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Catalytic mechanism of wild-type EndA: the proposed catalytic mechanism of EndA is based on the established catalytic mechanism of the Serratia nuclease (26). His160 acts as the general base for the activation of a water molecule. The magnesium ion is coordinated by Asn191, which is held in place by Asp157, and the proS-oxygen as well as the 3′-oxygen of the phosphate diester. A glutamic acid residue residue nearby (Glu196 or Glu205 in EndA) is assumed to be indirectly involved in water-mediated magnesium ion binding.