title: Substitution of Asp-210 in HAP1 (APE/Ref-1) eliminates endonuclease activity but stabilises substrate binding.
abstract: HAP1, also known as APE/Ref-1, is the major apurinic/apyrimidinic (AP) endonuclease in human cells. Previous structural studies have suggested a possible role for the Asp-210 residue of HAP1 in the enzymatic function of this enzyme. Here, we demonstrate that substitution of Asp-210 by Asn or Ala eliminates the AP endonuclease activity of HAP1, while substitution by Glu reduces specific activity approximately 500-fold. Nevertheless, these mutant proteins still bind efficiently to oligonucleotides containing either AP sites or the chemically unrelated bulky p-benzoquinone (pBQ) derivatives of dC, dA and dG, all of which are substrates for HAP1. These results indicate that Asp-210 is required for catalysis, but not substrate recognition, consistent with enzyme kinetic data indicating that the HAP1-D210E protein has a 3000-fold reduced K(cat )for AP site cleavage, but an unchanged K(m). Through analysis of the binding of Asp-210 substitution mutants to oligonucleotides containing either an AP site or a pBQ adduct, we conclude that the absence of Asp-210 allows the formation of a stable HAP1-substrate complex that exists only transiently during the catalytic cycle of wild-type HAP1 protein. We interpret these data in the context of the structure of the HAP1 active site and the recently determined co-crystal structure of HAP1 bound to DNA substrates.
authors.affiliation: Imperial Cancer Research Fund Laboratories, University of Oxford, UK.
authors.forename: D G
authors.initials: DG
authors.lastname: Rothwell
authors.forename: B
authors.initials: B
authors.lastname: Hang
authors.forename: M A
authors.initials: MA
authors.lastname: Gorman
authors.forename: P S
authors.initials: PS
authors.lastname: Freemont
authors.forename: B
authors.initials: B
authors.lastname: Singer
authors.forename: I D
authors.initials: ID
authors.lastname: Hickson
mesh_terms: D016227:Benzoquinones / Q000378:metabolism; D001665:Binding Sites; D019757:Carbon-Oxygen Lyases / Q000737:chemistry / Q000235:genetics*; D043603:DNA-(Apurinic or Apyrimidinic Site) Lyase; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism; D004926:Escherichia coli; D006801:Humans; D006860:Hydrogen Bonding; D007700:Kinetics; D008958:Models, Molecular; D009154:Mutation; D009838:Oligodeoxyribonucleotides / Q000378:metabolism; D017433:Protein Structure, Secondary; D013379:Substrate Specificity
chemical_list: D016227:Benzoquinones; D004268:DNA-Binding Proteins; D009838:Oligodeoxyribonucleotides; D019757:Carbon-Oxygen Lyases; C085108:APEX1 protein, human; D043603:DNA-(Apurinic or Apyrimidinic Site) Lyase