Smirnoff N, Wheeler G L
School of Biological Sciences, University of Exeter, Hatherly Laboratories, UK.
Crit Rev Biochem Mol Biol. 2000;35(4):291-314. doi: 10.1080/10409230008984166.
Ascorbic acid (vitamin C) is an abundant component of plants. It reaches a concentration of over 20 mM in chloroplasts and occurs in all cell compartments, including the cell wall. It has proposed functions in photosynthesis as an enzyme cofactor (including synthesis of ethylene, gibberellins and anthocyanins) and in control of cell growth. A biosynthetic pathway via GDP-mannose, GDP-L-galactose, L-galactose, and L-galactono-1,4-lactone has been proposed only recently and is supported by molecular genetic evidence from the ascorbate-deficient vtc 1 mutant of Arabidopsis thaliana. Other pathways via uronic acids could provide minor sources of ascorbate. Ascorbate, at least in some species, is a precursor of tartrate and oxalate. It has a major role in photosynthesis, acting in the Mehler peroxidase reaction with ascorbate peroxidase to regulate the redox state of photosynthetic electron carriers and as a cofactor for violaxanthin de-epoxidase, an enzyme involved in xanthophyll cycle-mediated photoprotection. The hypersensitivity of some of the vtc mutants to ozone and UV-B radiation, the rapid response of ascorbate peroxidase expression to (photo)-oxidative stress, and the properties of transgenic plants with altered ascorbate peroxidase activity all support an important antioxidative role for ascorbate. In relation to cell growth, ascorbate is a cofactor for prolyl hydroxylase that posttranslationally hydroxylates proline residues in cell wall hydroxyproline-rich glycoproteins required for cell division and expansion. Additionally, high ascorbate oxidase activity in the cell wall is correlated with areas of rapid cell expansion. It remains to be determined if this is a causal relationship and, if so, what is the mechanism. Identification of the biosynthetic pathway now opens the way to manipulating ascorbate biosynthesis in plants, and, along with the vtc mutants, this should contribute to a deeper understanding of the proposed functions of this multifaceted molecule.
抗坏血酸（维生素C）是植物中的一种丰富成分。它在叶绿体中的浓度超过20 mM，并存在于包括细胞壁在内的所有细胞区室中。它在光合作用中作为酶辅因子（包括乙烯、赤霉素和花青素的合成）以及在细胞生长控制方面具有推测的功能。仅在最近才提出了一条经由GDP - 甘露糖、GDP - L - 半乳糖、L - 半乳糖和L - 半乳糖醛酸 - 1,4 - 内酯的生物合成途径，并且得到了拟南芥抗坏血酸缺陷型vtc 1突变体的分子遗传学证据的支持。经由糖醛酸的其他途径可能提供少量的抗坏血酸来源。抗坏血酸，至少在某些物种中，是酒石酸和草酸的前体。它在光合作用中起主要作用，在与抗坏血酸过氧化物酶的梅勒过氧化物酶反应中调节光合电子载体的氧化还原状态，并作为紫黄质脱环氧化酶的辅因子，该酶参与叶黄素循环介导的光保护作用。一些vtc突变体对臭氧和UV - B辐射的超敏反应、抗坏血酸过氧化物酶表达对（光）氧化应激的快速响应以及具有改变的抗坏血酸过氧化物酶活性的转基因植物的特性均支持抗坏血酸具有重要的抗氧化作用。关于细胞生长，抗坏血酸是脯氨酰羟化酶的辅因子，该酶在翻译后羟基化细胞壁富含羟脯氨酸的糖蛋白中的脯氨酸残基，这些糖蛋白是细胞分裂和扩展所必需的。此外，细胞壁中高抗坏血酸氧化酶活性与细胞快速扩展的区域相关。这是否是一种因果关系以及如果是，其机制是什么仍有待确定。生物合成途径的鉴定现在为操纵植物中抗坏血酸的生物合成开辟了道路，并且与vtc突变体一起，这应该有助于更深入地理解这个多面分子的推测功能。
