| Abstract: | Hydrophobic interactions as structure determining factors for macromolecules are well-known in biochemistry. Considerably less recognized is the fact that these law-energy interactions may also determine the structure of low-molecular species. The same applies for weak ligand-metal ion interactions. That both these factors may lead to intramolecular, contraction-independent equilibria between isomeric metal-ion complexes is demostrated with α-lipoic acid as ligand. This cofactor affers metal ions two different binding sites: the carboxylate group and the disulfide linkage. The carboxylate group dominates the coordinating properties of this ligand towards the biologically important metal ions, but a disulfide-metal ion interaction is still possible and under sterically favourable conditions may becomevery important; this could also be true under enzymic conditions here the carboxyl group is no longer free but amide-liked to the protein. Furthermore, due to the valeric acid side chain, the lipoyl moiety is ideally suited to undergo hydrophobic ligand-ligand interactions in mixed-ligand complexes. Such hydrophobic interactions seem to be ideal to allwe migration, e.g., of the 14 Å long lipoyllysyl moiety, and also to caciliutate the correct ‘fixation’ at the surface of the enzyme. |
