Identification of residues essential for catalytic activity of a Bombyx mori arginase

Arginase is an oligometric protein that catalyzes the conversion of arginine to ornithine and urea in the presence of metal. Arginase in the silkworm, Bombyx mori, has been shown to participate in sperm maturation. In this study, we examine the enzymatic properties of B. mori arginase identified previously. The recombinant enzyme (bmArg-r) was purified to near homogeneity by ammonium sulfate fractionation, anion-exchange chromatography and gel-filtration chromatography. The isolated bmArg-r exhibits activity toward arginine in the presence of manganese ions, whereas it was unable to catalyze lysine hydrolysis. Copper and magnesium ions did not accelerate the activity toward arginine. Mutagenesis of His¹⁰⁷ in the metal-binding site of bmArg-r revealed that this residue is important for enzymatic function. We found that the mutation of His¹³², another histidine residue present in the metal-binding site, reduced the solubility of recombinant bmArg-r. It was considered that these histidine residues located in the metal-binding site of bmArg-r play a crucial role in catalytic activity.