Department of Chemistry and Biochemistry, UCLA, Los Angeles, California 90095, USA.
J Biol Chem. 2012 Apr 20;287(17):14234-45. doi: 10.1074/jbc.M112.341982. Epub 2012 Mar 5.
The L-galactose (Smirnoff-Wheeler) pathway represents the major route to L-ascorbic acid (vitamin C) biosynthesis in higher plants. Arabidopsis thaliana VTC2 and its paralogue VTC5 function as GDP-L-galactose phosphorylases converting GDP-L-galactose to L-galactose-1-P, thus catalyzing the first committed step in the biosynthesis of L-ascorbate. Here we report that the L-galactose pathway of ascorbate biosynthesis described in higher plants is conserved in green algae. The Chlamydomonas reinhardtii genome encodes all the enzymes required for vitamin C biosynthesis via the L-galactose pathway. We have characterized recombinant C. reinhardtii VTC2 as an active GDP-L-galactose phosphorylase. C. reinhardtii cells exposed to oxidative stress show increased VTC2 mRNA and L-ascorbate levels. Genes encoding enzymatic components of the ascorbate-glutathione system (e.g. ascorbate peroxidase, manganese superoxide dismutase, and dehydroascorbate reductase) are also up-regulated in response to increased oxidative stress. These results indicate that C. reinhardtii VTC2, like its plant homologs, is a highly regulated enzyme in ascorbate biosynthesis in green algae and that, together with the ascorbate recycling system, the L-galactose pathway represents the major route for providing protective levels of ascorbate in oxidatively stressed algal cells.
L-半乳糖（斯米尔诺夫-惠勒）途径是高等植物 L-抗坏血酸（维生素 C）生物合成的主要途径。拟南芥 VTC2 及其同源物 VTC5 作为 GDP-L-半乳糖磷酸化酶发挥作用，将 GDP-L-半乳糖转化为 L-半乳糖-1-P，从而催化 L-抗坏血酸生物合成的第一步。在这里，我们报告在高等植物中描述的抗坏血酸生物合成的 L-半乳糖途径在绿藻中是保守的。莱茵衣藻的基因组编码通过 L-半乳糖途径合成维生素 C 所需的所有酶。我们已经鉴定了重组莱茵衣藻 VTC2 作为一种活性 GDP-L-半乳糖磷酸化酶。暴露于氧化应激下的莱茵衣藻细胞显示出增加的 VTC2 mRNA 和 L-抗坏血酸水平。编码抗坏血酸-谷胱甘肽系统（例如抗坏血酸过氧化物酶、锰超氧化物歧化酶和脱氢抗坏血酸还原酶）酶成分的基因也被上调以响应增加的氧化应激。这些结果表明，与植物同源物一样，莱茵衣藻 VTC2 是绿藻中抗坏血酸生物合成中高度调控的酶，并且与抗坏血酸回收系统一起，L-半乳糖途径是为氧化应激的藻类细胞提供保护水平抗坏血酸的主要途径。
