Flash Photolysis of Enzymes

Summary

The photoionization of aromatic residues constitutes a major initial photochemical reaction in the flash photolysis of proteins at λ > 250 nm. The ejected electrons have been observed as e_aq⁻ and the disulphide bridge electron adduct, and also must be trapped at unidentified sites. The number or tryptophyl (or tyrosyl) residues photo-ionized at 5 μsec delay is approximately equal to the number of exposed residues. The flash photolysis data have been related to inactivation by considering how photolysis of these ‘photolabile’ residues can affect enzymic activity, based on the microstructure and available information about permanent alterations and residue specificities. This analysis indicates that hen lysozyme and papain are inactivated by photolysis of an essential Trp residue, that bovine trypsin is inactivated by photolysis of a Trp residue adjacent to the key catalytic Ser and other pathways initiated by excitation of Tyr and Cys, that the efficient photoionization of Tyr in RNase A is not an important inactivating reaction, and that aromatic residues in subtilisin Carlsberg are photosensitive.