Structural Modification and Aggregation of Mucin by Chromium(III) Complexes

Abstract

Metal ions binding to proteins regulate the functions of proteins and may also lead to structural changes. In this communication we demonstrate the interaction and subsequent conformational changes induced in pig gastric mucin (PGM) upon binding to certain chromium(III) complexes like, [Cr(salen)(H₂O)₂](ClO₄) (1), [Cr(en)₃]Cl₃ (2) and [Cr(EDTA)(H₂O)]Na (3) which vary in charge and ionic character. Complexes 1 and 3 have been shown to interact coordinately with PGM whereas complex 2 binds through electrostatic interaction and hydrogen bonding. Steady state fluorescence experiment reveals that at lower concentration of complex 2 there is partial quenching of the tyrosine emission, whereas at higher concentration of the complex the emission intensity is enhanced. On the other hand with complexes 1 and 3 a decrease in fluorescence intensity was observed. PGM viscosity was found to decrease in the presence of complex 1 and 3 due to the formation of flexible fibres through coordinate interaction. Complex 2 was found to facilitate metal induced intertangling of PGM fibres which tends to stabilize the interaction and leads to sol-gel transition with subsequent increase in viscosity. A significant change in CD spectrum of PGM was observed in the presence of complex 2 where random coil spectrum became typical of a α-helical structure with 80% alpha helix content. In the case of complexes 1 and 3 only minor changes in the amplitude of the spectrum were observed. Histochemical analysis supports the contention that complex 2 favors the oligomerisation of PGM and leads to the formation of aggregated mass of macromolecules.