Friedman Jonathan, Meharenna Yergalem T, Wilks Angela, Poulos Thomas L
Department of Molecular Biology and Biochemistry, and the Center for Chemical and Structural Biology, University of California, Irvine, California 92697, USA.
J Biol Chem. 2007 Jan 12;282(2):1066-71. doi: 10.1074/jbc.M609112200. Epub 2006 Nov 9.
Heme oxygenases have an increased binding affinity for O2 relative to CO. Such discrimination is critical to the function of HO enzymes because one of the main products of heme catabolism is CO. Kinetic studies of mammalian and bacterial HO proteins reveal a significant decrease in the dissociation rate of O2 relative to other heme proteins such as myoglobin. Here we report the kinetic rate constants for the binding of O2 and CO by the heme oxygenases from Neisseria meningitidis (nmHO) and Pseudomonas aeruginosa (paHO). A combination of stopped-flow kinetic and laser flash photolysis experiments reveal that nmHO and paHO both maintain a similar degree of ligand discrimination as mammalian HO-1 and the HO from Corynebacterium diphtheriae. However, in addition to the observed decrease in dissociation rate for O2 by both nmHO and paHO, kinetic analyses show an increase in dissociation rate for CO by these two enzymes. The crystal structures of nmHO and paHO both contain significant differences from the mammalian HO-1 and bacterial C. diphtheriae HO structures, which suggests a structural basis for ligand discrimination in nmHO and paHO.
相对于一氧化碳，血红素加氧酶对氧气具有更高的结合亲和力。这种区分对于血红素加氧酶的功能至关重要，因为血红素分解代谢的主要产物之一是一氧化碳。对哺乳动物和细菌血红素加氧酶蛋白的动力学研究表明，相对于其他血红素蛋白（如肌红蛋白），氧气的解离速率显著降低。在此，我们报告了脑膜炎奈瑟菌（nmHO）和铜绿假单胞菌（paHO）的血红素加氧酶与氧气和一氧化碳结合的动力学速率常数。停流动力学和激光闪光光解实验相结合表明，nmHO和paHO与哺乳动物HO-1以及白喉棒状杆菌的血红素加氧酶一样，都保持着相似程度的配体区分能力。然而，除了观察到nmHO和paHO对氧气的解离速率降低外，动力学分析还表明这两种酶对一氧化碳的解离速率增加。nmHO和paHO的晶体结构与哺乳动物HO-1和细菌白喉棒状杆菌血红素加氧酶的结构均存在显著差异，这表明nmHO和paHO中配体区分存在结构基础。
