ROLE OF AROMATIC RESIDUES IN STABILIZATION OF THE [FE4S4] CLUSTER IN HIGH-POTENTIAL IRON PROTEINS (HIPIPS) - PHYSICAL CHARACTERIZATION AND STABILITY STUDIES OF TYR-19 MUTANTS OF CHROMATIUM-VINOSUM HIPIP

The functional role of residue Tyr-19 of Chromatium vinosum HiPIP has been evaluated by site-directed mutagenesis experiments. The stability of the [Fe4S4] cluster prosthetic center is sensitive to side-chain r eplacements. Polar residues result in significant instability, while n onpolar residues (especially with aromatic side chains) maintain clust er stability. Two-dimensional NMR data of native and mutant HiPIPs are consistent with a model where Tyr-19 serves to preserve the structura l rigidity of the polypeptide backbone, thereby maintaining a hydropho bic barrier for exclusion of water from the cluster cavity. Solvent ac cessibility results in more facile oxidation of the cluster by atmosph eric oxygen, with subsequent rapid hydrolysis of the [Fe4S4](3+) core.