Carbon monoxide as an intrinsic ligand to iron in the active site of te iron-sulfur-cluster-free hydrogenase H(2)-forming methylenetetrahydromethanopterin dehydrogenase as revealed by infrared spectroscopy.

The iron-sulfur-cluster-free hydrogenase Hmd (H2-forming methylenetetrahydromethanopterin
dehydrogenase) from methanogenic archaea has recently been found to contain one iron associated tightly with an extractable cofactor of yet unknown structure. We report here that Hmd contains intrinsic CO bound to the Fe. Chemical analysis of Hmd revealed the presence of 2.4 ( 0.2 mol of CO/mol of iron. Fourier transform infrared spectra of the native enzyme showed two bands of almost equal intensity at 2011 and 1944 cm-1, interpreted as the stretching frequencies of two CO molecules bound to the same iron in an angle of 90°. We also report on the effect of extrinsic 12CO, 13CO, 12CN-, and 13CN- on the IR spectrum of Hmd.