1－胺基環丙烷－1－羧酸合成酶之生化特性及其基因結構

Biochemical Properties and Gene Structure of the 1-Aminocyclopropane-1-carboxylate Synthase

乙烯是主要植物荷爾蒙之一，它對植物的生長與發育具有相當廣泛的 生理效應。目前已知其生合成途徑為：甲硫胺酸（methionine）→S-腺苷甲硫胺酸（S-adenosylmethionine, SAM）→1－胺基環丙院－1－羧酸（1-aminocyclopro-pane-l-carboxylic acid, ACC）→乙烯（ethylene）。其中SAM轉變成ACC為速率限制步驟；而ACC合成酶正是擔任催化此步驟之酵素；故ACC合成酶在乙烯合成途徑中扮演著非常重要的角色；是為一速率限制酵素。本文針對此酵素介紹其性質、構造及其調節的機制；總結如下：ACC合成酶是以多種形式存在的；但其活性部位之胺基酸序列具高度保守性；其分子量大約55KDa，且以雙元体的方式存在，但在純化的過程中會降解成45KDa左右；在進行催化時，以維生素六輔酶（pyridoxal phosphate, PLP）為輔助因子，且具有被其受質SAM失活化之特性；此外ACC合成酶基因是由多重基因所組成，並且解碼（encode）許多異型態之ACC合成酶；其酵素活性可被不同的因子誘發；包括生長素、逆境、癒傷及各種金屬物質；此誘發作用被認為是由於轉錄作用加強而使蛋白質重新合成。

Ethylene is a plant hormone that has a wide impact on plant growth and development. Methionine is the biological precursor of ethylene in higher plants, and it is converted to ethylene via the following route: methionine→ S-adenosylmethionine (SAM) → 1-aminocyclopropane- 1-carboxylic acid (ACC) → ethylene. The conversion of SAM to ACC is catalyzed by the key regulatory enzyme, ACC synthase (E. C4. 1. 1. 14), in the biosynthesis of ethylene. This review presents an overview of characteristics, structure and regulation of this enzyme. It shows that there are multiple forms of ACC synthase, but the active-site peptide reveals highly conserved dodecapeptides; the molecular size of ACC synthase, a dimer, is 55 KDa, and the enzyme is partially degraded to a 45 KDa peptide during purification procedures; it requires pyridoxal phosphate as cofactor, and can be inactivated by its substrate SAM; and ACC synthase gene is a gene family, encoding multiple forms of ACC synthase, which can be induced by a diverse group of inducers, including auxin, stress, wounding, and various metals, and the induction was suggested to be due to enhanced transcription.