Catalyzes the dehydration of D-mannonate. 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 1.3 sec(-1) with D-mannonate. Carbohydrate metabolism; pentose and glucuronate interconversion. Homotetramer. Belongs to the mandelate racemase/muconate lactonizing enzyme family. GalD subfamily.