A combined experimental and theoretical study of the catalytic activity of a [NiFeSe] hydrogenase has been performed by H/D exchange mass spectrometry and molecular dynamics simulations. Hydrogenases are enzymes that catalyze the heterolytic cleavage or production of H-2. The [NiFeSe] hydrogenases belong to a subgroup of the [NiFe] enzymes in which a selenocysteine is a ligand of the nickel atom in the active site instead of cysteine. The aim of this research is to determine how much the specific catalytic properties of this hydrogenase are influenced by the replacement of a sulfur by selenium in the coordination of the bimetallic active site versus the changes in the protein structure surrounding the active site. The pH dependence of the D-2/H+ exchange activity and the high isotope effect observed in the Michaelis constant for the dihydrogen substrate and in the single exchange/double exchange ratio suggest that a "cage effect" due to the protein structure surrounding the active site is modulating the enzymatic catalysis. This "cage effect" is supported by molecular dynamics simulations of the diffusion of H-2 and D-2 from the outside to the inside of the protein, which show different accumulation of these substrates in a cavity next to the active site.