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 x 10(5) 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,S-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 x 10(4) 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 X 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.