Probing electron transfer reactions between two azurins from Alcaligenes xylosoxidans GIFU 1051 with optically active Ru complexes as molecular recognition probes: Importance of the 43rd residue

Abstract

Electron transfer reactions between optically-active Ru II/III complexes incorporating ( S)-/( R)-amino acids, and the two azurins, azurin-1 (az-1Cu) and azurin-2 (az-2Cu) isolated from Alcaligenes xylosoxidans GIFU 1051, have been studied to probe molecular recognition sites on the two azurins. The Ru II/III complexes are K[Ru II(L)(bpy)] and [Ru III(L)(bpy)], and have a tripodal ligand (L) derived from the ( S)-/( R)-amino acids, which are in turn exchanged for other functional substituent groups, such as ( S)-/( R)-phenylalanine, -leucine, -valine, -alanine, and -glutamic acid (L = ( S)-/( R)- BCMPA, - BCMLE, - BCMVA, - BCMAL, and - BCMGA). In the oxidation reaction of az-1Cu I promoted by the Ru III complexes, the kinetic parameters exhibited enantio- and stereo-selectivities, while the same reaction of az-2Cu I was less enantio- and stereo-selective. These differences suggest that the processes of formation of the activated states are different for the two azurins. On the other hand, such a difference has not been observed for az-1 and az-2 with respect to the reduction reactions promoted by both azurins Cu II by the Ru II complexes within the experimental error. This suggests that the neutrality of the Ru complexes is important for precise molecular recognition of azurins. His117 has been proposed as the electron transfer site. The local structures in the vicinity of the His117 side chain in the two azurins, are essentially identical with the exception of the 43rd residue, Val43 and Ala43 for az-1 and az-2, respectively. Electron transfer reactions between Ru III complexes and a mutant azurin, V43A-az-1, were also carried out. Interestingly, the activation parameters estimated were very similar to those of az-2, indicating that the 43rd residue acts as the electron transfer site in azurins and provides rationalization for the different mechanisms of az-1 and az-2 in redox reactions.