with C2H4 bound as a ferracycle to a single Fe of the FeMo-cofactor active site. 35 GHz CW and Mims pulsed Fe-57 ENDOR of Fe-57-enriched SEPR1 cofactor indicates that it exhibits the same valencies as those of the CO-bound cofactor of the lo-CO intermediate formed during turnover with CO, [Mo4+, Fe3+, Fe-6(2+), S-9(2-)(d(43))](+1), reduced by m = 2 electrons relative to the resting-state cofactor. Consideration of Fe-57 hyperfine coupling in SEPR1 and lo-CO leads to a picture in which CO bridges two Fe of lo-CO, while the C2H4 Of SEPR1 binds to one of these. To correlate these and other intermediates with Lowe-Thorneley (LT) kinetic schemes for substrate reduction, we introduce the concept of an "electron inventory". It partitions the number of electrons a MoFe protein intermediate has accepted from the Fe protein (n) into the number transmitted to the substrate (s), the number that remain on the intermediate cofactor (m), and the additional number delivered to the cofactor from the P clusters (p): n = m + s - p (with p = 0 here). The cofactors of lo-CO and SEPR1 both are reduced by m = 2 electrons, but the intermediates are not at the same LT reduction stage (E): (n = 2; m = 2, s = 0) for lo-CO; (n = 4; s = 2, m = 2) for SEPR1. This is the first proposed correlation of an LT E kinetic state with a well-defined chemical state of the enzyme.