PHBaccumulating

PHB accumulating refers to the ability of certain microorganisms to synthesize and store polyhydroxybutyrate as intracellular granules when carbon is plentiful but other nutrients are limited. The polymer serves as a carbon and energy reserve and can be mobilized later.

Most PHB-accumulating organisms are bacteria. The classic model is Cupriavidus necator (formerly Ralstonia eutropha), which can accumulate PHB to a large fraction of cell dry weight under nutrient limitation. Other producers include certain Bacillus species and various Rhizobiales; some archaea also form PHB granules. Accumulation levels vary by strain and conditions, with high-yield strains reaching substantial percentages of dry weight.

Biosynthesis involves three enzymes: PhaA condenses two acetyl-CoA molecules to acetoacetyl-CoA; PhaB reduces it to 3-hydroxybutyryl-CoA; PhaC polymerizes it into PHB. Granules are stabilized by phasin proteins (PhaP). PHB can be mobilized by depolymerase PhaZ when needed. Regulation is controlled by carbon flux and nutrient status, especially nitrogen and phosphorus limitation.

Properties and applications: PHB is a biodegradable thermoplastic with properties similar to some petrochemical plastics, but brittleness and processability depend on polymer composition and copolymers such as PHBV. It is of interest for bioplastics, packaging, and medical devices. Production often uses renewable substrates or waste streams to improve sustainability.

Limitations: production costs, processing challenges, and competing plastics. Ongoing research aims to improve yields, create copolymers with desirable properties, and enable use of inexpensive substrates through metabolic engineering and synthetic biology.