aldose-ketose互换机制由D-xylose异构酶,包括开环,后跟一个1,2-hydride转变。
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aldose-ketose互换机制由D-xylose异构酶,包括开环,后跟一个1,2-hydride转变。
J杂志。1990年3月5日,212 (1):211 - 35。
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2319597 (在PubMed]
- 文摘
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D-xylose异构酶的活性部位和机理探讨了测定酶的晶体结构绑定到各种基质、抑制剂和阳离子。开环是一个必须的第一步反应,被认为是醛糖酮糖转换速率决定步骤。复杂的结构和循环thio-glucose已经决定,认为这是一个模拟的米歇利斯复杂。在-10摄氏度基质晶体中观察到的伸展链形式。缺乏适当的位于基地循环或伸展链形式从底物结合位点表明异构化没有发生由enediol或enediolate机制。绑定三价阳离子的地方额外收费的活性部位,产生基质复杂,类似于一个可能的过渡态。两个结合位点的二价阳离子,[1]是永久占领在催化条件下,协调四羧酸盐组。在缺乏底物暴露于溶剂,和米歇利斯复杂的模拟,网站[1]是八面体的协调,与配体O-3和O-4 thiopyranose。在复杂的碳链底物仍是八面体的协调,与配体-和O-4。绑定在一个阳离子催化网站[2]也是必要和这个网站被认为结合Co2 +比网站[1]更强烈。 This site is octahedrally co-ordinated to three carboxylate groups (bidentate co-ordination to one of them), an imidazole and a solvent molecule. It is proposed that during the hydride shift the C-O-1 and C-O-2 bonds of the substrate are polarized by the close approach of the site [2] cation. In the transition-state analogue this cation is observed at a site [2'], 1.0 A from site [2] and about 2.7 A from O-1 and O-2 of the substrate. It is likely that co-ordination of the cation to O-1 and O-2 would be concomitant with ionisation of the sugar hydroxyl group. The polarisation of C-O-1 and C-O-2 is assisted by the co-ordination of O-2 to cation [1] and O-1 to a lysine side-chain.(ABSTRACT TRUNCATED AT 400 WORDS)