Characterization of a Keystone Enzyme in the Metabolism of 3,6-Anhydro-L-Galactose, a Main Sugar Component of Red Macroalgae

Red macroalgae are emerging as sustainable sources for producing biofuels and chemicals. 3,6-Anhydro-L-galactose (AHG) is the major constituent of agar, which is the main carbohydrate of red macroalgae. Recently, the metabolic pathway of AHG and the enzymes involved in the AHG metabolism were discovered in a marine bacterium. In this study, a novel aldehyde dehydrogenase, AHG dehydrogenase (AHGD) which oxidizes AHG to 3,6-anhydrogalactonate (AHGA) as the first enzyme in the AHG metabolism was characterized. Both NAD⁺ and NADP⁺ were found to act as a cofactor for the enzymatic reaction of AHGD with AHG. The optimal reaction pH and temperature and substrate specificity of AHGD were investigated, and the kinetic parameters of AHGD were determined in this study. Since AHGD is involved in the first step of the AHG pathway, AHGD may be the keystone enzyme which enables AHG to enter its catabolic pathway of AHG in marine bacteria utilizing red macroalgal biomass.