L-galactose metabolism in bacteroides vulgatus from the human gut microbiota

A previously unknown metabolic pathway for the utilization of l-galactose was discovered in a prevalent gut bacterium, Bacteroides vulgatus. The new pathway consists of three previously uncharacterized enzymes that were found to be responsible for the conversion of l-galactose to d-tagaturonate. Bvu0219 (l-galactose dehydrogenase) was determined to oxidize l-galactose to l-galactono-1,5-lactone with k_cat and k_cat/K_m values of 21 s^-1 and 2.0 × 10⁵ M^-1 s ^-1, respectively. The kinetic product of Bvu0219 is rapidly converted nonenzymatically to the thermodynamically more stable l-galactono-1,4-lactone. Bvu0220 (l-galactono-1,5-lactonase) hydrolyzes both the kinetic and thermodynamic products of Bvu0219 to l-galactonate. However, l-galactono-1,5-lactone is estimated to be hydrolyzed 300-fold faster than its thermodynamically more stable counterpart, l-galactono-1,4-lactone. In the final step of this pathway, Bvu0222 (l-galactonate dehydrogenase) oxidizes l-galactonate to d-tagaturonate with k_cat and k_cat/K _m values of 0.6 s^-1 and 1.7 × 10⁴ M ^-1 s^-1, respectively. In the reverse direction, d-tagaturonate is reduced to l-galactonate with values of k_cat and k_cat/K_m of 90 s^-1 and 1.6 × 10 ⁵ M^-1 s^-1, respectively. d-Tagaturonate is subsequently converted to d-glyceraldehyde and pyruvate through enzymes encoded within the degradation pathway for d-glucuronate and d-galacturonate.