Resonance Raman evidence that distal histidine protonation removes the steric hindrance to upright binding of carbon monoxide by myoglobin.
The resonance Raman band assigned to Fe--CO stretching in the sperm whale myoglobin CO adduct shifts from 507 cm-1 at neutral pH to 488 cm-1 at low pH, in concert with a shift of the C-O stretching infrared band from 1947 to 1967 cm-1 (Fuchsman & Appleby, 1979), while the 575-cm-1 Fe-C-O bending RR band loses intensity. The pKa that characterizes these changes is approximately 4.4. The vibrational frequencies at low pH are well modeled by the protein-free CO, imidazole adduct of protoheme in a nonpolar solvent while those at high pH are modeled by the adduct of a heme with a covalent strap (Yu et al., 1983) which inhibits upright CO binding. It is inferred that the Fe-C-O unit changes from a tilted to an upright geometry when the distal histidine is protonated, because its side chain swings out of the heme pocket due to electrostatic repulsion with a nearby arginine residue. A different protonation step (pKa = 5.7), which has been shown to modulate the CO rebinding kinetics (Doster et al., 1982) as well as the optical spectrum (Fuchsman & Appleby, 1979), is suggested to involve a global structure change associated with protonation of histidine residues distant from the heme.
Ramsden
J
J
Department of Chemistry, Princeton University, New Jersey 08544-1009.
Spiro
T G
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D000494:Allosteric Regulation; D000818:Animals; D001665:Binding Sites; D002248:Carbon Monoxide; D006639:Histidine; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D009211:Myoglobin; D011522:Protons; D013059:Spectrum Analysis, Raman; D014907:Whales
D009211:Myoglobin; D011522:Protons; D006639:Histidine; D002248:Carbon Monoxide