52
dimethylallyl diphosphate. They are consecutively condensed producing geranyl,
farnesyl, and geranylgeranyl diphosphate; the series of reactions are catalyzed by
enzymes geranyl diphosphate synthase, farnesyl diphosphate synthase, and geranylgeranyl diphosphate synthase, respectively (Liao et al. 2016).
These carbon precursors are rapidly transformed into different terpenoids, as
carotenoids and their oxidative and enzymatic cleavage products, for example, volatile organic compounds as α-ionone, β-ionone, and β-cyclocitral (Durme et al. 2013;
Santos et al. 2016a; Lee et al. 2017; Hosoglu 2018).
Through geranyl diphosphate, the sesquiterpenes can be formed as
2- methylisoborneol (Watson et al. 2016; Lee et al. 2017). In microalgae system, the
cyclization of farnesyl diphosphate can produce geosmin, in three distinct phases:
where farnesyl diphosphate form germacradienol which is converted the
8,10-dimethyl-1-octalin to form geosmin finally, this mechanism is catalyzed by
geosmin synthase (Giglio et al. 2008).
In the keto acid pathway, any longer-structure keto acid can be decarboxylated
and reduced to higher alcohols. This reaction comprises chain extension at the level
of 2-keto acids, which in turn are utilized as building blocks in branched-chain
amino acid synthesis. Keto acid structure extension is catalyzed through the acetohydroxyacid enzyme in the leucine synthesis pathway, or the valine, followed by
reactions of isomerization, reduction, dehydration, and esterification. Besides, it can
produce aldehydes, carboxylic acids, esters, and alcohols. For example, in the isobutanol, 2-methyl-1-butanol, 3-methyl-1-butanol, and 1-butanol, the reaction can be
extended to form 1-hexanol and other alcohols (Hasegawa et al. 2012; Lan and Liao
2012; Liao et al. 2016).
The de novo fatty acid pathway starts with acetyl-CoA using malonyl-CoA as a
building block based on cyclic series mechanisms catalyzed by the multienzyme
system, denominated fatty acid synthase (Peralta-Yahya et al. 2012; Zhou et al.
2018). Aldehydes, hydrocarbons, and ketones can be fatty acid derivatives (Santos
et al. 2016a).
Aldehyde compounds 2,4-decadienal and 2,4,7-decatrienal are degradation
products of arachidonic or eicosapentaenoic acid, catalyzed by lipoxygenase/hydroperoxide lyase. The fatty acids, linoleic or linolenic acid, are the precursors of aldehydes such as nonanal, hexanal, and 2-pentanal (Adolph et al. 2003; Yu et al. 2014;
Santos et al. 2016b; Jerković et al. 2018). The alkanes such as heptadecane and
pentadecane, along with alkenes, presumably derived from unsaturated fatty aldehydes and aliphatic ketones can be lipid oxidation products (Schirmer et al. 2010;
Santos et al. 2016a, b).
Sulfur compounds are another group of volatile organic compounds that are
released by many microalgae, such as dimethyl sulfide, dimethyl disulfide, and
dimethyl trisulfide, generated by a diversity of biota, biochemical pathways,
enzymes, and precursors. Interestingly, in microalgae (in species that this pathway
has been reported), dimethyl sulfide is dependent of dimethylsulfoniopropionate by
senescing microalgae (Giordano et al. 2005; Achyuthan et al. 2017).
In the dimethylsulfoniopropionate biosynthesis, the methionine is the forerunner
of the 2-keto acid, 4-methylthio-2-oxobutyrate, through a transamination (perhaps
I. A. Severo et al.
dimethylallyl diphosphate. They are consecutively condensed producing geranyl,
farnesyl, and geranylgeranyl diphosphate; the series of reactions are catalyzed by
enzymes geranyl diphosphate synthase, farnesyl diphosphate synthase, and geranylgeranyl diphosphate synthase, respectively (Liao et al. 2016).
These carbon precursors are rapidly transformed into different terpenoids, as
carotenoids and their oxidative and enzymatic cleavage products, for example, volatile organic compounds as α-ionone, β-ionone, and β-cyclocitral (Durme et al. 2013;
Santos et al. 2016a; Lee et al. 2017; Hosoglu 2018).
Through geranyl diphosphate, the sesquiterpenes can be formed as
2- methylisoborneol (Watson et al. 2016; Lee et al. 2017). In microalgae system, the
cyclization of farnesyl diphosphate can produce geosmin, in three distinct phases:
where farnesyl diphosphate form germacradienol which is converted the
8,10-dimethyl-1-octalin to form geosmin finally, this mechanism is catalyzed by
geosmin synthase (Giglio et al. 2008).
In the keto acid pathway, any longer-structure keto acid can be decarboxylated
and reduced to higher alcohols. This reaction comprises chain extension at the level
of 2-keto acids, which in turn are utilized as building blocks in branched-chain
amino acid synthesis. Keto acid structure extension is catalyzed through the acetohydroxyacid enzyme in the leucine synthesis pathway, or the valine, followed by
reactions of isomerization, reduction, dehydration, and esterification. Besides, it can
produce aldehydes, carboxylic acids, esters, and alcohols. For example, in the isobutanol, 2-methyl-1-butanol, 3-methyl-1-butanol, and 1-butanol, the reaction can be
extended to form 1-hexanol and other alcohols (Hasegawa et al. 2012; Lan and Liao
2012; Liao et al. 2016).
The de novo fatty acid pathway starts with acetyl-CoA using malonyl-CoA as a
building block based on cyclic series mechanisms catalyzed by the multienzyme
system, denominated fatty acid synthase (Peralta-Yahya et al. 2012; Zhou et al.
2018). Aldehydes, hydrocarbons, and ketones can be fatty acid derivatives (Santos
et al. 2016a).
Aldehyde compounds 2,4-decadienal and 2,4,7-decatrienal are degradation
products of arachidonic or eicosapentaenoic acid, catalyzed by lipoxygenase/hydroperoxide lyase. The fatty acids, linoleic or linolenic acid, are the precursors of aldehydes such as nonanal, hexanal, and 2-pentanal (Adolph et al. 2003; Yu et al. 2014;
Santos et al. 2016b; Jerković et al. 2018). The alkanes such as heptadecane and
pentadecane, along with alkenes, presumably derived from unsaturated fatty aldehydes and aliphatic ketones can be lipid oxidation products (Schirmer et al. 2010;
Santos et al. 2016a, b).
Sulfur compounds are another group of volatile organic compounds that are
released by many microalgae, such as dimethyl sulfide, dimethyl disulfide, and
dimethyl trisulfide, generated by a diversity of biota, biochemical pathways,
enzymes, and precursors. Interestingly, in microalgae (in species that this pathway
has been reported), dimethyl sulfide is dependent of dimethylsulfoniopropionate by
senescing microalgae (Giordano et al. 2005; Achyuthan et al. 2017).
In the dimethylsulfoniopropionate biosynthesis, the methionine is the forerunner
of the 2-keto acid, 4-methylthio-2-oxobutyrate, through a transamination (perhaps
I. A. Severo et al.