Inclusion of an Outer Sphere Effect in Solvent Proton Relaxation Enhancement Studies of the Enzyme Myeloperoxidase

Nova Southeastern University, Davie, Florida, USA

Objective: This study was conducted in order to estimate the contribution of outer sphere effects on the solvent proton paramagnetic relaxation enhancement rates in solutions of the enzyme myeloperoxidase. The purpose of this is to ultimately better understand the dynamics of small molecules that participate in reactions of myeloperoxidase. Background: Myeloperoxidase normally found in neutrophils, is well known for its protective antibacterial and cytotoxic effects. However, it is now known that this enzyme also plays important roles in more than 20 different diseases. In previous studies it was assumed that the paramagnetic relaxation enhancement effects on the solvent protons in solutions of myeloperoxidase were caused only by exchange of solvent protons with binding sites near the paramagnetic heme iron. In the cases of some other hemeproteins arguments have been presented in the literature that an outer sphere effect caused by solvent protons just passing near, but outside the heme site might be significant. Making a theoretical estimate of this effect can be very problematic. For that reason, an empirical approach has been used here to estimate this effect. Methods: In order to estimate this effect, a complex was made with the enzyme that is known to block the access channel to the heme iron without changing the iron spin state. Proton relaxation enhancement was measured. Conclusion: The outer sphere effect is a major contributor to the paramagnetic relaxation enhancement. Grants: The author acknowledges previous HPD Grant support for the study.