The Influence of the Heme Sixth Ligand on the Backbone Dynamics of an Endogenously Hexacoordinate Hemoglobin

The hemoglobin of the cyanobacterium Synechococcus sp. PCC 7002 (GlbN) protects the cell from reactive oxygen/nitrogen species.1 GlbN coordinates the heme group with two histidines in the absence of an exogenous ligand and undergoes an unusual post-translational attachment of the heme group to the H helix. GlbN is well behaved by NMR spectroscopic standards, making it an excellent target for structural and dynamic studies aimed at characterizing the differential lability of the axial histidines, the perturbations caused by the post-translational modification, and the effect of exogenous ligand binding. The structure of GlbN is largely unperturbed by the covalent heme-protein cross-link,2 but binding of diatomic ligands such as CO (to ferrous GlbN) and CN- (to ferric GlbN) induces the formation of a distal hydrogen bond network and causes a shift of the B and E helices. 15N relaxation measurements indicate that the differences in ps-ns dynamics between the proximal and distal sides of the heme cofactor are minimal and also independent of heme covalent attachment. Upon CO or CN- binding the us-ms timescale motions are enhanced in the B and E helices, suggesting that the preferential displacement of the distal histidine is due to the stability of the final bound state rather than an intrinsic bond strength difference. The implications of increased dynamics on the distal side of the pocket after ligand binding will be discussed in terms of ligand migration inside GlbN and related globins.1) Scott et al., Biochemistry, 2010, 49: 7000-7011.2) Pond et al., Biomolecular NMR Assignments, 2009, 3: 211-214.Supported by NSF grant MCB 0843439.