Ligand binding to heme proteins .6. Interconversion of taxonomic substates in carbonmonoxymyoglobin

The kinetic properties of the three taxonomic A substates of sperm whale carbonmonoxy myoglobin in 75% glycerol/buffer are studied by flash photolysis with monitoring in the infrared stretch bands of bound CO at nu(A(0)) approximate to 1967 cm(-1), nu(A(1)) approximate to 1947 cm(-1), and nu(A(3)) approximate to 1929 cm(-1) between 60 and 300 K. Below 160 K the photodissociated CO rebinds from the heme pocket, no interconversion among the A substates is observed, and rebinding in each A substate is nonexponential in time and described by a different temperature-independent distribution of enthalpy barriers with a different preexponential. Measurements in the electronic bands, e.g., the Soret, contain contributions of all three A substates and can, therefore, be only approximately modeled with a single enthalpy distribution and a single preexponential. The bond formation step at the heme is fastest for the A(0) substate, intermediate for the A(1) substate, and slowest for A(3). Rebinding between 200 and 300 K displays several processes, including geminate rebinding, rebinding after ligand escape to the solvent, and interconversion among the A substates. Different kinetics are measured in each of the A bands for times shorter than the characteristic time of fluctuations among the A substates. At longer times, fluctuational averaging yields the same kinetics in all three A substates. The interconversion rates between A(1) and A(3) are determined from the time when the scaled kinetic traces of the two substates merge. Fluctuations between A(1) and A(3) are much faster than those between A(0) and either A(1) or A(3), so A(1) and A(3) appear as one kinetic species in the exchange with A(0). The maximum-entropy method is used to extract the distribution of rate coefficients for the interconversion process A(0) <----> A(1) + A(3) from the flash photolysis data. The temperature dependencies of the A substate interconversion processes are fitted with a non-Arrhenius expression similar to that used to describe relaxation processes in glasses. At 300 K the interconversion time for A(0) <----> A(1) + A(3) is 10 mu s, and extrapolation yields similar to 1 ns for A(1) <----> A(3). The pronounced kinetic differences imply different structural rearrangements. Crystallographic data support this conclusion: They show that formation of the A(0) substate involves a major change of the protein structure; the distal histidine rotates about the C-alpha-C-beta bond, and its imidazole sidechain swings out of the heme pocket into the solvent, whereas it remains in the heme pocket in the A(1) <----> A(3) interconversion. The fast A(1) <----> A(3) exchange is inconsistent with structural models that involve differences in the protonation between A(1) and A(3).