Low molecular weight complexed poly(3-hydroxybutyrate): a dynamic and versatile molecule in vivo.

It is increasingly clear that poly(3-hydroxybutyrate) (PHB) is not just an inert storage polymer, confined to certain bacteria, but a ubiquitous, interactive, solvating biopolymer involved in important physiological functions. Low molecular weight PHB, complexed to other macromolecules (c-PHB), is widely distributed in biological cells, being found in representative organisms of nearly all phyla. Complexation modifies the physical and chemical properties of c-PHB, allowing it to pervade aqueous as well as hydrophobic regions of the cell, and as a result c-PHB can be found in cytoplasm and intracellular fluids as well as in membranes and lipoproteins. The lipidic homopolymer associates with other macromolecules primarily via its ester carbonyl oxygens, which can act as hydrogen-bond acceptors or as ligands for coordinate bonds to cations. The spacing of the electron-donating groups along the flexible backbone allows for multiple bonding interactions, and forms the basis for the ability of c-PHB to bind to proteins, or to form ion-conducting complexes with salts. The singular ability of c-PHB to dissolve salts and facilitate their transfer across hydrophobic barriers defines a potential physiological niche for c-PHB in cell metabolism.