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PHOTOOXIDATION OF LANTHANIDE ION‐LYSOZYME COMPLEXES. A NEW APPROACH TO THE EVALUATION OF INTRAMOLECULAR DISTANCES IN PROTEINS
Author(s) -
Jori Giulio,
Folin Marcella,
Gennari Giorgio,
Galiazzo Guido,
Buso Orfeo
Publication year - 1974
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1111/j.1751-1097.1974.tb06534.x
Subject(s) - lysozyme , chemistry , lanthanide , intramolecular force , ion , crystallography , metal ions in aqueous solution , metal , side chain , excited state , muramidase , photochemistry , stereochemistry , organic chemistry , biochemistry , physics , nuclear physics , polymer
— Rare‐earth metal ions give 1:1 complexes with hen's egg‐white lysozyme. Spectroscopic and enzymic activity measurements suggest that the binding site consists of the side chains of glutamic‐35 and aspartic‐52. The spatial conformation of these complexes is practically identical to that of native lysozyme, especially as concerns the environment of the tryptophyl side chains. Irradiation of La 3+ ‐lysozyme by visible light, in the presence of proflavine as photosensitizer, causes the oxidative modification of all the tryptophyl and methionyl residues at almost the same rate as in uncomplexed protein. On the other hand, when lanthanide ions with nonvanishing magnetic moments were coordinated with lysozyme, at least some tryptophans and methionines were protected from photooxidative attack. The distance of the protected residues from the coordination site increased with increasing magnetic moment of the bound metal ion, which suggests that inhibition of the photoprocess was mainly due to perturbation of the lifetime of the electronically excited intermediate species. On the basis of the atomic coordinates of lysozyme in the crystal state, it is thus possible to define a “quenching radius” for the various lanthanide ions; these radii could in turn be used to evaluate intramolecular distances in proteins of unknown tertiary structure, by identifying the amino acid residues that are protected or photooxidized upon irradiation of complexes between the given protein and several different lanthanide ions. Our studieson lysozyme allow us to define five radii of protection, ranging from 6·7 Å for Sm 3+ to over 17 Å for Dy 3+ , Ho 3+ , Er 3+ and Tb 3+ . Therefore, this technique opens the possibility of mapping appreciably large regions of a protein molecule.