Involved in the degradation of 3,6-anhydro-L-galactose, which is the major monomeric sugar of red macroalgae. Catalyzes the isomerization of 3,6-anhydrogalactonate (AHGA) to 2-keto-3-deoxy-galactonate (KDGal). 3,6-anhydro-L-galactonate = 2-dehydro-3-deoxy-L-galactonate Could be used for bioconversion of red macroalgal biomass into biofuels or industrial chemicals. Belongs to the mandelate racemase/muconate lactonizing enzyme family.