Apurinic/apyrimidinic endonuclease Apn1 from Saccharomyces cerevisiae is recruited to the nucleotide incision repair pathway: Kinetic and structural features.

Apurinic/apyrimidinic endonuclease Apn1 of Saccharomyces cerevisiae is known as a key player of the base excision DNA repair (BER) pathway in yeast. BER is initiated by DNA glycosylases, whereas Apn1 can start DNA repair individually in the nucleotide incision repair (NIR) pathway. The aim of this research was to elucidate kinetic and structural dynamic aspects of Apn1 involvement in the NIR process. One of the key characteristics of AP endonuclease's interactions is known to be divalent metal ions playing a part of a cofactor. Well-studied human APE1 employs Mg ions, with metal ion concentration's affecting enzymatic activity exerted by APE1. In our study, we aimed to test the effect of the Mg ion on Apn1's NIR catalysis by examining structural dynamics of DNA during the interaction in real time using the stopped-flow technique. To test NIR activity of Apn1, deoxyribooligonucleotide duplexes containing a 5,6-dihydro-2'-deoxyuridine (DHU) residue were employed as substrates. A 2-aminopurine (2-aPu) residue was a reporter group fluorescence intensity of which was detected during Apn1-DNA interactions. NIR activity of both WT and H83A Apn1 was found to be arrested during the interaction with a DNA duplex containing the 2-aPu residue upstream of DHU. We conducted molecular dynamics simulations to elucidate the structural features of complexes of the enzyme with DHU-containing DNAs. The NIR recruiting S. cerevisiae Apn1 proceeds via multistep rearrangements of the complex of Apn1 with a DHU-containing DNA substrate and results in the incised product of the reaction. For wild-type Apn1, the catalytic rate constants do not depend on the Mg concentration, i.e., they are equal in NIR and BER buffers, with equilibrium association constant K being 10-fold higher in NIR buffer. Our data reveal more delicate regulation of Apn1's NIR activity due to the more complicated kinetic mechanism, as compared to BER.