EXAFS STUDIES OF FE(III)-PHOSVITIN AT HIGH METAL TO PROTEIN RATIOS

The stereochemistry of the Fe(III) binding sites in chicken egg phosvi tin (PST) at very high iron content, in solution and as a powder, has been investigated through EXAFS spectroscopy. We found that the EXAFS spectra obtained for aqueous PST solutions at metal:protein ratios of 20:1 and 40:1 are very similar to those previously obtained by us on a Fe10PST sample. In all cases the iron ions are octahedrally coordinat ed by oxygen atoms of the serine-bound phosphate groups and by other l igands from either the protein or the solvent. The average metal-donor atom distance is 1.94 Angstrom. At variance, the EXAFS results for a Fe50PST powder sample suggest the occurrence of a switch in iron coord ination from octahedral to lower coordination numbers (5,4). The avera ge iron-oxygen distance is virtually unchanged; apparently, four iron ligands are provided by four different coordinate phosphate groups fro m the phosphorylated serine residues abundant in the protein. This fin ding contains interesting implications for the structure-function rela tionships of this intriguing protein.