Faculty of Biology, University of Freiburg, D-79104 Freiburg, Germany.
J Biol Chem. 2010 Apr 16;285(16):12109-20. doi: 10.1074/jbc.M109.091843. Epub 2010 Feb 23.
The cyclization of lycopene generates provitamin A carotenoids such as beta-carotene and paves the way toward the formation of cyclic xanthophylls playing distinct roles in photosynthesis and as precursors for regulatory molecules in plants and animals. The biochemistry of lycopene cyclization has been enigmatic, as the previously proposed acid-base catalysis conflicted with the possibility of redox catalysis as predicted by the presence of a dinucleotide binding site. We show that reduced FAD is the essential lycopene cyclase (CrtY) cofactor. Using flavin analogs, mass spectrometry, and mutagenesis, evidence was obtained based on which a catalytic mechanism relying on cryptic (net) electron transfer can be refuted. The role of reduced FAD is proposed to reside in the stabilization of a transition state carrying a (partial) positive charge or of a positively charged intermediate via a charge transfer interaction, acid-base catalysis serving as the underlying catalytic principle. Lycopene cyclase, thus, ranks among the novel class of non-redox flavoproteins, such as isopentenyl diphosphate:dimethylallyl diphosphate isomerase type 2 (IDI-2) that requires the reduced form of the cofactor.
番茄红素的环化生成维生素 A 前体类胡萝卜素，如β-胡萝卜素，并为在光合作用中发挥独特作用的环状叶黄素的形成铺平了道路，同时也是植物和动物中调节分子的前体。番茄红素环化的生物化学一直是个谜，因为之前提出的酸碱催化与存在二核苷酸结合位点所预测的氧化还原催化的可能性相矛盾。我们表明，还原型 FAD 是必需的番茄红素环化酶（CrtY）辅因子。使用黄素类似物、质谱和突变分析，获得了证据，据此可以反驳依赖于隐式（净）电子转移的催化机制。还原型 FAD 的作用被认为在于通过电荷转移相互作用稳定携带部分正电荷或带正电荷的中间体，酸碱催化是其潜在的催化原理。因此，番茄红素环化酶属于新型非氧化还原黄素蛋白类，如异戊烯二磷酸：二甲基烯丙基二磷酸异构酶 2（IDI-2），它需要辅因子的还原形式。
