Cloning, expression and characterization of a phosphoglucomutase/phosphomannomutase from sphingan-producing Sphingomonas sanxanigenens

Several strains of the genus Sphingomonas produce sphingans, extracellular polysaccharides used as thickeners, emulsifiers and gelling agents. The pgmG gene from Sphingomonas sanxanigenens, which encodes a bifunctional protein with phosphoglucomutase and phosphomannomutase activities, was cloned and sequenced. The predicted amino acid sequence of the PgmG protein possessed 460 amino acids and a calculated molecular mass of 49.8 kDa, and it was 80 % identical to PGM/PMM from S. elodea. We overexpressed pgmG in Escherichia coli, and the purified protein displayed a K ₘ of 0.2 mM and a V ₘₐₓ of 1.3 μmol min⁻¹ mg⁻¹ with glucose 1-phosphate as substrate. The catalytic efficiency (K cₐₜ/K ₘ) of PgmG was about 15-fold higher for glucose 1-phosphate than for mannose 1-phosphate. Overexpression of pgmG in S. sanxanigenens resulted in a 17 ± 0.3 % increase in sphingan production to ~12.5 g l⁻¹.