Cryptoregiochemistry of a Brassica napus fatty acid desaturase (FAD3): a kinetic isotope effect study
作者:Christopher K. Savile、Darwin W. Reed、Dauenpen Meesapyodsuk、Patrick S. Covello、Peter H. Buist
DOI:10.1039/b008111f
日期:——
α-Linolenic acid ((Z,Z,Z)-octadeca-9,12,15-trienoic acid) is biosynthesized by a series of regio- and stereoselective dehydrogenation reactions which are catalyzed by a set of enzymes known as fatty acid desaturases. As part of ongoing research into the mechanism of these remarkable catalysts, we have examined the cryptoregiochemistry (site of initial oxidation) of extraplastidial ω − 3 desaturation as it occurs in the commercially important plant Brassica napus (oilseed rape or canola). The individual deuterium kinetic isotope effects associated with the C–H bond cleavages at C-15 and C-16 of a thiaoleoyl analogue were measured using a convenient in vivo yeast expression system. Competition experiments using appropriately deuterium-labelled 7-thia substrates revealed a large kinetic isotope effect (KIE) (kH/kD = 7.5 ± 0.4)
for the C–H bond-breaking step at C-15 while the C–H bond cleavage at C-16 was found to be insensitive to deuterium substitution (kH/kD = 1.0 ± 0.14). These results point to C-15 as the site of initial oxidation in ω − 3 desaturation since the first chemical step in this type of reaction is rupture of a strong, unactivated C–H bond—an energetically difficult process which typically exhibits a large KIE.
α-亚麻酸((Z,Z,Z)-十八碳-9,12,15-三烯酸)是由一系列区域选择性和立体选择性脱氢反应生物合成的,这些反应由一组被称为脂肪酸去饱和酶的酶催化。作为对这些卓越催化剂机理的持续研究的一部分,我们研究了重要经济作物油菜(油籽或菜籽)中质体外ω-3去饱和作用的隐秘化学选择性(初始氧化位点)。通过一个方便的活体内酵母表达系统,测量了噻酯类似物C-15和C-16处的C-H键断裂所伴随的单独氘动力学同位素效应。使用适当氘标记的7-噻基底物的竞争实验揭示了C-15处的C-H键断裂步骤具有较大的动力学同位素效应(KIE)(kH/kD = 7.5 ± 0.4),而C-16处的C-H键断裂则对氘置换不敏感(kH/kD = 1.0 ± 0.14)。这些结果表明,C-15是ω-3去饱和作用中初始氧化的位点,因为在这种类型的反应中,第一个化学步骤是断裂一个强力的、未活化的C-H键——这是一个能量上困难的过程,通常表现出较大的KIE。