The reaction mechanism of the dinuclear cobalt enzyme arginase is investigated using density functional theory. As an arginase-containing binuclear Mn-2(2+) cluster, it catalyzes the hydrolysis of L-arginine in L-ornithine and urea. The bridging hydroxide is capable of performing nucleophilic attack on the iminium carbon ion from its bridging position, resulting in the formation of a tetrahedral intermediate, as was already obtained in a previous theoretical study on the manganese enzyme. Our theoretical investigation allows us to obtain an accurate potential energy profile and confirms that the coordination mode of the substrate to the dimetallic center is quite similar to that present in the manganese enzyme. In agreement with the experimental observations, our results show that both Mn- and Co-containing enzymes catalyze the same reaction with quite comparable energy barriers.