Contribution of separate tryptophan residues to intrinsic fluorescence of actin. Analysis of 3D structure

The location of tryptophan residues in the actin macromolecule was studied on the basis of the known 3D structure. For every tryptophan residue the po larity and packing density of their microenvironments were evaluated. To es timate the accessibility of the tryptophan residues to the solvent molecule s it was proposed to analyze the radial dependence of the packing density o f atoms in the macromolecule about the geometric center of the indole rings of the tryptophan residues. The proposed analysis revealed that the microe nvironment of tryptophan residues Trp-340 and Trp-356 has a very high densi ty, So these residues can be regarded as internal and inaccessible to solve nt molecules. Their microenvironment is mainly formed by non-polar groups o f protein. Though the packing density of the Trp-86 microenvironment is low er, this tryptophan residue is apparently also inaccessible to solvent mole cules, as it is located in the inner region of macromolecule. Tryptophan re sidue Trp-79 is' external and accessible to the solvent. All residues that can affect tryptophan fluorescence were revealed. It was found that in the close vicinity of tryptophan residues Trp-79 and Trp-86 there are a number of sulfur atoms of cysteine and methionine residues that are known to be ef fective quenchers of tryptophan fluorescence. The most essential is the loc ation of SG atom of Cys-10 near the NE1 atom of the indole ring of tryptoph an residue Trp-86, On the basis of microenvironment analysis of these trypt ophan residues and the evaluation of energy transfer between them it was co ncluded that the contribution of tryptophan residues Trp-79 and Trp-86 must be low. Intrinsic fluorescence of actin must be mainly determined by two o ther tryptophan residues - Trp-340 and Trp-356, It is possible that the uns trained conformation of tryptophan residue Trp-340 and the existence of aro matic rings of tyrosine and phenylalanine and proline residues in the micro environments of tryptophan residues Trp-340 and Trp-356 are also essential to their blue fluorescence spectrum. (C) 1999 Federation of European Bioche mical Societies.