title: Transient resonance Raman spectroscopy shows unrelaxed heme following CO photodissociation from cytochrome-c peroxidase.
abstract: The 7 ns 436 nm pulses of an H2-shifted YAG laser have been used to photolyze the CO adduct of cytochrome-c peroxidase and produce the resonance Raman spectrum of the photoproduct. A 3 cm-1 downshift, relative to the spectrum of reduced enzyme, was observed for the porphyrin C-N breathing mode, v4. The downshift diminishes with decreasing CO /protein ratio, implying, in conjunction with a recent study of CO binding, that the unrelaxed heme is associated with adduct having a tilted, H-bonded FeCO unit. The downshift is eliminated when the phosphate buffer concentration is increased from 0.01 to 0.1 M. It is proposed that the heme relaxation under study involves a transition between two conformations, B and A, differing in the disposition of the distal residues, and having different v4 frequencies for unligated Fe(II) heme. Conformation B allows H-bonding to bound CO, and is favored at high CO and phosphate concentrations, while conformation A, which is unfavorable to CO H-bonding, is favored at low CO and phosphate concentrations. The recently reported absence of unrelaxed frequencies in the 7 ns photo-product of the CO adduct of horseradish peroxidase has been confirmed, and is attributed to lower stability for conformation B and a smaller A - B v4 difference.
mesh_terms: D002248:Carbon Monoxide / Q000378:metabolism; D003578:Cytochrome-c Peroxidase / Q000032:analysis* / Q000528:radiation effects; D006418:Heme / Q000032:analysis* / Q000378:metabolism; D010544:Peroxidases / Q000032:analysis*; D010782:Photolysis; D011485:Protein Binding; D011487:Protein Conformation; D013059:Spectrum Analysis, Raman
chemical_list: D006418:Heme; D002248:Carbon Monoxide; D010544:Peroxidases; D003578:Cytochrome-c Peroxidase