[Fe]-Hydrogenase catalyses the reversible hydrogenation of a methenyltetrahydromethanopterin substrate, which is an intermediate step during the methanogenesis from CO2 and H2. The active site contains an iron-guanylylpyridinol cofactor, in which Fe(2+) is coordinated by two CO ligands, as well as an acyl carbon atom and a pyridinyl nitrogen atom from a 3,4,5,6-substituted 2-pyridinol ligand. However, the mechanism of H2 activation by [Fe]-hydrogenase is unclear. Here we report the reconstitution of [Fe]-hydrogenase from an apoenzyme using two FeGP cofactor mimics to create semisynthetic enzymes. The small-molecule mimics reproduce the ligand environment of the active site, but are inactive towards H2 binding and activation on their own. We show that reconstituting the enzyme using a mimic that contains a 2-hydroxypyridine group restores activity, whereas an analogous enzyme with a 2-methoxypyridine complex was essentially inactive. These findings, together with density functional theory computations, support a mechanism in which the 2-hydroxy group is deprotonated before it serves as an internal base for heterolytic H2 cleavage.