Extracellular PHA

Extracellular PHA depolymerases are ubiquitous in the environment (Tokiwa and Calabia 2004). The earliest discovery of PHA-degrading bacteria belongs to... 

Many extracellular PHA depolymerases have been pirtilied from elifferent microorganisms anel/or characterized. The purified PHA depolymerases... 

Due to the differences between the physical structures of the intracellular native granules and extracellular denatured PHA, intracellular PhaZ is unable to hydrolyse extracellular PHA, and extracellular PhaZ cannot hydrolyse intracellular PHA [14]. The enzymatic degradation of P(3HB) is known as a heterogeneous reaction because the polyhydroxybutyrate (PHB) depolymerase is water-soluble, whereas the P(3HB) polymer is water-insoluble. Therefore, the enzymatic degradation of P(3HB) involves two steps, namely adsorption and hydrolysis. During adsorption, the enzyme is attached to the surface of P(3HB) via the binding domain of the polymer. This is followed by hydrolysis of the polymer chain at the active site of the enzyme [5]. 

The extracellular PHA depolymerases have been purified from different isolates, and their properties have been studied. (Mukai et al, 1993, 1994). Almost all... 

The mechanism of extracellular degradation of PHA is not yet well understood. A wide variety of microorganisms (fungi, aerobic and anaerobic bacteria) have been implicated but only Pseudomonas lemoignei, Alcaligenes faecalis, and Comamonas sp. have been studied in detail. Production of extracellular PHA depolymerases is induced by the presence of PHA as the sole carbon source. Generally, a PHB depolymerase acts from the hydroxyl terminus to form dimers (or trimers, depending on the depolymerase) and a trace amoimt of monomer. An... 

Figure 7.14 Immobilization of targeted protein onto PHA using extracellular PHA depolymerase, reprinted from Ihssen et al with permission from the American Chemical Society.

The biodegradation of PHA can occur inside the cell, by microorganisms that produce PHA, or outside the cell by microorganisms that do not produce PHA, but can also use it as carbon and energy sources. However, in both cases the hydrolysis is catalyzed by specific enzymes, intracellular, or extracellular PHA depolymerases. 

Intracellular PHA depolymerases are unable to hydrolyse extracellular PHA, and vice versa. This is due to differences in the two biophysical conformations of PHA when present in different conditions. Within the cell, PHA exists in an amorphous (native) form and is covered by a monolayer of phospholipid embedded with several proteins. Within the cell, such granules are known as native PHA granules or nPHA . After the release of these native granules from the cell either by cell lysis or solvent extraction processes or after the damage of the surface layer, the polymer denatures and becomes crystalline. This type of PHA granules is known as denatured PHA granules or dPHA . ... 

The extracellular PHA depolymerases of many bacteria as well as fungi have been purified and characterized. For further details refer to review articles quoted in the references. 

Intracellular degradation of PHA by depolymerases has not been studied in as much depth as compared to extracellular PHA degradation. C. necator, an organism able to accumulate PHB, was the organism of research to study the intracellular mobilization. This is mediated by a cyclic metabolic route from... 

Native (intact) PHA granules are not degraded by extracellular PHA depolymerases, indicating a structural difference between them and the intracellular depolymerases [169]. 

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