Fox Brian G, Lyle Karen S, Rogge Corina E
Department of Biochemistry, College of Agricultural and Life Sciences, University of Wisconsin, Madison, Wisconsin 53706, USA.
Acc Chem Res. 2004 Jul;37(7):421-9. doi: 10.1021/ar030186h.
Stearoyl-acyl carrier protein Delta(9) desaturase (Delta9D) produces oleic acid, a nutritionally valuable fatty acid containing a cis double bond between C-9 and C-10. This multiprotein diiron enzyme complex reacts with stearoyl-acyl carrier protein, reduced [2Fe-2S] ferredoxin, and O(2) to complete the highly regiospecific and stereoselective desaturation reaction. Interactions with the acyl chain provide stability to the enzyme-substrate complex, give an energetic contribution to catalytic selectivity, and help to order the electron transfer, O(2) binding, and C-H bond cleavage steps of catalysis. Reactions with natural acyl chains indicate the involvement of a highly reactive diiron intermediate capable of oxidizing secondary C-H bonds (bond dissociation energy approximately 95 kcal/mol), but also capable of diagnostic O-atom transfer reactions with the appropriate substrate analogues. For soluble Delta9D, the natural reaction may initiate at the C-10 position, in contrast to the well-established initial reactivity of the membrane enzyme homologue stearoyl-coenzyme A (CoA) Delta(9) desaturase at the C-9 position.
硬脂酰-酰基载体蛋白Δ9去饱和酶（Δ9D）可产生油酸，这是一种在营养方面具有重要价值的脂肪酸，其在C-9和C-10之间含有一个顺式双键。这种多蛋白二铁酶复合物与硬脂酰-酰基载体蛋白、还原型[2Fe-2S]铁氧还蛋白以及O₂发生反应，以完成高度区域特异性和立体选择性的去饱和反应。与酰基链的相互作用为酶-底物复合物提供稳定性，对催化选择性做出能量贡献，并有助于催化过程中电子转移、O₂结合和C-H键裂解步骤的有序进行。与天然酰基链的反应表明，存在一种高反应性的二铁中间体，它能够氧化仲C-H键（键解离能约为95千卡/摩尔），但也能够与合适的底物类似物发生诊断性的O原子转移反应。对于可溶性Δ9D，天然反应可能在C-10位置起始，这与膜酶同系物硬脂酰辅酶A（CoA）Δ9去饱和酶在C-9位置已确定的初始反应性形成对比。
