Alginate Production: Precursor Biosynthesis, Polymerization and Secretion
The activated intracellular precursor of alginate biosynthesis is GDP-mannuronic acid. Carbon sources are oxidized to acetyl-CoA, which enters the citric acid cycle, providing the intermediate oxaloacetate. Oxaloacetate is converted via gluconeogenesis into fructose 6-phosphate. The central metabolite fructose 6-phosphate derived from gluconeogenesis is then converted to the activated alginate precursor GDP-mannuronic acid employing alginate-specific biosynthesis enzymes. This conversion requires four enzymatic steps catalysed by bifunctional phosphomannose isomerase:GDP-mannose pyrophosphorylase, phosphomannomutase and GDP-mannose dehydrogenase. GDP-mannuronic acid is polymerized to alginate by a membrane-anchored glycosyltransferase which is presumably represented by Alg8, which has been suggested to be a subunit of a multiprotein complex spanning the cytoplasmic membrane (Alg44), the periplasm (AlgX, AlgK, AlgG, AlgL) and the outer membrane (AlgE). These periplasmic proteins have been proposed to form a scaffold guiding the nascent alginate chain through the periplasm to the alginate-specific channel protein AlgE in the outer membrane. AlgE has been suggested to facilitate export of alginate through the outer membrane. The extended periplasmic C terminus of membrane-anchored Alg44 shows similarities to membrane fusion proteins and might colocalize the alginate polymerase (Alg8) with the export protein AlgE. The cytoplasmic N-terminal loop of Alg44 comprises a PilZ domain required for binding of the secondary messenger bis-(3′–5′)-cyclic dimeric guanosine monophosphate, the binding of which was found to be required for alginate production.
Alginates: Biology and Applications
Biochemistry and Cell Biology not elsewhere classified