Has low D-mannonate dehydratase activity (in vitro), suggesting that this is not a physiological substrate and that it has no significant role in D-mannonate degradation in vivo. Has no detectable activity with a panel of 70 other acid sugars (in vitro). D-mannonate = 2-dehydro-3-deoxy-D-gluconate + H2O Binds 1 Mg(2+) ion per subunit. kcat is 0.01 sec(-1) with D-mannonate. Belongs to the mandelate racemase/muconate lactonizing enzyme family. GalD subfamily.