ATP + [L-Asp(4-L-Arg)]n + L-Asp ADP + phosphate + [L-Asp(4-L-Arg)]n-L-Asp
This enzyme requires Mg2+ for activity. All enzymes known to have this activity also catalyze the addition of arginine, i.e. cyanophycin synthase (L-arginine-adding) activity. It is structurally similar to Muramyl ligases.
^Aboulmagd E, Oppermann-Sanio FB, Steinbüchel A (November 2000). "Molecular characterization of the cyanophycin synthetase from Synechocystis sp. strain PCC6308". Archives of Microbiology. 174 (5): 297–306. doi:10.1007/s002030000206. PMID11131019.