The D14 and R138 ion pair is involved in dimeric arginine kinase activity, structural stability and folding

Arginine kinase (AK) is a key enzyme in cellular energy metabolism of invertebrates. There are two conserved amino acid residues D14 and R138 in dimeric AK which form inter-subunit hydrogen bond. In Stichopus japonicus AK, mutations in these residues caused pronounced loss of activity, conformational changes and distinct substrate synergism alteration. Mutations (R138G, R138A and D14G) abolished D14 and R138 interaction disrupted the structure or conformation of S. japonicus AK. These R138G, R138A and D14G mutations changed their native assembles of dimeric AK and caused them in a partially unfolded state. The partially unfolded state of these mutant AKs made them prone to aggregate under environmental stress. The D14E/R138K and R138K mutant AKs showed similar characteristics to those of WT AK for forming the interaction which could replacement roles of D14 and R138 interaction. These results suggested that D14 and R138 interaction is involved in AK's activity, substrate synergism and structural stability.