Ramos Daniel R, García M Victoria, Canle L Moisés, Santaballa J Arturo, Furtmüller Paul G, Obinger Christian
Chemical Reactivity and Photoreactivity Group, Department of Physical Chemistry and Chemical Engineering I, University of A Coruña, Alejandro de la Sota 1, E-15008 A Coruña, Spain.
J Inorg Biochem. 2008 May-Jun;102(5-6):1300-11. doi: 10.1016/j.jinorgbio.2008.01.003. Epub 2008 Jan 9.
Myeloperoxidase (MPO) is a dominating enzyme of circulating polymorphonuclear neutrophils that catalyzes the two-electron oxidation of chloride, thereby producing the strong halogenating agent hypochlorous acid (ClO(-)/HOCl). In absence of MPO the tripeptide Pro-Gly-Gly reacts with HOCl faster than the amino acid taurine (2-aminoethanesulfonic acid, Tau), while the MPO-mediated chlorination shows reverse order. A comparative study of the enzymatic oxidation of both substrates at pH 4.0-6.0, varying H2O2 concentration is presented. Initial and equilibrium rates studies have been carried on, reaction rates in the latter being slower due to the chemical equilibrium between MPO-I and MPO-II-HO2. A maximum of chlorination rate is observed for Pro-Gly-Gly and Tau when [H2O2] approximately 0.3-0.7 mM and pH approximately 4.5-5.0. Several mechanistic possibilities are considered, the proposed one implies that chlorination takes place via two pathways. One, for bulkier substrates, involves chlorination by free HOCl outside the heme cavity; ClO(-) is released from the active center, diffuses away the heme cavity, and undergoes protonation to HOCl. The other implies the existence of compound I-Cl(-) complex (MPO-I-Cl), capable of chlorinating smaller substrates in the heme pocket. Electronic structure calculations show the size of Pro-Gly-Gly comparable to the available gap in the substrate channel, this tripeptide being unable to reach the active site, and its chlorination is only possible by free HOCl outside the enzyme.
髓过氧化物酶（MPO）是循环多形核中性粒细胞中的主要酶，它催化氯离子的双电子氧化，从而产生强卤化剂次氯酸（ClO(-)/HOCl）。在缺乏MPO的情况下，三肽Pro-Gly-Gly与HOCl的反应速度比氨基酸牛磺酸（2-氨基乙烷磺酸，Tau）快，而MPO介导的氯化反应则呈现相反的顺序。本文介绍了在pH 4.0 - 6.0、不同H2O2浓度下对两种底物酶促氧化的比较研究。进行了初始速率和平衡速率研究，由于MPO-I和MPO-II-HO2之间的化学平衡，后者的反应速率较慢。当[H2O2]约为0.3 - 0.7 mM且pH约为4.5 - 5.0时，观察到Pro-Gly-Gly和Tau的氯化速率最高。考虑了几种可能的机制，提出的机制意味着氯化反应通过两条途径发生。一种途径是对于较大的底物，通过血红素腔外的游离HOCl进行氯化；ClO(-)从活性中心释放，扩散离开血红素腔，并质子化为HOCl。另一种途径意味着存在化合物I-Cl(-)复合物（MPO-I-Cl），它能够在血红素口袋中氯化较小的底物。电子结构计算表明，Pro-Gly-Gly的大小与底物通道中的可用间隙相当，这种三肽无法到达活性位点，其氯化只能通过酶外的游离HOCl进行。
