Atomic mutations at the single tryptophan residue of human recombinant annexin V: Effects on structure, stability, and activity

The single tryptophan residue (Trp187) of human recombinant annexin V, containing 320 residues and 5328 atoms, was replaced with three different isosteric analogues where hydrogen atoms at positions 4, 5, and 6 in the indole ring were exchanged with fluorine. Such single atom exchanges of H --> F represent atomic mutations that result in slightly increased covalent bond lengths and inverted polarities in the residue side-chain structure. These minimal changes in the local geometry do not affect the secondary and tertiary structures of the mutants, which were identical to those of wild-type protein in the crystal form. But the mutants exhibit significant differences in stability, folding cooperativity, biological activity, and fluorescence properties if compared to the wild-type protein. These rather large global effects, resulting from the minimal local changes, have to be attributed either to the relatively strong changes in polar interactions of the indole ring or to differences in the van der Waals radii or to a combination of both facts, The changes in local geometry that are below resolution of protein X-ray crystallographic studies are probably of secondary importance in comparison to the strong electronegativity introduced by the fluorine atom. Correspondingly, these types of mutations provide an interesting approach to study cooperative functions of integrated residues and modulation of particular physicochemical properties, in the present case of electronegativity, in a uniquely structured and hierarchically organized protein molecule.