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[NiFe]-hydrogenases are highly efficient catalysts for the heterolytic splitting of molecular hydrogen (H 2 ). The heterobimetallic cysteine-coordinated active site of these enzymes is covered by a highly conserved arginine residue, whose role in the reaction is not fully resolved yet. The structural and catalytic role of this arginine is investigated here using QM/MM calculations with various exchange-correlation functionals. All of them give a very consistent picture of the thermodynamics of H 2  oxidation. The concept of the presence of a neutral arginine and its direct involvement as a Frustrated Lewis Pair (FLP) in the reaction is critically evaluated. The arginine, however, would exist in its standard protonation state and perform a critical role in positioning and slightly polarizing the substrate H 2 . It is not directly involved in the heterolytic processing of H 2  but guides its approach and reduces its flexibility during binding. Upon substitution of the positively charged arginine by a charge-conserving lysine residue, the H 2  binding position remains unaffected. However, critical hydrogen bonding interactions with nearby aspartate residues are lost. In addition, the H 2  polarization is unfavorable and the reduced side-chain volume may negatively affect the kinetics of the catalytic process.

QM/MM Investigation of the Role of a Second Coordination Shell Arginine in [NiFe]-Hydrogenases