INTERACTION OF ESCHERICHIA-COLI 8-OXOGUANINE-DNA GLYCOSYLASE WITH SINGLE-STRANDED OLIGODEOXYRIBONUCLEOTIDES AND THEIR COMPLEXES

The affinity of E. coli 8-oxoguanine-DNA glycosylase to oligonucleotid es (ON) d(pC)(n), d(pA)(n), d(pT)(n), and their duplexes was studied. All ss- and dsON were competitive inhibitors of the reaction catalyzed by this glycosylase. The Ki values for different ON and their complex es were determined. Enhanced affinity of the enzyme active center to d NMP or a unit in ss- or dsON was shown. Elongation of d(pN)(n), and th eir duplexes by one unit increases the affinity to glycosylase 1.4-1.6 and 2.2 times, respectively. The affinity of ssON increases with n in crease till 10, whereas for dsON the affinity increases till n = 14, w hich correlates with the size of a site protected from nuclease action , 9 and 13 units in ss- and dsON, respectively. DNA recognition by the enzyme is mainly due to the additivity of weak nonspecific contacts o f the enzyme with 9-10 and 13-14 units of the sugar-phosphate backbone of ss- and dsDNA, respectively, which provides for seven orders of af finity. Unlike nonmodified ligands, inclusion of 8-oxoguanine causes o nly a 4-10-fold increase of the ssON and dsON affinity to the enzyme. It is concluded that in case of ON containing 8-oxoguanine, complex fo rmation cannot provide for enzyme specificity, which is ensured at the stage of catalysis and is due to the reaction rate constant increase by several orders of magnitude.