A Second Divalent Metal Ion in the Active Site of a New Crystal Form of Human Apurinic/Apyridinimic Endonuclease, Ape1, and its Implications for the Catalytic Mechanism
Two Divalent Metal Ions in the Active Site of a New Crystal Form of Human Apurinic/Apyrimidinic Endonuclease, Ape1: Implications for the Catalytic Mechanism
Primary Citation of Related Structures: 1E9N, 1HD7
PubMed Abstract:
The major human abasic endonuclease, Ape1, is an essential DNA repair enzyme that initiates the removal of apurinic/apyrimidinic sites from DNA, excises 3' replication-blocking moieties, and modulates the DNA binding activity of several transcriptional regulators ...
The major human abasic endonuclease, Ape1, is an essential DNA repair enzyme that initiates the removal of apurinic/apyrimidinic sites from DNA, excises 3' replication-blocking moieties, and modulates the DNA binding activity of several transcriptional regulators. We have determined the X-ray structure of the full-length human Ape1 enzyme in two new crystal forms, one at neutral and one at acidic pH. The new structures are generally similar to the previously determined structure of a truncated Ape1 protein, but differ in the conformation of several loop regions and in spans of residues with weak electron density. While only one active-site metal ion is present in the structure determined at low pH, the structure determined from a crystal grown at the pH optimum of Ape1 nuclease activity, pH 7.5, has two metal ions bound 5 A apart in the active site. Enzyme kinetic data indicate that at least two metal-binding sites are functionally important, since Ca(2+) exhibits complex stimulatory and inhibitory effects on the Mg(2+)-dependent catalysis of Ape1, even though Ca(2+) itself does not serve as a cofactor. In conjunction, the structural and kinetic data suggest that Ape1 catalyzes hydrolysis of the DNA backbone through a two metal ion-mediated mechanism.
Organizational Affiliation:
Molecular and Structural Biology Division, Biology and Biotechnology Research Program, Lawrence Livermore National Laboratory, Livermore, CA, 94550, USA.
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