Biomedical Engineering Reference
In-Depth Information
chains, the PHA granules enlarge in size, and then the surfaces of PHA granules
become covered by PhaP and other specifi c proteins before the other nonspecifi c
proteins bind to the PHA granules. Under these conditions, the authors concluded
that PhaR is a sensor for PHA synthesis in the cell.
According to the conventional classifi cation of PHA granule-associated proteins
proposed by Steinbü chel
et al
. [66], the following four distinct proteins can be
defi ned functionally: class I comprises the PHA synthases, which catalyze the
polymerization of the monomers of hydroxyacyl-CoA; class II comprises the PHA
depolymerases, which are responsible for the intracellular degradation and mobi-
lization of PHA; class III comprises the phasins (designated as PhaP), which
probably form a protein layer at the surface of the PHA granule with phospholi-
pids, lipids, and other proteins; and class IV comprises all other proteins. Figure
2.1 shows a likely model for the PHA granules.
Based on these observations, two models have been proposed for granule forma-
tion. The fi rst one is the micelle model, in which the extended PHA chains cova-
lently attached to the synthase aggregate initially into a micelle structure [56, 67].
The physical properties of the polymer are thus proposed to be the driving force
for inclusion formation. The second model is the maturing model that was pro-
posed by Stubbe and Tian [68], in which the hydrophobic synthase binds to the
inner face of the plasma membrane, leading to a granule surface covered with a
lipid monolayer. In this model, the biology of the system and the physical proper-
ties of the polymer are required for granule formation.
Figure 2.1
Model for the PHA granules. PhaC
protein is a PHA synthase, PhaP protein is a
phasin, PhaZ protein is a PHA depolymerase,
PhaR protein is a sensor for PHA synthesis in
the cell, the phospholipidic monolayer, and
other proteins that surround the granule
(based on [65] , with modifi cations).
