Pines O, Even-Ram S, Elnathan N, Battat E, Aharonov O, Gibson D, Goldberg I
Department of Molecular Biology, Hebrew University-Hadassah Medical School, Jerusalem, Israel.
Appl Microbiol Biotechnol. 1996 Nov;46(4):393-9. doi: 10.1007/BF00166235.
Saccharomyces cerevisiae accumulates L-malic acid but not only minute amounts of fumaric acid. A 13C-nuclear magnetic resonance study following the label from glucose to L-malic acid indicates that the L-malic acid is synthesized from pyruvic acid via oxaloacetic acid. From this, and from previously published studies, we conclude that a cytosolic reductive pathway leading from pyruvic acid via oxaloacetic acid to L-malic acid is responsible for the L-malic acid production in yeast. The non-production of fumaric acid can be explained by the conclusion that, in the cell, cytosolic fumarase catalyzes the conversion of fumaric acid to L-malic but not the reverse. This conclusion is based on the following findings. (a) The cytosolic enzyme exhibits a 17-fold higher affinity towards fumaric acid than towards L-malic acid; the Km for L-malic acid is very high indicating that L-malic acid is not an in vivo substrate of the enzyme. (b) Overexpression of cytosolic fumarase does not cause accumulation of fumaric acid (but rather more L-malic acid). (c) According to 13C NMR studies there is no interconversion of cytosolic L-malic and fumaric acids.
酿酒酵母会积累L-苹果酸，但仅积累微量的富马酸。一项跟踪从葡萄糖到L-苹果酸标记的13C核磁共振研究表明，L-苹果酸是由丙酮酸经草酰乙酸合成的。据此以及先前发表的研究，我们得出结论，从丙酮酸经草酰乙酸到L-苹果酸的胞质还原途径是酵母中L-苹果酸产生的原因。不产生富马酸可以解释为，在细胞中，胞质富马酸酶催化富马酸转化为L-苹果酸，而不是相反的过程。这一结论基于以下发现。(a) 胞质酶对富马酸的亲和力比对L-苹果酸高17倍；L-苹果酸的Km值非常高，表明L-苹果酸不是该酶的体内底物。(b) 胞质富马酸酶的过表达不会导致富马酸积累（而是更多的L-苹果酸）。(c) 根据13C NMR研究，胞质L-苹果酸和富马酸之间没有相互转化。
