Pig heart nucleosidediphosphate kinase. Phosphorylation and interaction with Cibacron blue 3GA

The nucleosidediphosphate kinase phosphorylation reaction led to the incorporation of 0.95 +/- 0.1 phosphate groups per enzyme subunit. The equilibrium constant of the phosphorylation reaction was 0.26. The inhibition of the nucleosidediphosphate kinase activity by Cibacron blue 3GA was competitive with respect to ATP, the donor nucleotide (apparent Ki = 0.28 microM) and uncompetitive with respect to 8-bromoinosine 5'-diphosphate, the acceptor nucleotide (apparent Ki = 0.31 microM). By difference spectroscopy it was shown that each enzyme subunit bound one Cibacron blue 3GA molecule, whereas the phosphorylated enzyme had no affinity for the dye. ATP was an effective competitor, being able to displace the dye from its bound state. The complex behaviour noted was taken as evidence for cooperative interaction between the enzyme subunits. The data obtained using polarographic techniques agreed with these results.