Catalytic domain, repeat 2, of metazoan tyrosyl-DNA phosphodiesterase
Catalytic domain, repeat 2, of metazoan tyrosyl-DNA phosphodiesterase (Tdp1, EC 3.1.4.-). Human Tdp1 (hTdp1) acts as an important DNA repair enzyme with a preference for single-stranded or blunt-ended duplex oligonucleotides. It can remove stalled topoisomerase I-DNA complexes by catalyzing the hydrolysis of a phosphodiester bond between a tyrosine side chain and a DNA 3'-phosphate. It is therefore a potential molecular target for new anti-cancer drugs. hTdp1 has been shown to associate with additional proteins, such as XRCC1, to form a multi-enzyme complex. These additional proteins may be involved in recognizing 3'-phoshotyrosyl DNA in vivo. hTdp1 is a monomeric protein containing two copies of a variant HKD motif (H-x-K-x(4)-D, where x represents any amino acid residue), which consists of the highly conserved histidine and lysine residues, but lacks the aspartate residue that is well conserved in other phospholipase D (PLD, EC 3.1.4.4) enzymes. Like other PLD enzymes, hTdp1 may utilize a common two-step general acid/base catalytic mechanism, involving a DNA-enzyme intermediate to cleave phosphodiester bonds. A single active site involved in phosphatidyl group transfer would be formed by the two variant HKD motifs from the N- and C-terminal domains in a pseudodimeric way.
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