respective genes, i.e., acyl CoA dehydrogenase, acyl CoA synthase,
acyl CoA oxidase, and carnitine acyl transferase I [27, 28]. Diatom
Cyclotella cryptica was the first algae engineered to improve fatty
acid production by overexpression of the ACCase gene. However,
in spite of the two- to three-fold increase observed in ACCase
activity, fatty acid accumulation remained constant [29]. The
subsequent attempts to increase lipid content in various microalgal
systems by overexpression of the ACCase were also somewhat
disappointing [30, 31]. This might indicate that the ACCase activity might not be as critical as originally thought or, in better words,
the reaction catalyzed by this enzyme might not be a rate-limiting
step in microalgal fatty acid production pathway. This would necessitate further research to better scrutinize the various aspects of this
enzyme.
Malonyl-CoA ACP transacylase (MAT) has also been recognized as a promising enzyme to be targeted for manipulation of the
fatty acid synthesis pathway. This enzyme catalyzes malonyl-ACP
formation by transferring a malonyl-CoA to the acyl carrier protein
(ACP) [32]. Some successful attempts have been made so far,
through which, lipid contents of different microalgal species were
improved by overexpression of MAT (Table 1).
Following the formation of malonyl-ACP, this complex enters a
series of reactions also known as FAS that leads to the production of
fatty acids. One of the targeted enzymes in FAS complex to achieve
higher lipid production is β-ketoacyl-ACP synthase (KAS), i.e., the
first enzyme in the FAS cycle [24]. However, the overexpression of
KAS in Phaeodactylum tricornutum did not cause any increase in
lipid production [33]. It should be mentioned that contrary to
expectations, KAS overexpression in plants led to 5–10% decreases
in TAG contents [34]. Thus, it could be deduced that KAS would
not be a suitable candidate gene for microalagal metabolic engineering to improve lipid production. In fact, in addition to KAS,
the other subunits of the FAS complex have been found as challenging targets to enhance lipid production in microalgae
[35]. Accordingly, future research works should more thoroughly
take into account the roles and mechanism of action of these
enzymes throughout cellular networks.
Apart from the fatty acid production pathway, manipulation of
the genes involved in the TAG production pathway (Fig. 1) has also
been practiced to increase lipid content in different microalagal
species. TAGs are in fact the major storage lipids in microalgae.
Two main pathways have been proposed for TAG formation:
(1) acylation of diacylglycerol (DAG) via acyl CoA-dependent
pathway (Kennedy Pathway), and (2) acylation of DAG via acyl
CoA-independent pathway (also known as recycling of membrane
lipids into TAGs). A great deal of studies have been focused on
improving the metabolic flux throughput the TAG production
Metabolic Engineering of Microalgae
157
acyl CoA oxidase, and carnitine acyl transferase I [27, 28]. Diatom
Cyclotella cryptica was the first algae engineered to improve fatty
acid production by overexpression of the ACCase gene. However,
in spite of the two- to three-fold increase observed in ACCase
activity, fatty acid accumulation remained constant [29]. The
subsequent attempts to increase lipid content in various microalgal
systems by overexpression of the ACCase were also somewhat
disappointing [30, 31]. This might indicate that the ACCase activity might not be as critical as originally thought or, in better words,
the reaction catalyzed by this enzyme might not be a rate-limiting
step in microalgal fatty acid production pathway. This would necessitate further research to better scrutinize the various aspects of this
enzyme.
Malonyl-CoA ACP transacylase (MAT) has also been recognized as a promising enzyme to be targeted for manipulation of the
fatty acid synthesis pathway. This enzyme catalyzes malonyl-ACP
formation by transferring a malonyl-CoA to the acyl carrier protein
(ACP) [32]. Some successful attempts have been made so far,
through which, lipid contents of different microalgal species were
improved by overexpression of MAT (Table 1).
Following the formation of malonyl-ACP, this complex enters a
series of reactions also known as FAS that leads to the production of
fatty acids. One of the targeted enzymes in FAS complex to achieve
higher lipid production is β-ketoacyl-ACP synthase (KAS), i.e., the
first enzyme in the FAS cycle [24]. However, the overexpression of
KAS in Phaeodactylum tricornutum did not cause any increase in
lipid production [33]. It should be mentioned that contrary to
expectations, KAS overexpression in plants led to 5–10% decreases
in TAG contents [34]. Thus, it could be deduced that KAS would
not be a suitable candidate gene for microalagal metabolic engineering to improve lipid production. In fact, in addition to KAS,
the other subunits of the FAS complex have been found as challenging targets to enhance lipid production in microalgae
[35]. Accordingly, future research works should more thoroughly
take into account the roles and mechanism of action of these
enzymes throughout cellular networks.
Apart from the fatty acid production pathway, manipulation of
the genes involved in the TAG production pathway (Fig. 1) has also
been practiced to increase lipid content in different microalagal
species. TAGs are in fact the major storage lipids in microalgae.
Two main pathways have been proposed for TAG formation:
(1) acylation of diacylglycerol (DAG) via acyl CoA-dependent
pathway (Kennedy Pathway), and (2) acylation of DAG via acyl
CoA-independent pathway (also known as recycling of membrane
lipids into TAGs). A great deal of studies have been focused on
improving the metabolic flux throughput the TAG production
Metabolic Engineering of Microalgae
157