The crystal structure of the human arginase I-thiosemicarbazide complex reveals an unusual thiocarbonyl mu-sulfide ligand in the binuclear manganese cluster. The CS moiety of thiosemicarbazide bridges Mn-A(2+) and Mn-B(2+) with coordination distances of 2.6 and 2.4 A, respectively. Otherwise, the binding of thiosemicarbazide to human arginase I does not cause any significant structural changes in the active site. The crystal structure of the unliganded enzyme reveals a hydrogen-bonded water molecule that could support proton transfer between a mu-water molecule and H141 to regenerate the nucleophilic mu-hydroxide ion in the final step of catalysis.