The al., 2010), refined and crude glycerol (Ashby et

The bacteria such as A.
vinelandii UWD (Chou et al., 1997)
and A. eutrophus, A. latus and a mutant Azotobacter
vinelandii (Ojumu
et al., 2004) found to
accumulate PHAs under non-limiting conditions. Biosynthesis of PHA is produces
by microorganis through microbial fermentation from many feed stocks containg
carbon substrates including simple sugars (Huijberts et al., 1992), free
fatty acids (Eggink
et al., 1993), simple alkanes and
alkanols (De
Smet et al., 1983), and
triacylglycerols (Ashby and Foglia, 1998; Solaiman et al., 2002). They are generally categorized into two groups based
on the length of their monomeric side-chains. Short-chain-length PHA (scl-PHA)
polymers consist of 3-hydroxyalkanoic acids with monomeric repeat units of 4–5
carbons, while medium-chain-length PHA (mcl-PHA) polymers are composed of
monomeric repeat units that are C6-C14 carbons in length and it was first
detected in Pseudomonas oleovorans (De Smet et al., 1983) and
later in a variety of Pseudomonads (Solaiman et al, 2006).The only drawback of these
PHAs is their high production cost (waqas et all, 2010) to overcome this the use of
less valuable, renewable materials as substrates for PHA production (Ashby et al.,
2011), as soy molasses (Solaiman et al., 2006), wheat-based
co-products (Koutinas
et al., 2007; Xu et al., 2010), refined and crude glycerol (Ashby et al.,
2004; Cavalheiro
et al,, 2009; Shrivastav et al., 2010; Zhu et al., 2010; Mothes et al., 2007;
Kawata et al., 2010; Ashby et al., 2011).


           The hydrolysis of esters is a reaction of ester with
waters, wherein the molecule is split, and carboxylic acid and alcohol are
formed, that is, the units that form ester (it is an analogous process of
protein hydrolysis – proteins are ‘chemically-wise’ polyamides, linked to the
amide group). Esters are compounds with the ester functional group -COO, which
is on both sides of the group bound to the random organic group (R-COO-R1 ).
PHAs  are formed during esterification.
This is a reaction between carboxylic acid and alcohol. The reaction usually
happens at the presence of a strong acid (such as H2SO4 )
as a catalyst. There are eight pathways have been reported but first pathways
is most common for PHA synthesis. PHA synthases are the key enzymes involved in
PHA biosynthesis. They use coenzyme A (CoA) thioesters of hydroxyalkanoic acids
(HA) as substrates and catalyze the polymerization of HAs into PHA with the
concomitant release of CoA. PHA synthases are specific for the synthesis
of either PHA (SCL) or PHA (MCL) (Rehm and Steinbuchel, 1999). The biosynthetic
pathway of P(3HB) consists of three enzymatic reactions catalyzed by three
different enzymes. The first reaction consists of the condensation of two
acetyl coenzyme A (acetyl-CoA) molecules into acetoacetyl-CoA by b-ketoacylCoA
thiolase (encoded by phbA). The second reaction is the reduction of acetoacetyl-CoA
to (R)-3-hydroxybutyryl-CoA by an NADPH-dependent acetoacetyl-CoA dehydrogenase
(encoded by phbB). Lastly, the (R)-3-hydroxybutyrylCoA monomers are polymerized
into PHB by P(3HB) polymerase, encoded by phbC (Huisman et al., 1989).

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