Barrel Rigidity, Heme Distortion, Ligand Protection and Nitric Oxide Interactions with Heme in Nitrophorin 4

Abstract

Nitrophorins are nitric oxide transport proteins that aid in an insect obtaining a blood meal. The nitrophorins from the saliva of Rhodnius prolixus, a blood-sucking insect, consist of seven closely related proteins (rNP1-7). These proteins have the classic hallmarks that define the lipocalin protein family: an eight-stranded beta-barrel, binds a hydrophobic molecule, contains a flexible omega loop that lines the entrance to the binding cavity, contains a N-terminal helix and a conserved Trp-Arg/Lys interaction, and a disulfide bridge pattern. This dissertation explores the functional implications of the rNP4 structure. An apo structure of rNP4 to 1.11 angstrom resolution revealed the beta-barrel of rNP4 remains a rigid structure in the absence of its heme cofactor and is similar to that of holo rNP4. An X-ray crystallographic study of alterations to protein and heme cofactor was used to explore how heme distortion arises. In all altered proteins, local changes were seen at the point of alteration. The protein has evolved so as to have all surrounding contacts, as well as electronic interactions between ligand and heme, contribute to distortion. An omega loop triple alanine mutant of rNP4 that was unable to collapse over the binding cavity was used to investigate ligand (nitric oxide) protection crystallographically and kinetically. A 1.00 angstrom resolution structure confirmed the mutant protein was unable to collapse. The loop mutant, as well as wild-type protein, display NO binding to heme that was unaffected by oxygen, indicating that NO binds to heme before reacting with surrounding oxygen. NO-protein complex decay was very slow (hours) under conditions where NO was allowed to rebind to heme, highlighting the protection properties of rNP4. In the presence of oxygen and/or glutathione, which can react with NO, decay was accelerated but still slow (minutes to hours). Taken together, these data suggest that NO binds to the heme iron and then dissociates and associates hundreds of times before reacting with any surrounding compound.