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首页> 外文期刊>The biochemical journal >A competitive labelling method for determining the ionization constants and reactivity of individual histidine residues in proteins. The histidines of α-chymotrypsin
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A competitive labelling method for determining the ionization constants and reactivity of individual histidine residues in proteins. The histidines of α-chymotrypsin

机译：一种竞争标记方法，用于测定蛋白质中单个组氨酸残基的电离常数和反应性。 α-chymotrypsin的组氨酸

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pA competitive labelling method (Kaplan iet al./i, 1971), using tritiated 1-fluoro-2,4-dinitrobenzene as the labelling reagent, is described for determining the ionization constants and reactivities of individual histidine residues in proteins. When this method was applied to the two histidines of α-chymotrypsin, histidine-57 was found to have piKsuba/sub/i 6.8 and a reactivity ten times that of α-iN/i-acetyl-l-histidine. Histidine-40 had piKsuba/sub/i 6.7 and a reactivity approximately six times that of α-iN/i-acetyl-l-histidine. Between pH7.5 and 8 the reactivities of both histidines decrease simultaneously to approximately that of α-iN/i-acetyl-l-histidine. The high reactivities of the histidines are attributed to hydrogen bonding, which increases the nucleophilicity of the imidazole ring. The sharp decrease in reactivity between pH7.5 and 8 is attributed to a conformational change that disrupts the hydrogen bonding by these residues. The reactivity data support the proposal of a charge-relay mechanism involving histidine-57 (Blow iet al./i, 1969), which makes serine-195 more nucleophilic but indicates that this system is fully operative only in the enzyme–substate complex./p
机译：竞争标记方法（Kaplan 等人。使用氚化的1-氟-2,4-二硝基苯作为标记试剂，用于确定个体的电离常数和反应性蛋白质中的组氨酸残基。当将该方法施加到α-chymotrypsin的两个组氨酸时，发现组氨酸-57具有P k A 6.8和反应性十倍的α- N - 乙酰-1-组氨酸。组氨酸-40具有P K A 6.7，反应性约为α- n-乙酰-1-组氨酸的六倍。 PH7.5和8之间的反应性两种组氨酸的反应同时降低至-Cetyl-L-组氨酸的大约。组氨酸的高反应性归因于氢键，其增加了咪唑环的亲核性。 PH7.5和8之间的反应性的急剧降低归因于通过这些残基破坏氢键的构象变化。反应性数据支持涉及组氨酸-57的电荷继电器机制的提议（吹嘘等，1969），这使得丝氨酸-195更亲核，但表明该系统仅在于酶 - 代位复合物。

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