The control of ascorbate biosynthesis: the role of light and GDP-L-galactose phosphorylase

抗坏血酸(维生素C)是植物生长所必需的，由植物叶片大量合成。它是一种重要的抗氧化剂(抗氧化剂是一种化合物，可以防止所有活着的生物体从氧气中产生的活性氧物种的破坏作用)，也是(动物和植物中)胶原蛋白和激素产生的酶辅助因子。了解控制叶片中维生素C积累量的因素很重要，原因有两个。首先，它在人类饮食中提供维生素C。其次，维生素C含量低的植物容易受到各种环境胁迫的伤害。研究发现，叶片中维生素C的含量与光照强度密切相关，这很可能是因为叶片中的维生素C是保护植物免受光合作用过程中不可避免地产生的具有潜在破坏性的活性氧的需要。该项目将确定控制叶片中维生素C浓度的生化机制。部分研究将集中于确定维生素C合成途径中的每一步对增加暴露在强光下的叶片中维生素C的贡献。另一部分将是对两个基因(VTC2和VTC5)的详细研究，这两个基因都编码与维生素C合成有关的酶(国内生产总值-L-半乳糖磷酸化酶)。虽然维生素C等产物的合成速度的控制通常在其合成途径的几个步骤之间共享，但有证据强烈表明VTC2和VTC5可能是该途径的主要控制点。在这个项目中，我们的目标是确定VTC2和VTC5的活性是如何与光和光合作用有关的控制的，以及这与该途径中的其他步骤如何有助于非常精确地控制叶片中的维生素C浓度。

Ascorbic acid (vitamin C) is essential for plant growth and is synthesised by plant leaves in large amounts. It is important as an antioxidant (antioxidants are compounds that protect against the damaging effects of reactive oxygen species that are produced from oxygen by all living organisms) and is also needed as an enzyme cofactor for production of collagen (in animals) and hormones (in animals and plants). Understanding the factors that control how much vitamin C accumulates in leaves is important for two reasons. Firstly, it provides vitamin C in the human diet. Secondly, plants with low vitamin C are susceptible to damage caused by various environmental stresses. It has been found that leaf vitamin C content is very closely related to light intensity, most probably because it is needed for protection against potentially damaging reactive oxygen species that are inevitably formed during photosynthesis. This project will determine the biochemical mechanisms that control the concentration of vitamin C in leaves. Part of the research will focus on identifying the contribution of each step in the pathway of vitamin C synthesis to increasing vitamin C in leaves exposed to high light. The other part will be a detailed investigation of two genes (VTC2 and VTC5) which both encode an enzyme involved in vitamin C synthesis (GDP-L-galactose phosphorylase). While the control of the rate of synthesis of products such as vitamin C is generally shared between a number of steps in its synthresis pathway, the evidence strongly implicates VTC2 and perhaps VTC5 as major control points in the pathway. In this project we aim to determine how the activity of VTC2 and VTC5 are controlled in relation to light and photosynthesis and how this contributes, along with other steps in the pathway, to the very precise control of vitamin C concentration in leaves.