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 kcat and kcat/Km 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 kcat and kcat/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 kcat and kcat/Km 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.
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