Atomic Structure of the DNA Repair [4Fe-4S] Enzyme Endonuclease III

Abstract

The crystal structure of the DNA repair enzyme endonuclease III, which recognizes and cleaves DNA at damaged bases, has been solved to 2.0 angstrom resolution with an R factor of 0.185. This iron-sulfur [4Fe-4S] enzyme is elongated and bilobal with a deep cleft separating two similarly sized domains: a novel, sequence-continuous, six-helix domain (residues 22 to 132) and a Greek-key, four-helix domain formed by the amino-terminal and three carboxyl-terminal helices (residues 1 to 21 and 133 to 211) together with the [4Fe-4S] cluster. The cluster is bound entirely within the carboxyl-terminal loop with a ligation pattern (Cys-X 6 -Cys-X 2 -Cys-X 5 -Cys) distinct from all other known [4Fe-4S] proteins. Sequence conservation and the positive electrostatic potential of conserved regions identify a surface suitable for binding duplex B-DNA across the long axis of the enzyme, matching a 46 angstrom length of protected DNA. The primary role of the [4Fe-4S] cluster appears to involve positioning conserved basic residues for interaction with the DNA phosphate backbone. The crystallographically identified inhibitor binding region, which recognizes the damaged base thymine glycol, is a seven-residue β-hairpin (residues 113 to 119). Location and side chain orientation at the base of the inhibitor binding site implicate Glu 112 in the N -glycosylase mechanism and Lys 120 in the β-elimination mechanism. Overall, the structure reveals an unusual fold and a new biological function for [4Fe-4S] clusters and provides a structural basis for studying recognition of damaged DNA and the N -glycosylase and apurinic/apyrimidinic-lyase mechanisms.