N-terminal DEDDh 3'-5' exonuclease domain of Escherichia coli exonuclease I and similar proteins
This subfamily is composed of the N-terminal domain of Escherichia coli exonuclease I (ExoI) and similar proteins. ExoI is a monomeric enzyme that hydrolyzes single stranded DNA in the 3' to 5' direction. It plays a role in DNA recombination and repair. It primarily functions in repairing frameshift mutations. The N-terminal domain of ExoI is a DEDDh-type DnaQ-like 3'-5 exonuclease containing three conserved sequence motifs termed ExoI, ExoII and ExoIII, with a specific Hx(4)D conserved pattern at ExoIII. These motifs are clustered around the active site and contain four conserved acidic residues that serve as ligands for the two metal ions required for catalysis. The ExoI structure is unique among DnaQ family enzymes in that there is a large distance between the two metal ions required for catalysis and the catalytic histidine is oriented away from the active site.