Solution structure and dynamics of myoglobin and its mutants

• Main Author: Chiu, Mark Lawrence
• Other Authors: Sligar, Stephen G.
• Format: Text
• Language: English
• Published: 1992
• Subjects: Chemistry; Biochemistry; Sperm whale
• Online Access: http://hdl.handle.net/2142/20541

This thesis discusses the solution structure and function of sperm whale myoglobin. The solution structure of sperm whale myoglobin, with atomic resolution is obtained by using nuclear magnetic resonance spectroscopy (NMR). In solution, the protein samples a wider array of conformational substates. The carbon monoxide ligand has served as a model system for resolving the difference between equilibrium fluctuations and deterministic functionally important motions. By using C$\sp{17}$O, one can study the interactions of the heme pocket residues and the heme ligand. An explanation for some of myoglobin's conformational substates lies in the possibility that the heme bound ligand can be bent or oriented differently with respect to the heme plane. From Oxygen-17 NMR studies, another explanation for the source of myoglobin's conformational heterogeneity emerges. This thesis presents an argument for the ability of the histidine 64, which has van der Waals contact with the heme-bound carbon monoxide, to tautomerize and flip with respect to its beta and gamma carbon axis. The different states and orientations of this histidine 64 can explain the earlier discussions of conformational heterogeneity. Since the heme pocket of crystalline myoglobin appears to be solvent inaccessible to small ligands, some form of protein fluctuation must take place in order for ligands to enter and exit the heme pocket. By determining the hydrogen exchange rates of the backbone amide protons, a map of the flexibility of the protein is obtained. By correcting for local primary structure effects, the amide proton exchange rates also provide guides to the potential mechanism of ligand accessibility. Site-directed mutagenesis on sperm whale myoglobin has permitted the alteration of the heme iron coordination scheme. The His64Tyr Mb mutant has a six-coordinate heme iron with four heme pyrroles, proximal histidine, and distal tyrosine as ligands. Since the pK of the binding of the tyrosine to the heme iron is at 5.0, there is a possibility ...