Steady-state and time-resolved fluorescence studies on wild type and mutant Chromatium vinosum high potential iron proteins: holo- and apo-forms

Abstract

Detailed circular dichroism (CD), steady-state and time-resolved tryptophan fluorescence studies on the holo- and apo- forms of high potential iron protein (HiPIP) from Chromatium vinosum and its mutant protein have been carried out to investigate conformational properties of the protein. CD studies showed that the protein does not have any significant secondary structure elements in the holo- or apo- HiPIP, indicating that the metal cluster does not have any effect on formation of secondary structure in the protein. Steady-state fluorescence quenching studies however, suggested that removal of the iron-sulfur ([Fe₄S₄]³⁺) cluster from the protein leads to an increase in the solvent accessibility of tryptophans, indicating change in the tertiary structure of the protein. CD studies on the holo- and apo- HiPIP also showed that removal of the metal prosthetic group drastically affects the tertiary structure of the protein. Time-resolved fluorescence decay of the wild type protein was fitted to a four-exponentials model and that of the W80N mutant was fitted to a three-exponentials model. The time-resolved fluorescence decay was also analyzed by maximum entropy method (MEM). The results of the MEM analysis agreed with those obtained from discrete exponentials model analysis. Studies on the wild type and mutants helped to assign the fast picosecond lifetime component to the W80 residue, which exhibits fast fluorescence energy transfer to the [Fe₄S₄]³⁺ cluster of the protein. Decay-associated fluorescence spectra of each tryptophan residues were calculated from the time-resolved fluorescence results at different emission wavelengths. The results suggested that W80 is in the hydrophobic core of the protein, but W60 and W76 are partially or completely exposed to the solvent.