Chipman D, Barak Z, Schloss J V
Department of Life Sciences, Ben Gurion University of the Negev, Beer Sheva, Israel.
Biochim Biophys Acta. 1998 Jun 29;1385(2):401-19. doi: 10.1016/s0167-4838(98)00083-1.
Two groups of enzymes are classified as acetolactate synthase (EC 4. 1.3.18). This review deals chiefly with the FAD-dependent, biosynthetic enzymes which readily catalyze the formation of acetohydroxybutyrate from pyruvate and 2-oxobutyrate, as well as of acetolactate from two molecules of pyruvate (the ALS/AHAS group). These enzymes are generally susceptible to inhibition by one or more of the branched-chain amino acids which are ultimate products of the acetohydroxyacids, as well as by several classes of herbicides (sulfonylureas, imidazolinones and others). Some ALS/AHASs also catalyze the (non-physiological) oxidative decarboxylation of pyruvate, leading to peracetic acid; the possible relationship of this process to oxygen toxicity is considered. The bacterial ALS/AHAS which have been well characterized consist of catalytic subunits (around 60 kDa) and smaller regulatory subunits in an alpha2beta2 structure. In the case of Escherichia coli isozyme III, assembly and dissociation of the holoenzyme has been studied. The quaternary structure of the eukaryotic enzymes is less clear and in plants and yeast only catalytic polypeptides (homologous to those of bacteria) have been clearly identified. The presence of regulatory polypeptides in these organisms cannot be ruled out, however, and genes which encode putative ALS/AHAS regulatory subunits have been identified in some cases. A consensus sequence can be constructed from the 21 sequences which have been shown experimentally to represent ALS/AHAS catalytic polypeptides. Many other sequences fit this consensus, but some genes identified as putative 'acetolactate synthase genes' are almost certainly not ALS/AHAS. The solution of the crystal structures of several thiamin diphosphate (ThDP)-dependent enzymes which are homologous to ALS/AHAS, together with the availability of many amino acid sequences for the latter enzymes, has made it possible for two laboratories to propose similar, reasonable models for a dimer of catalytic subunits of an ALS/AHAS. A number of characteristics of these enzymes can now be better understood on the basis of such models: the nature of the herbicide binding site, the structural role of FAD and the binding of ThDP-Mg2+. The models are also guides for experimental testing of ideas concerning structure-function relationships in these enzymes, e.g. the nature of the substrate recognition site. Among the important remaining questions is how the enzyme suppresses alternative reactions of the intrinsically reactive hydroxyethylThDP enamine formed by the decarboxylation of the first substrate molecule and specifically promotes its condensation with 2-oxobutyrate or pyruvate.
两类酶被归类为乙酰乳酸合酶（EC 4.1.3.18）。本综述主要涉及依赖黄素腺嘌呤二核苷酸（FAD）的生物合成酶，这类酶能够轻易催化丙酮酸和2-氧代丁酸生成乙酰羟丁酸，以及两分子丙酮酸生成乙酰乳酸（ALS/AHAS组）。这些酶通常易受一种或多种支链氨基酸（乙酰羟酸的最终产物）以及几类除草剂（磺酰脲类、咪唑啉酮类等）的抑制。一些ALS/AHAS还催化丙酮酸的（非生理性）氧化脱羧反应，生成过氧乙酸；本文探讨了该过程与氧毒性之间可能存在的关系。已得到充分表征的细菌ALS/AHAS由催化亚基（约60 kDa）和较小的调节亚基组成，呈α2β2结构。以大肠杆菌同工酶III为例，对全酶的组装和解离进行了研究。真核生物酶的四级结构尚不清楚，在植物和酵母中，仅明确鉴定出了与细菌催化多肽同源的催化多肽。然而，不能排除这些生物体中存在调节多肽的可能性，并且在某些情况下已鉴定出编码假定的ALS/AHAS调节亚基的基因。从21个经实验证明代表ALS/AHAS催化多肽的序列中可以构建一个共有序列。许多其他序列符合这个共有序列，但一些被鉴定为假定“乙酰乳酸合酶基因”的基因几乎肯定不是ALS/AHAS。几种与ALS/AHAS同源的依赖硫胺素二磷酸（ThDP）的酶的晶体结构的解析，以及后者许多氨基酸序列的可得性，使得两个实验室能够为ALS/AHAS催化亚基的二聚体提出相似且合理的模型。基于这些模型，现在可以更好地理解这些酶的一些特性：除草剂结合位点的性质、FAD的结构作用以及ThDP-Mg2+的结合。这些模型也是对这些酶结构-功能关系相关观点进行实验验证的指导，例如底物识别位点的性质。尚待解决的重要问题之一是，该酶如何抑制由第一个底物分子脱羧形成的具有内在反应活性的羟乙基ThDP烯胺的替代反应，并特异性促进其与2-氧代丁酸或丙酮酸的缩合反应。
