Multifunctionality of lipoamide dehydrogenase: lysine residue and cationic environment

Chemical modifications are carried out to investigate cationic residues of lipoamide dehydrogenase. Amidinations with imidoesters which introduce amidino groups with various substituents, alter the dehydrogenase activity without significantly affecting other functional activities. Correlation analyses of kinetic parameters (lipoamide reduction catalyzed by amidinated enzymes) for substituent effects offer a useful technique for studying structure-function relationship of the lysine residues. The specificity of phosphopyridoxylation and subsequent photoinactivation of the phosphopyridoxylated enzyme enable us to identify the lysine residue at the proximity of the active site histidine. Sensitized photoinactivation of glyoxalated enzyme together with relevant results suggest that the lysine residue provides cationic environment to the hydrophobic active site, and thereby, affects the reactivity of the active site histidine in the dehydrogenase reaction.