Uptake and Release of O_2 by Myohemerythrin. Evidence for Different Rate-Determining Steps and a Caveat

摘要

The biological transport of molecular oxygen is carried out by iron- and copper-containing proteins. Refined X-ray structures of members of the iron O_2 carriers, hemoglobins and hemerythrins, reveal no channels for exogenous ligands to directly access the iron sites from the solvent, demonstrating the physiological importance of protein fluctuations. The oxy adducts of these proteins are photosensitive, enabling the study of O_2 recombination with deoxy forms produced by laser flash photolysis. Results to date have been interpreted in terms of consecutive O_2 recombination equilibria for members of both protein families, indicating that O_2 passage through these protein matrices involves multiple barriers. In relating rate and equilibrium data at physiological temperatures for these proteins, it has been tacitly assumed by many workers that K_(eq) = k_(on)/k_(off). in this communication, we report results for Themiste zosteri-cola myohemerythrin (Mhr) and demonstrate that this assumption is not valid.