DYNAMICS OF PROTEIN RELAXATION IN SITE-SPECIFIC MUTANTS OF HUMAN MYOGLOBIN

We have recently reported spectroscopic evidence for structural relaxa tion of myoglobin (Mb) following photodissociation of MbCO [Lambright, D. G., Balasubramanian, S., & Boxer, S. G. (1991) Chem. Phys. 158, 24 9-260]. In this paper we report measurements for a series of single am ino acid mutants of human myoglobin on the distal side of the heme poc ket (positions 45, 64, and 68) in order to examine specific structural determinants involved in this conformational relaxation and to determ ine the nature of the coupling between relaxation and the functional p rocess of ligand binding. The kinetics of ligand binding and conformat ional relaxation were monitored by transient absorption spectroscopy i n the Soret spectral region, and the results are analyzed using a four -state ligand binding model. Two principal results emerge: (1) amino a cid substitutions in the distal heme pocket affect the kinetics of the nonequilibrium conformational relaxation and (2) the rate of ligand e scape from the protein matrix is not significantly perturbed by the di stal heme pocket mutations.