The role of two-state reactivity at the enzyme active site with respect to binding of molecular H(2) for the high- and low-spin of [NiFe] hydrogenase (Ni-SI forms) is examined by density functional theory. In addition to examination of a single H(2) molecule binding at either the Ni or Fe of the active site, the possibility that H(2) binds simultaneously at each metal center in the active site of this enzyme is examined. The concurrent binding of two molecules of H(2) suggests a potential hydrogen bottleneck in which high concentrations might lead to a decrease in the rate of hydrogen oxidation.