Expression and functional studies on lignin and manganese peroxidase

Efficient expression systems for recombinant MnP and LiP are required for structure/function studies. Previously, we developed the first efficient homologous expression system for MnP which is based on expression of the enzyme during primary growth under the control of the glyceraldehyde-P-dehydrogenase (gpd) gene promoter. In recent work, we have expressed recombinant LiP using a modification of this system. We have carried out site-directed mutagenesis on Phe190 which is located on the proximal side of the heme pocket of MnP. While the Tyr and Leu variants function almost normally, the Ile and Ala variants show significant decreases in thermal stability from the wild-type enzyme and increases in the rates of the spontaneous reduction of compounds I and II. Furthermore, at high pH, the wild-type enzyme and variants shift from a high-spin, pentacoordinate form to a low-spin, hexacoordinate, bis-His form where the distal His becomes the sixth ligand to the heme iron. In an extension of our previous studies, we have carried out new mutation studies on Arg177 of MnP. Variant proteins containing single mutations of this residue all exhibit altered steady-state and transient-state kinetic properties, demonstrating that this residue plays an important role in Mn binding.