Metal ions-induced stability and function of bimetallic human arginase-I, a therapeutically important enzyme

Recent studies have highlighted the therapeutic importance of bimetallic human arginase-I against hyperargininemia and L-arginine auxotrophic cancers. The longer retention of catalytic activity of the Co²⁺-enzyme than that of the Mn²⁺ in human serum is associated with its enhanced therapeutic potential. To understand the basis of this and also to explore the role of a bimetallic center as well as the role of individual metal ions in the stability, we performed a detailed biochemical and biophysical investigation. The thermodynamic and kinetic stabilities of both the holo proteins are found to be significantly higher than the apo form, indicating that an intact bimetallic centre is vital for the enhanced stability of the holo proteins. The Co²⁺-protein is found to be more stable than that of the Mn²⁺, which might explain its longer retention of activity observed in the serum. Mutational studies demonstrated that the metal ions are individually crucial for both the enhanced stability and catalytic activity. Furthermore, we investigated the underlying mechanism for the effect of heat activation on the holo protein for higher catalytic activity, which is not yet known for arginases. Our data reveal that heat activation significantly increases the stability of the holo protein through a metal-induced increase in the helical content leading to the formation of a kinetically competent enzyme. Thus, the present study provides an in-depth insight into the significance of heat activation and the role of metal ions in human arginase, which may be useful for better understanding of its therapeutic use.