Histidine at the active centre of pig heart lipoamide dehydrogenase

Abstract

Dye sensitized photooxidation of one or more histidine residues in lipoamide dehydrogenase adversely affects enzymic activity with lipoamide as acceptor. Other enzymic activities are relatively unaffected. Evidence is presented that phosphorylation of a common histidine residue enhances activity with lipoamide, possibly by stabilizing an imidazolium cation. The magnetic circular dichroism spectra of reduced enzyme species display an A term attributable to charge transfer from a thiolate anion to an imidazolium cation. Upon photooxidation this term disappears.