Chlamydomonas chloroplast ferrous hemoglobin - Heme pocket structure and reactions with ligands

We report the optical and resonance Raman spectral characterization of ferr ous recombinant Chlamydomonas LI637 hemoglobin. We show that it is present in three pH dependent equilibrium forms including a 4-coordinate species at acid pH, a 6-coordinate high spin species at neutral pH, and a 6-coordinat e low spin species at alkaline pH, The proximal ligand to the heme is the i midazole group of a histidine, Kinetics of the reactions with ligands were determined by stopped-flow spectroscopy. At alkaline pH, combination with o xygen, nitric oxide, and carbon monoxide displays a kinetic behavior that i s interpreted as being rate-limited by conversion of the 6-coordinate form to a reactive 5-coordinate form. At neutral pH, combination rates of the 5- coordinate form with oxygen and carbon monoxide were much faster (>10(7) mu M-1 s(-1)). The dissociation rate constant measured for oxygen is among th e slowest known, 0.014 s(-1), and is independent of pH, Replacement of the tyrosine 63 (B10) by leucine or of the putative distal glutamine by glycine increases the dissociation rate constant 70- and 30-fold and increases the rate of autoxidation 20- and 90-fold, respectively. These results are cons istent with at least two hydrogen bonds stabilizing the bound oxygen molecu le, one from tyrosine B10 and the other from the distal glutamine, In addit ion, the high frequency (232 cm(-1)) of the iron-histidine bond suggests a structure that lacks any proximal strain thus contributing to high ligand a ffinity.