Kinetic study of the inhibition of CK2 by heparin fragments of different length

The structure-activity relationships for the inhibition of protein kinase C K2 by heparin were investigated using purified heparin fragments of differe nt length, varying from 4 to 24 oligosaccharide sugar units. The inhibitory potency was shown to decrease concomitant with the shortening of the hepar in fragment length. The fragment of 24 oligosaccharide sugar units was the most potent inhibitor with a K-i value of 22 nM which is close to the K-i v alue for the commercial heparin mixture available, Shortening of the hepari n from 24 to 12 sugar units had a moderate influence on the inhibitory pote ncy causing an increase in K-i values up to 151 nM while fragments shorter than 12 sugar units showed a more drastic increase in K-i, values reaching up to micromolar range. The mode of inhibition was studied in respect to th e protein substrate beta-casein and it was shown to be competitive for the long as well as for the short heparin fragments. In contrast, the inhibitio n mode in respect to a synthetic peptide substrate RRRADDSDDDDD was found t o be hyperbolic partial non-competitive mixed-type. Such a kinetic model su ggests that heparin binds to a site on CK2 which does not overlap with the peptide substrate binding site and that a productive enzyme-complex exists where both heparin and peptide substrate are simultaneously bound. This is in contrast to the competitive inhibition model of the phosphorylation of p rotein substrate beta-casein where the binding of the protein substrate and inhibitor was mutually exclusive. (C) 1999 Elsevier Science B.V. All right s reserved.