Department of Vegetable Crops, University of California, Davis, California, 95616.
Arch Biochem Biophys. 2022 Sep 15;726:109238. doi: 10.1016/j.abb.2022.109238. Epub 2022 Jun 6.
1-Aminocyclopropanecarboxylate (ACC) synthase, which catalyzes the conversion of S-adenosylmethionine (SAM) to ACC and methylthioadenosine, was demonstrated in tomato extract. Methylthioadenosine was then rapidly hydrolyzed to methylthioribose by a nucleosidase present in the extract. ACC synthase had an optimum pH of 8.5, and a K of 20 μM with respect to SAM. S-Adenosylethionine also served as a substrate for ACC synthase, but at a lower efficiency than that of SAM. Since S-adenosylethionine had a higher affinity for the enzyme than SAM, it inhibited the reaction of SAM when both were present. S-Adenosylhomocysteine was, however, an inactive substrate. The enzyme was activated by pyridoxal phosphate at a concentration of 0.1 μM or higher, and competitively inhibited by aminoethoxyvinylglycine and aminooxyacetic acid, which are known to inhibit pyridoxal phosphate-mediated enzymic reactions. These results support the view that ACC synthase is a pyridoxal enzyme. The biochemical role of pyridoxal phosphate is catalyzing the formation of ACC by α,γ-elimination of SAM is discussed.
1-氨基环丙烷羧酸（ACC）合酶，它催化 S-腺苷甲硫氨酸（SAM）转化为 ACC 和甲硫腺苷，在番茄提取物中得到证实。然后，提取物中存在的核酶将甲硫腺苷迅速水解为甲硫核糖。ACC 合酶的最适 pH 值为 8.5，对 SAM 的 K 值为 20 μM。S-腺苷乙硫氨酸也可以作为 ACC 合酶的底物，但效率低于 SAM。由于 S-腺苷乙硫氨酸对酶的亲和力高于 SAM，因此当两者都存在时，它会抑制 SAM 的反应。然而，S-腺苷同型半胱氨酸是一种无活性的底物。该酶在 0.1 μM 或更高浓度的吡哆醛磷酸盐的作用下被激活，并被氨基乙氧基乙烯基甘氨酸和氨基氧乙酸竞争性抑制，这两种物质已知可抑制吡哆醛磷酸盐介导的酶反应。这些结果支持 ACC 合酶是一种吡哆醛酶的观点。讨论了吡哆醛磷酸盐通过 SAM 的α，γ-消除来催化 ACC 形成的生化作用。
