η1-N-succinimidato complexes of iron, molybdenum and tungsten as reversible inhibitors of papain

Recently we have found that the metallocarbonyl complexes (η5-C5H5)M(CO)x(η1-N-maleimidato) (M = Fe, Mo, W; x = 2 or 3) bearing a maleimide function were irreversible inhibitors of the enzyme papain. To get further insight into the binding mechanism of these compounds we synthesized the related complexes (η5-C5H5)M(CO)x(η1-N-succinimidato) (M = Fe, Mo, W; x = 2 or 3) that lacked the ethylenic bond responsible for alkylation of the cysteine 25 thiol group in the papain‘s catalytic pocket. We performed kinetic studies of the interaction of the synthesized complexes towards papain. We found that they act as reversible inhibitors of the enzyme with IC50 values in the range 480–1700 μM. Docking experiments confirmed binding of these complexes to the enzyme’s catalytic pocket.