His84 rather than His35 is the active site histidine in the corrinoid protein MrtA of the energy conserving methyltransferase complex from Methanobacterium thermoautotrophicum.

The energy conserving corrinoid containing MtrA-H complex from Methanobacterium thermoautotrophicum is composed of eight different subunits of which MtrA harbors the corrinoid prosthetic group, the corrinoid being bound in the base-off/His-on configuration. Based on sequence comparisons it was recently proposed that His35 of MtrA is the active site histidine. We report here that His84 rather than His35 is the axial ligand to the cobamide in MtrA.