Catalyzes the dehydration of D-mannonate. D-mannonate = 2-dehydro-3-deoxy-D-gluconate + H2O Mn(2+) can substitute for iron, but in higher concentrations. Cannot use Fe(3+), Ni(2+) or Mg(2+). Is inhibited by high concentrations of Fe(2+) (> 2 mM), and by EDTA or other iron chelators in vitro. Optimum pH is about 7.5. The activity at pH 6.5 and 8.6 is only about 20% of the maximal achievable activity at optimum pH. Carbohydrate metabolism; pentose and glucuronate interconversion. By fructuronate. Its expression is subjected to catabolite repression by glucose. Belongs to the mannonate dehydratase family.