RESUMEN
AP endonuclease (AP endo), a key enzyme in repair of abasic sites in DNA, makes a single nick 5' to the phosphodeoxyribose of an abasic site (AP-site). We recently proposed a novel mechanism, whereby the enzyme uses a key tyrosine (Tyr(171)) to directly attack the scissile phosphate of the AP-site. We showed that loss of the tyrosyl hydroxyl from Tyr(171) resulted in dramatic diminution in enzymatic efficiency. Here we extend the previous work to compare binding/recognition of AP endo to oligomeric DNA with and without an AP-site by wild type enzyme and several tyrosine mutants including Tyr(128), Tyr(171) and Tyr(269). We used single turnover and electrophoretic mobility shift assays. As expected, binding to DNA with an AP-site is more efficient than binding to DNA without one. Unlike catalytic cleavage by AP endo, which requires both hydroxyl and aromatic moieties of Tyr(171), the ability to bind DNA efficiently without an AP-site is independent of an aromatic moiety at position 171. However, the ability to discriminate efficiently between DNA with and without an AP-site requires tyrosine at position 171. Thus, AP endo requires a tyrosine at the active site for the properties that enable it to behave as an efficient, processive endonuclease.
Asunto(s)
ADN-(Sitio Apurínico o Apirimidínico) Liasa/química , ADN-(Sitio Apurínico o Apirimidínico) Liasa/metabolismo , ADN/metabolismo , Tirosina/química , Sitios de Unión , ADN/química , Reparación del ADN , ADN-(Sitio Apurínico o Apirimidínico) Liasa/genética , Ensayo de Cambio de Movilidad Electroforética , Humanos , Cinética , Factores de Tiempo , Tirosina/metabolismoRESUMEN
Apurinic/apyrimidinic endonuclease (AP endo, HAP1) recognizes abasic sites in ds DNA and makes a single nick in the backbone 5' to the abasic site. In this report we examine the roles of three conserved tyrosine residues in close proximity to the active site. We show that Tyr(128) and Tyr(269), which interact upstream and downstream of the abasic site, respectively, are involved in recognition and binding of abasic site-containing double stranded DNA. However, the two residues are not equivalent, as their effects are differentiated by changes in salt concentration. In sharp contrast, Tyr(171) is directly involved in catalysis as well as binding. Y171F, Y171H, and Y171A all show decreased catalytic efficiencies 25,000-50,000-fold from the WT enzyme. Both imidazole and basic pH markedly stimulate the WT enzyme. Imidazole stimulates Tyr(171) mutant enzymes when tyrosine is also present but basic pH eliminates remaining mutant activity. These results underscore the importance of tyrosines in AP endo catalysis. They render the current hypotheses regarding enzyme action unlikely and allow us to consider the possibility that the phenolate of Tyr(171) is the nucleophile that attacks the scissile phosphate.