141
O-acetyl-esterified at O-2 or O-3. Sometimes one or two α-1, 2-linked
l- rhamnopyranose units can join the smooth region and can be seen in most of the
pectin structures. Additionally at C-2 or C-3 positions, GalA units may be substituted with xylose or apiose residues, forming domains known as xylogalacturonan
or apiogalacturo-nan, respectively. As a result of these modifications, the functional
properties of HGA are altered.
Rhamnogalacturonan I (RG-I)
In case of RG-I, 100 repeating units of the dis-accharide α-1,2-linked-lrhamnose -α-1,4-d-GalA are linked. In other words, every twenty-fifth GalA of the
main chain is substituted with rhamnose and the backbone comprising of the repeating units of α-1,2-linked-l-rhamnose -α-1,4-d-GalA (Fig. 4ii). More that 30% of the
pectin is comprised of the rhamnose residues (Renard et al. 1995). It has been estimated that 50–78% of the rhamnose is RGI. But the GalA residues of RG-I can be
O-acetyl- esterified and not methyl-esterfied (Caffall and Mohnen 2009). It has been
reported that 20–80% of rhamnose residues have neutral side chains substituted at
the C-4 position. There can even be the attachments of side chains of neutral sugars
at the C-2, C-3 and/or C-4 positions of rhamnose residues. The major part of these
side chains include arabinose, forming galactan, arabinogalactans, ara-binan and
galactose. The covalent linkage of the other sugars like glucose, mannose, fucose,
xylose, and glucuronic acid can also be found as side chains. The neutral side chains
can be made up of a single glycosyl residue up to 50 or more, which can result in
polysaccharide having high branching and complexity (Voragen et al. 2009).The
massive branching characteristics of RG-I is the reason that it is called as “hairy
region”.
Rhamnogalacturonan II (RG-II)
One of the most striking features of the RG-II is that it is the only domain of pectin
that does not have structural diversification. It consists of around nine α-1, 4-linked
GalA units having four different polymeric side chains. Some of the side chains
may be methyl- esterfied and these contain eleven rare sugars that include
2-O-methyl-d-xylose,
3-deoxy-d-manno-octulosonic
acid,
3-deoxy-dlyxoheptulosaric acid, 2-O-methyl-l-fucose, 3-C-carboxy-5-deoxy-l-xylose (aceric
acid) and apiose. Its cleavage by endopolygalactouronase signifies its covalent
bonding to HGA and it also corsslinks the two pectin molecules (apiosyl residues)
within the cell wall by borate ester links. RG-II has whole togetherly different structure than RG-I athough their names resemble in sequence and it is also included in
the hairy region. The ends of RG-II are believed to be glycosidically linked to the
HGA domains (Fig. 4iii).
Pectin
O-acetyl-esterified at O-2 or O-3. Sometimes one or two α-1, 2-linked
l- rhamnopyranose units can join the smooth region and can be seen in most of the
pectin structures. Additionally at C-2 or C-3 positions, GalA units may be substituted with xylose or apiose residues, forming domains known as xylogalacturonan
or apiogalacturo-nan, respectively. As a result of these modifications, the functional
properties of HGA are altered.
Rhamnogalacturonan I (RG-I)
In case of RG-I, 100 repeating units of the dis-accharide α-1,2-linked-lrhamnose -α-1,4-d-GalA are linked. In other words, every twenty-fifth GalA of the
main chain is substituted with rhamnose and the backbone comprising of the repeating units of α-1,2-linked-l-rhamnose -α-1,4-d-GalA (Fig. 4ii). More that 30% of the
pectin is comprised of the rhamnose residues (Renard et al. 1995). It has been estimated that 50–78% of the rhamnose is RGI. But the GalA residues of RG-I can be
O-acetyl- esterified and not methyl-esterfied (Caffall and Mohnen 2009). It has been
reported that 20–80% of rhamnose residues have neutral side chains substituted at
the C-4 position. There can even be the attachments of side chains of neutral sugars
at the C-2, C-3 and/or C-4 positions of rhamnose residues. The major part of these
side chains include arabinose, forming galactan, arabinogalactans, ara-binan and
galactose. The covalent linkage of the other sugars like glucose, mannose, fucose,
xylose, and glucuronic acid can also be found as side chains. The neutral side chains
can be made up of a single glycosyl residue up to 50 or more, which can result in
polysaccharide having high branching and complexity (Voragen et al. 2009).The
massive branching characteristics of RG-I is the reason that it is called as “hairy
region”.
Rhamnogalacturonan II (RG-II)
One of the most striking features of the RG-II is that it is the only domain of pectin
that does not have structural diversification. It consists of around nine α-1, 4-linked
GalA units having four different polymeric side chains. Some of the side chains
may be methyl- esterfied and these contain eleven rare sugars that include
2-O-methyl-d-xylose,
3-deoxy-d-manno-octulosonic
acid,
3-deoxy-dlyxoheptulosaric acid, 2-O-methyl-l-fucose, 3-C-carboxy-5-deoxy-l-xylose (aceric
acid) and apiose. Its cleavage by endopolygalactouronase signifies its covalent
bonding to HGA and it also corsslinks the two pectin molecules (apiosyl residues)
within the cell wall by borate ester links. RG-II has whole togetherly different structure than RG-I athough their names resemble in sequence and it is also included in
the hairy region. The ends of RG-II are believed to be glycosidically linked to the
HGA domains (Fig. 4iii).
Pectin