Dennis M W, Kolattukudy P E
Ohio State Biotechnology Center, Ohio State University, Columbus 43210.
Arch Biochem Biophys. 1991 Jun;287(2):268-75. doi: 10.1016/0003-9861(91)90478-2.
The final step in the synthesis of n-hydrocarbons in an animal and a higher plant involves enzymatic decarbonylation of aldehydes to the corresponding alkanes by loss of the carbonyl carbon. Whether such a novel reaction is involved in hydrocarbon synthesis in the colonial microalga, Botryococcus braunii, which is known to produce unusually high levels (up to 32% of dry weight) of n-C27, C29, and C31 alka-dienes and -trienes, was investigated. Dithioerythritol severely inhibited the incorporation of [1-14C]acetate into these hydrocarbons with accumulation of the label in the aldehyde fraction in the B. braunii cells. Microsomal preparations of the alga synthesized alkane from fatty acid and aldehyde in the absence of O2. Conversion of fatty acid to alkane required CoA, ATP, and NADH, whereas conversion of aldehyde to alkane did not require the addition of cofactors. That the alkane synthesis involves a decarbonylation was shown by the production of CO and heptadecane from octadecanal. CO was identified by adsorption to RhCl[(C6H6)3P]3. The decarbonylase had a pH optimum at 7.0, an apparent Km of 65 microM, a Vmax of 1.36 nmol/min/mg and was inhibited by the metal chelators EDTA, O-phenanthroline and 8-hydroxyquinoline. It was stimulated nearly threefold by 2 mM ascorbate and inhibited by the presence of O2. A partial (28%) retention of the aldehydic hydrogen of [1-3H]octadecanal in the heptadecane was observed; the remaining 3H was lost to H2O. The microsomal preparation also catalyzed the oxidation of 14CO to 14CO2, with a pH optimum of 7.0. This accounts for the nonstoichiometry of CO to heptadecane observed. In vivo studies with 14CO showed that the label was incorporated into metabolic products. This metabolic conversion of CO, not found in the previously examined hydrocarbon synthesizing systems, may be necessary for organisms that produce large amounts of hydrocarbons such as the present alga. The mechanism of the decarbonylation and the nature of the decarbonylase remain to be elucidated.
动物和高等植物中 n - 烃类合成的最后一步涉及醛通过羰基碳的损失经酶促脱羰作用生成相应的烷烃。在已知能产生异常高水平（高达干重的 32%）的 n - C27、C29 和 C31 链状二烯和三烯的群居微藻布朗葡萄藻（Botryococcus braunii）中，是否存在这样一种新颖的反应参与烃类合成，对此进行了研究。二硫苏糖醇严重抑制了 [1 - 14C] 乙酸盐掺入这些烃类中，标记物在布朗葡萄藻细胞的醛部分积累。该藻类的微粒体制剂在无氧条件下能从脂肪酸和醛合成烷烃。脂肪酸转化为烷烃需要辅酶 A、ATP 和 NADH，而醛转化为烷烃则不需要添加辅因子。十八醛生成一氧化碳和十七烷表明烷烃合成涉及脱羰作用。一氧化碳通过吸附到 RhCl[(C6H6)3P]3 上得以鉴定。脱羰酶的最适 pH 为 7.0，表观 Km 为 65 μM，Vmax 为 1.36 nmol/min/mg，并且受到金属螯合剂乙二胺四乙酸（EDTA）、邻菲啰啉和 8 - 羟基喹啉的抑制。2 mM 的抗坏血酸能使其活性提高近三倍，而氧气的存在则会抑制其活性。观察到 [1 - 3H] 十八醛的醛氢在十七烷中有部分（28%）保留；其余的 3H 则以水的形式损失。微粒体制剂还催化了 14CO 氧化为 14CO2，最适 pH 为 7.0。这就解释了所观察到的一氧化碳与十七烷的非化学计量关系。用 14CO 进行的体内研究表明，标记物被掺入代谢产物中。这种一氧化碳的代谢转化在之前研究的烃类合成系统中未被发现，对于像当前这种能产生大量烃类的生物体可能是必要的。脱羰作用的机制和脱羰酶的性质仍有待阐明。
