The diheme enzyme MauG catalyzes a six-electron oxidation required for posttranslational modification of a precursor of methylamine dehydrogenase (preMADH) to complete the biosynthesis of its protein-derived tryptophan tryptophylquinone (TTQ) cofactor. One heme is low-spin with ligands provided by His205 and Tyr294, and the other is high-spin with a ligand provided by His35. The side chain methyl groups of Thr67 and Leu70 are positioned at a distance of 3.4 Å on either side of His35, maintaining a hydrophobic environment in the proximal pocket of the high-spin heme and restricting the movement of this ligand. Mutation of Thr67 to Ala in the proximal pocket of the high-spin heme prevented reduction of the low-spin heme by dithionite, yielding a mixed-valent state. The mutation also enhanced the stabilization of the charge-resonance-transition of the high-valent bis-FeIV state that is generated by addition of H2O2. The rates of electron transfer from TTQ biosynthetic intermediates to the high-valent form of T67A MauG were similar to that of wild-type MauG. These results are compared to those previously reported for mutation of residues in the distal pocket of the high-spin heme that also affected the redox properties and charge resonance transition stabilization of the high-valent state of the hemes. However, given the position of residue 67, the structure of the variant protein and the physical nature of the T67A mutation, the basis for the effects of the T67A mutation must be different from those of the mutations of the residues in the distal heme pocket.
Bibliographical noteFunding Information:
This research was supported by the National Institute of General Medical Sciences of the National Institutes of Health under award numbers R37GM41574 (VLD), R01GM66569 (CMW), and Mississippi INBRE, funded by an Institutional Development Award (IDeA) from the National Institute of General Medical Sciences of the National Institutes of Health under grant number P20GM103476 (MF).
Computer resources were provided by the Basic Sciences Computing Laboratory of the University of Minnesota Supercomputing Institute, and we thank Nancy Rowe for her support. X-ray data were collected at the Kahlert Structural Biology Laboratory (KSBL) at The University of Minnesota and GM/CA-CAT at the Advanced Photon Source (APS), Argonne National Laboratory, Argonne, IL. GM/CA CAT has been funded in whole or in part with Federal funds from the National Cancer Institute ( Y1-CO-1020 ) and the National Institute of General Medical Science ( Y1-GM-1104 ). The use of the APS was supported by the U.S. Department of Energy, Basic Energy Sciences, Office of Science , under contract No. DE-AC02-06CH11357 . We thank Ed Hoeffner for KSBL support, and the staff at Sector 23, APS for their support. Resonance Raman spectra were collected at the Analytical Core Laboratory (RCMI core facility) at Jackson State University (supported by NIH-RCMI program , grant number 8G12MD007581 ).
© 2015 Elsevier B.V.
- Electron transfer