Moe O A, Butler L G
J Biol Chem. 1983 Jun 10;258(11):6941-6.
Extensive kinetic studies of bovine intestinal 5'-nucleotide phosphodiesterase as a function of pH have confirmed and amplified the catalytic mechanism previously proposed on the basis of isolation of a covalent phosphorylated intermediate (Landt, M., and Butler, L.G. (1978) Biochemistry 17, 4130-4135). An enzyme-ionizing group with apparent pKa = 6.85 controls the rate-determining step. Electrostatic interactions between anionic substrate and two or more ionic groups on the enzyme have a major role in substrate binding. Binding of strongly inhibitory 5'-AMP is controlled by an ionizing group, probably on the enzyme, with pKa less than or equal to 5.9. At pH 6.0, imidazole is a classic uncompetitive inhibitor, in agreement with independent evidence that it stabilizes the covalent intermediate form of the enzyme. KI values for phosphonate analogs, which are competitive inhibitors, indicate that phosphodiesterase binds its products and product analogs more strongly than it binds substrate analogs. Some of the results presented here can be interpreted as indicating that 5'-nucleotide phosphodiesterase is the evolutionary precursor of alkaline phosphatase, with which it has many structural and catalytic properties in common, and which is found in relatively large amounts in the same tissue.
对牛肠5'-核苷酸磷酸二酯酶随pH值变化的广泛动力学研究，证实并扩展了先前基于共价磷酸化中间体的分离而提出的催化机制（兰特，M.，和巴特勒，L.G.（1978年）《生物化学》17，4130 - 4135）。一个表观pKa = 6.85的酶电离基团控制着速率决定步骤。阴离子底物与酶上两个或更多离子基团之间的静电相互作用在底物结合中起主要作用。强抑制性5'-AMP的结合受一个电离基团控制，该基团可能在酶上，pKa小于或等于5.9。在pH 6.0时，咪唑是一种典型的非竞争性抑制剂，这与它能稳定酶的共价中间形式的独立证据一致。作为竞争性抑制剂的膦酸酯类似物的KI值表明，磷酸二酯酶与其产物和产物类似物的结合比与底物类似物的结合更强。此处呈现的一些结果可以解释为表明5'-核苷酸磷酸二酯酶是碱性磷酸酶的进化前体，它与碱性磷酸酶有许多共同的结构和催化特性，并且在同一组织中含量相对较高。
