Globins act as a nitrite reductase (NiR), converting nitrite (NO2-) to nitric oxide (NO), and have recently become a system of important interest. NO plays many key roles in biological processes of most mammals, however, an imbalance of NO can lead to various heath problems. Although the mechanism of nitrite reduction in globins is unknown, it is believed to follow a mechanism similar to cd1 NiRs. In this work, we focus on myoglobin (Mb) as a model compound for globins. One facet that may control the NiR mechanism is the H-bonding second-sphere coordination between Mb and the heme. To study the effect of this network, heme b in wt Mb is replaced with a non-H-bonding heme, protoheme IX dimethyl ester (PPIX-DME). The electronic structures of the various forms of this reconstituted Mb sample (e.g. Fe(III), Fe(II), Fe(II)-NO, Fe(III)-NO, Fe(III)-NO2-, and Fe(II)-O2) were first investigated in order to analyze the contributions of one of these forms to the final product from the two-step NiR reaction. Slow kinetics reactions between PPIX-DME Mb and NO2- were carried out using UV-vis spectroscopy, where the loss of the Fe(II) species was monitored at 432 nm as a function of time. Preliminary results indicate a pseudo-first order decay, with a bimolecular rate constant only slightly different from the reported value of wt Mb (~5.0 M-1 s-1 at 25 °C). Current studies on another reconstituted Mb sample containing a non-H-bonding heme, octaethylporphyrin, are underway. Although neither porphyrin formed an H-bonding network with Mb, the unique presence of ethyl groups, as opposed to dimethyl ester at the 6,7-heme positions, present an interesting comparison. Implications of this change on their NiR activity will be presented.

Influence of the second-sphere coordination on the nitrite reductase activity of globins

249th National Meeting (2015)

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