Biosynthesis of the cyanobacterial reserve polymer multi-L-arginyl-poly-L-aspartic acid (cyanophycin) - Mechanism of the cyanophycin synthetase reaction studied with synthetic primers

Biosynthesis of the cyanobacterial nitrogen reserve cyanophycin (multi-L-ar ginyl-poly-L-aspartic acid) is catalysed by cyanophycin synthetase, an enzy me that consists of a single kind of polypeptide. Efficient synthesis of th e polymer requires ATP, the constituent amino acids aspartic acid and argin ine, and a primer like cyanophycin. Using synthetic peptide primers, the co urse of the biosynthetic reaction was studied. The following results were o btained: (a) sequence analysis suggests that cyanophycin synthetase has two ATP-binding sites and hence probably two active sites; (b) the enzyme cata lyses the formation of cyanophycin-like polymers of 25-30 kDa apparent mole cular mass in vitro; (c) primers are elongated at their C-terminus; (d) the constituent amino acids are incorporated stepwise, in the order aspartic a cid followed by arginine, into the growing polymer. A mechanism for the cya nophycin synthetase reaction is proposed; (e) the specificity of the enzyme for its amino-acid substrates was also studied. Glutamic acid cannot repla ce aspartic acid as the acidic amino acid, whereas lysine can replace argin ine but is incorporated into cyanophycin at a much lower rate.