title: The nature of species prepared by photolysis of half-reduced, fully reduced and fully reduced carbonmonoxy-cytochrome c-551 peroxidase from Pseudomonas aeruginosa.
abstract: The half-reduced, fully reduced and fully reduced CO-bound forms of the enzyme cytochrome c-551 peroxidase isolated from Pseudomonas aeruginosa were examined by a combination of low-temperature absorption and magnetic-circular-dichroism spectroscopy. Deliberate low-temperature (4.2K) photolysis of these forms of the enzyme, in all of which the high-potential haem is in the ferrous state, revealed that this haem group, assigned to have a histidine-methionine ligand set, is photosensitive. The photolabile ligand is most likely to be the methionine residue, and the product of photolysis, namely the high-spin (S = 2) ferrous form, is stable at low temperature (4.2K). Warming to approx. 20K allows thermal recombination to occur, restoring the low-spin (S = 0) state. The low-potential haem (bis-histidine ligation) is photoinert in both ferric and ferrous states; however, the photosensitive CO adduct of this centre cannot be maintained as the photolysed (S = 2) product at 4.2K. This surprising observation may be due to quantum-mechanical tunnelling of the CO through the activation barrier even at 4.2K, implying that the activation barrier to thermal recombination is both narrow and low. Low-temperature absorption spectroscopy reveals that the high-potential haem has a very characteristic low-spin ferrous spectrum with intense highly structured beta- and split alpha-bands, whereas the spectrum of the low-potential ferrous haem contains alpha- and beta-bands devoid of fine structure.
mesh_terms: D002248:Carbon Monoxide / Q000378:metabolism; D002942:Circular Dichroism; D003578:Cytochrome-c Peroxidase / Q000378:metabolism*; D006418:Heme / Q000032:analysis; D008280:Magnetics; D010084:Oxidation-Reduction; D010544:Peroxidases / Q000378:metabolism*; D010782:Photolysis; D011550:Pseudomonas aeruginosa / Q000201:enzymology*; D013696:Temperature
chemical_list: D006418:Heme; D002248:Carbon Monoxide; D010544:Peroxidases; D003578:Cytochrome-c Peroxidase