Hohmann S, Meacock P A
Department of General and Marine Microbiology, Göteborg University, Medicinaregatan 9C, S-41390 Göteborg, Sweden.
Biochim Biophys Acta. 1998 Jun 29;1385(2):201-19. doi: 10.1016/s0167-4838(98)00069-7.
The yeast Saccharomyces cerevisiae utilises external thiamin for the production of thiamin diphosphate (ThDP) or can synthesise the cofactor itself. Prior to uptake into the cell thiamin phosphates are first hydrolysed and thiamin is taken up as free vitamin which is then pyrophosphorylated by a pyrophosphokinase. Synthesis of ThDP starts with the production of hydroxyethylthiazole and hydroxymethylpyrimidine. Those are linked to yield thiamin phosphate which is hydrolysed to thiamin and subsequently pyrophosphorylated. The THI genes encoding the enzymes of these final steps of ThDP production and of thiamin utilisation have been identified. Their expression is controlled by the level of thiamin and a number of regulatory proteins involved in regulated expression of the THI genes are known. However, the molecular details of the regulatory circuits need to be deciphered. Since the nucleotide sequence of the entire yeast genome is known we can predict the number of ThDP-dependent enzymes in S. cerevisiae. Eleven such proteins have been found: pyruvate decarboxylase (Pdc, three isoforms), acetolactate synthase, a putative alpha-ketoisocaproate decarboxylase with a regulatory role in ThDP synthesis and two proteins of unknown function form the group of Pdc related enzymes. In addition there are two isoforms for transketolase as well as the E1 subunits of pyruvate dehydrogenase and 2-oxoglutarate dehydrogenase. Expression of most of these genes is either induced or repressed by glucose. Surprisingly, it has been found recently that expression of one of the genes for Pdc is repressed by thiamin. In addition, the regulatory protein Pdc2p was shown to be required for high level expression of both the THI and the PDC genes. Apparently, the production of ThDP and of the enzymes using this cofactor is coordinately regulated. Future research will focus on the elucidation of the molecular mechanisms of this novel type of regulation.
酿酒酵母利用外源硫胺素生成硫胺素二磷酸（ThDP），或者自身合成这种辅因子。在硫胺素磷酸盐被细胞摄取之前，首先会被水解，硫胺素以游离维生素的形式被摄取，然后由焦磷酸激酶进行焦磷酸化。ThDP的合成始于羟乙基噻唑和羟甲基嘧啶的生成。它们连接形成硫胺素磷酸盐，后者被水解为硫胺素，随后进行焦磷酸化。编码ThDP生成和硫胺素利用这些最终步骤中酶的THI基因已被鉴定。它们的表达受硫胺素水平控制，并且已知一些参与THI基因调控表达的调节蛋白。然而，调控回路的分子细节仍需解读。由于整个酵母基因组的核苷酸序列已知，我们可以预测酿酒酵母中依赖ThDP的酶的数量。已发现11种此类蛋白：丙酮酸脱羧酶（Pdc，三种同工型）、乙酰乳酸合酶、一种在ThDP合成中具有调节作用的假定α-酮异己酸脱羧酶，以及两种功能未知的蛋白构成Pdc相关酶组。此外，转酮醇酶有两种同工型，以及丙酮酸脱氢酶和2-酮戊二酸脱氢酶的E1亚基。这些基因中的大多数表达受葡萄糖诱导或抑制。令人惊讶的是，最近发现Pdc的一个基因的表达受硫胺素抑制。此外，调节蛋白Pdc2p被证明是THI和PDC基因高水平表达所必需的。显然，ThDP的生成以及使用这种辅因子的酶的生成是协同调节的。未来的研究将集中于阐明这种新型调节的分子机制。
