95
D-galacturonic acid and α-1,2-linked rhamnose residue (Richard and Hilditch
2009). In the “hairy region,” xylogalacturonan comprising of D-xylose-substituted
galacturonan backbone and rhamnogalacturonan can be recognized (de Vries and
Visser 2001).
In fungus, D-galacturonic acid is catabolized through a pathway with four
enzymes and two reduction steps, whereas five enzymes are involved in catabolic
pathway for bacteria (Khosravi et al. 2015). The Initial steps of degradation in
eukaryotes invovle NADPH-dependent D-galacuronic reductase which is coded as
gaaA in A. niger or gar1 in T. reesei which govern the conversion of D-galacturonate
to L-galacturonic acid by utilizing NADPH or NADH as electron doner (Kuorelahti
et al. 2006). In this study, D-galacturonic reductase can use NADPH and NADH but
it showed a higher affinity to NADPH (Richard and Hilditch 2009). L-Galactonic
acid serves as intermediate in the eukaryotic route for D-galacturonic acid metabolism (Kuivanen et al. 2012). L-Galactonate dehydratase (LGD1) coded as gaaB in
A. niger further converts L-galactonic acid to 2-keto-3-deoxy-L-galactonic acid
(Biz et al. 2016). Water molecules from L-galactonic acid were split by dehydratase
to form this molecule (Kuorelahti et al. 2006). The third step in the pathway is catalyzed by 2-keto-3-deoxy-L-galactonate aldolase coded as GaaC in A. niger which
cut 2-keto-3-deoxy-L-galactonate between carbon 3 and 4 into pyruvate and
L-glyceraldehyde (Biz et al. 2016). Pyruvate can be utilized by different pathways
like the citric acid cycle, while L-glyceraldehyde cannot be utilized in any known
pathway. However, a specific NADPH-dependent glyceraldehyde reductase coded
as gaaD in A. niger has been reported in conversion of L-glyceraldehyde to glycerol
using NADPH as electron donor (Khosravi et al. 2015; Biz et al. 2016). In the
eukaryotic route, the second and fourth enzymes are activating only for one direction, whereas the first and third enzymes are reversible (Richard and Hilditch 2009).
Considering pectin as a crucial carbon source for bacteria and also fungi living
on decaying plant material, the metabolic pathway was concentrated on the main
backbone of pectin which was D-galacturonic acid (GalA) (Martens-Uzunova and
Schaap 2008). The metabolic pathway of filamentous fungi was different from the
bacteria, and a non-phosphorolytic pathway was proposed, leading to glyceraldehydes and pyruvate as end product of the pathway (Martens-Uzunova and Schaap
2008) (Fig. 6.2). A study from Sealy-Lewis and Fairhurst (1992) showed in filamentous fungus that an NADPH-dependent D-glyceraldehyde (D-GAD) reductase was
induced on D-galacturonate and an NADP
+
-dependent glycerol dehydrogenase was
also then discovered to induce on D-galactonate and helped in reducing glycerol.
6.3.3 Heterologous Pectinase Production
In order to meet the increasing demand for the production of pectinase enzyme,
there are several approaches applied in its production. There have been a number of
longitudinal studies in increasing the production through optimization methods as
6 Fungal Pectinases: Production and Applications in Food Industries
D-galacturonic acid and α-1,2-linked rhamnose residue (Richard and Hilditch
2009). In the “hairy region,” xylogalacturonan comprising of D-xylose-substituted
galacturonan backbone and rhamnogalacturonan can be recognized (de Vries and
Visser 2001).
In fungus, D-galacturonic acid is catabolized through a pathway with four
enzymes and two reduction steps, whereas five enzymes are involved in catabolic
pathway for bacteria (Khosravi et al. 2015). The Initial steps of degradation in
eukaryotes invovle NADPH-dependent D-galacuronic reductase which is coded as
gaaA in A. niger or gar1 in T. reesei which govern the conversion of D-galacturonate
to L-galacturonic acid by utilizing NADPH or NADH as electron doner (Kuorelahti
et al. 2006). In this study, D-galacturonic reductase can use NADPH and NADH but
it showed a higher affinity to NADPH (Richard and Hilditch 2009). L-Galactonic
acid serves as intermediate in the eukaryotic route for D-galacturonic acid metabolism (Kuivanen et al. 2012). L-Galactonate dehydratase (LGD1) coded as gaaB in
A. niger further converts L-galactonic acid to 2-keto-3-deoxy-L-galactonic acid
(Biz et al. 2016). Water molecules from L-galactonic acid were split by dehydratase
to form this molecule (Kuorelahti et al. 2006). The third step in the pathway is catalyzed by 2-keto-3-deoxy-L-galactonate aldolase coded as GaaC in A. niger which
cut 2-keto-3-deoxy-L-galactonate between carbon 3 and 4 into pyruvate and
L-glyceraldehyde (Biz et al. 2016). Pyruvate can be utilized by different pathways
like the citric acid cycle, while L-glyceraldehyde cannot be utilized in any known
pathway. However, a specific NADPH-dependent glyceraldehyde reductase coded
as gaaD in A. niger has been reported in conversion of L-glyceraldehyde to glycerol
using NADPH as electron donor (Khosravi et al. 2015; Biz et al. 2016). In the
eukaryotic route, the second and fourth enzymes are activating only for one direction, whereas the first and third enzymes are reversible (Richard and Hilditch 2009).
Considering pectin as a crucial carbon source for bacteria and also fungi living
on decaying plant material, the metabolic pathway was concentrated on the main
backbone of pectin which was D-galacturonic acid (GalA) (Martens-Uzunova and
Schaap 2008). The metabolic pathway of filamentous fungi was different from the
bacteria, and a non-phosphorolytic pathway was proposed, leading to glyceraldehydes and pyruvate as end product of the pathway (Martens-Uzunova and Schaap
2008) (Fig. 6.2). A study from Sealy-Lewis and Fairhurst (1992) showed in filamentous fungus that an NADPH-dependent D-glyceraldehyde (D-GAD) reductase was
induced on D-galacturonate and an NADP
+
-dependent glycerol dehydrogenase was
also then discovered to induce on D-galactonate and helped in reducing glycerol.
6.3.3 Heterologous Pectinase Production
In order to meet the increasing demand for the production of pectinase enzyme,
there are several approaches applied in its production. There have been a number of
longitudinal studies in increasing the production through optimization methods as
6 Fungal Pectinases: Production and Applications in Food Industries