A research group at Kyoto University reported that metabolic engineering of a native alginate-metabolizing Sphingomonas sp. A1 cells harboring pyruvate decarboxylase and alcohol dehydrogenase II from Zymomonas mobilis could produce bioethanol from alginate.
A pit in the range of 0.02e0.1 mmwas formed on the surface of the cell in the presence of alginate.
Alginate polymer was directly transported into the inner membrane of the cell through bacterial super channel and periplasmic alginate-binding proteins.
Alginate was transported into the cytoplasm via an ATP-binding cassette (ABC) transporter in the inner membrane.
The alginate was degraded into oligomer and monosaccharides by various alginate lyases inside the cytoplasm.
The unsaturated monosaccharide was nonenzymatically converted to a-keto acid, DEH.
In Sphingomonas sp. A1, DEH is converted to KDG, a metabolite of Entner-Doudoroff (ED) pathway, by NADPH-dependent uronate reductase(Fig. 4).
KDG was further degraded to pyruvate and glyceraldehyde-3-phosphate by kinase A1-K and aldolase A1-A.
Pyruvate was converted to ethanol by pyruvate decarboxylase and alcohol dehydrogenase II.
Metabolically engineered Sphingomonas sp. A1 with a homoethanol pathway could produce 0.25 kg bioethanol/kg alginate with a volumetric productivity of 13 g/L.