Ketopantoate hydroxymethyltransferase。二世。物理、催化和管理属性。
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斯奈尔权力SG, EE
Ketopantoate hydroxymethyltransferase。二世。物理、催化和管理属性。
J生物化学杂志。1976年6月25日,251 (12):3786 - 93。
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6463年(在PubMed]
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一些物理、催化和监管的性质ketopantoate hydroxymethyltransferase (5, 10-methylenetetrahydrofolate: alpha-ketoisovalerate hydroxymethyltranferase)从大肠杆菌。这种酶催化合成可逆的ketopantoate(反应1),泛酸的重要前体。(1)HC (CH3) 2 cocoo——+ 5, 10-methylene tetrahydrofolate f在平衡r HOCH2C (CH3) 2 cocoo - + tetrahydrofolate有分子量到255000年沉降平衡,沉降系数(S20 w) 11年代,部分具体体积0.74 ml / g,等电点为4.4,和吸光度(见文章),0.85。在十二烷基硫酸钠聚丙烯酰胺凝胶电泳和氨基酸分析的亚基分子量27000年和25700年分别;两个程序都表明10相同的子单元的存在。Met-Tyr——NH2-terminal序列。酶稳定和活跃的pH值范围内,有一个最佳的从7.0到7.6。它需要Mg2 +活动;Mn2 + Co2 + Zn2 +少逐渐活跃。硼氢化酶不灭活减少多余的基质的存在,即它是一个类二醛缩酶。 Reaction 1f is partially inhibited by concentrations of formaldehyde (0.8 mM) and tetrahydrofolate (0.38 mM) below or near the Km values, apparent Km values are 0.18, 1.1 and 5.9 mM for tetrahydrofolate, alpha-ketoisovalerate, and formaldehyde, respectively. For Reaction 1r, apparent Km values are 0.16 and 0.18 mM, respectively, for ketopantoate and tetrahydrofolate, and the saturation curves for both substrates show positive cooperativity. Forward and reverse reactions occur at similar maximum velocities (Vmax approximately equal to 8 mumol of ketopantoate formed or decomposed per min per mg of enzyme at 37 degrees). Only 1-tetrahydrofolate is active in Reaction 1; d-tetrahydrofolate, folate, and methotrexate were neither active nor inhibitory. However, 1-tetrahydrofolate was effectively replaced with conjugates containing 1 to 6 additional glutamate residues; of these, tetrahydropterolpenta-, tetra-, and triglutamate were effective at lower concentrations than tetrahydrofolate itself; they were also the predominant conjugates of tetrahydrofolate present in E. coli. Alpha-Ketobutyrate, alpha-ketovalerate, and alpha-keto-beta-methylvalerate replaced alpha-ketoisovalerate as substrates; pyruvate was inactive as a substrate, but like isovalerate, 3-methyl-2-butanone and D- or L-valine, inhibited Reaction 1. the transferase has regulatory properties expected of an enzyme catalyzing the first committed step in a biosynthetic pathway. Pantoate (greater than or equal to 500 muM) and coenzyme A (above 1 mM) all inhibit; the Vmax is decreased, Km is increased, and the cooperativity for substrate (ketopantoate) is enhanced. Catalytic activity of the transferase is thus regulated by the products of the reaction path of which it is one component; transferase synthesis is not repressed by growth in the presence of pantothenate.