H2 -forming methylenetetrahydromethanopterin dehydro-genase (Hmd) is an unusual hydrogenase present in many methanogenic archaea. The homodimeric enzyme dubbed metal-free hydrogenase does not contain iron–sulfur clus-ters or nickel and thus differs from [Ni-Fe] and [Fe-Fe] hydrogenases, which are all iron–sulfur proteins. Hmd preparations were found to contain up to 1 mol iron per 40 kDa subunit, but the iron was considered to be a con-taminant as none of the catalytic and spectroscopic pro-perties of the enzyme indicated that it was an essential component
