Enzymatic Properties and Spectroscopic Studies of Photoresponsive Lysozyme

Abstract

Photochromic compounds such as azobenzene derivatives change their chemical and physical properties reversibly with their structural change by photo irradiation. This character may be useful for regulation of functions of biomaterials, analysis of their reaction mechanism, and development of function materials. Previously, we have reported that photoresponsive lysozymes were modified with azobenzene derivatives at a position-specific residue (Lys33, or Lys116) on hen egg white lysozyme, and their enzymatic properties were reversibly modulated according to photoisomarization of azobenzene moiety. In this study, hen egg white lysozyme was modified with 4-(phenylazo) aniline at Asp 87, which forms an amphiphilic helix with hydrophilic residues directed toward the solvent. The enzyme reaction, and the fluorescence, transient absorption, and circular dichroism spectroscopies were studied to clarify the photoregulation mechanism. In MALDI TOF mass analysis, azobenzene moiety binds at Asp 87 through amide binding was revealed. The enzyme kinetics parameters of the modified lysozyme were modulated reversibly according to isomerization of azobenzene moiety: the catalytic efficiency for cell wall of Micrococcus lysodeikticus of trans form was more than tenfold that of cis form. The binding constant with a substrate analogue (tri-N-acetyl-D-glucosamine) was also reduced by isomerization from trans form to cis form. Moreover the efficiency of energy transfer to Eosin Y modulated reversibly by photo irradiation; in absence of the substrate analogue, the efficiency of trans form was higher than that of cis form, while in the presence of the substrate analogue, this efficiency shows opposite behavior. The fluorescence and circular dichroism spectroscopic studies suggest that the modulation of enzymatic properties is mainly induced by a steric hindrance due to the conformational changes in the vicinity of modification position.