A molecular electrocatalyst is reported that reduces protons to hydrogen (H-2) in neutral water under aerobic conditions. The biomolecular catalyst is made from cobalt substitution of microperoxidase-11, a water-soluble heme-undecapeptide derived from the protein horse cytochrome c. In aqueous solution at pH 7.0, the catalyst operates with near quantitative Faradaic efficiency, a turnover frequency similar to 6.7 s(-1) measured over 10 mm at an overpotential of 852 mV, and a turnover number of 2.5 x 10(4). Catalyst activity has low sensitivity to oxygen. The results show promise as a hydrogenase functional mimic derived from a biomolecule.