CONSIDERATIONS ON THE STRUCTURE AND BIOCHEMISTRY OF BACTERIAL POLYHYDROXYALKANOIC ACID INCLUSIONS

Some mathematical calculations were done that provided information abo ut the structure and biochemistry of polyhydroxyalkanoic acid (PHA) gr anules and about the amounts of the different constituents that contri bute to the PHA granules. The data obtained from these calculations ar e compared with data from the literature, which show that PHA granules consist not only of the polyester but also of phospholipids and prote ins. The latter are referred to as granule-associated proteins, and th ey are always located at the surface of the PHA granules. A concept is proposed that distinguishes four classes of structurally and function ally different granule-associated proteins: (i) class I comprises the PHA synthases, which catalyze the formation of ester linkages between the constituents; (ii) class Il comprises the PKA depolymerases, which are responsible for the intracellular degradation of PHA, (iii) class III comprises a new type of protein, which is referred to as phasins and which has most probably a function analogous to that of oleosins i n oilseed plants, and (iv) class IV comprises all other proteins, whic h have been found to be associated with the granules but do not belong to classes I-III. Particular emphasis is placed on the phasins, which constitute a significant fraction of the total cellular protein. Phas ins are assumed to form a close protein layer at the surface of the gr anules, providing the interface between the hydrophilic cytoplasm and the much more hydrophobic core of the PHA inclusion.