Abstract
A gram-negative and alginate-assimilating bacterium, Sphingomonas sp. A1, can incorporate the polysaccharide into the cytoplasm through the cell-surface pit and ABC transporter. Alginate is depolymerized into disaccharides to tetrasaccharides by cytoplasmic endotype alginate lyases (A1-I, II, III). An exotype alginate lyase A1-IV degrades the oligosaccharides to unsaturated monosaccharides. α-Keto acids nonenzymatically formed from monosaccharides are converted to 2-keto-3-deoxy-^D-gluconic acid (KDG) by NADPH-dependent reductase A1-R. KDG is eventually metabolized to glyceraldehyde-3-phosphate and pyruvate through the sequential reaction catalyzed by KDG kinase (A1-K) and 2-keto-3-deoxy 6-phosphogluconate aldolase (A1-A). This review deals with the structure and function of bacterial alginate-metabolizing enzymes, especially structure determinants responsible for the catalytic reaction and mode of action of endotype lyases A1-II, II', III in strain A1 and exotype lyase Atu3025 in Agrobacterium tumefaciens as well as for the coenzyme-binding mode of strain A1 α-keto acid reductase (A1-R).
