Vidnoye, г. Москва и Московская область, Россия
Vidnoye, г. Москва и Московская область, Россия
Vidnoye, г. Москва и Московская область, Россия
Introduction. The functional basis of protopectin complex can be represented as a network of regions that consist of homogalacturonan sequences and a base of rhamnogalacturonans-I, i.e. rhamnosyl-containing branching sites. Enzymatic isolation of these regions is possible only at a certain minimal native degree of polymerization. The research objective was to develop a system of criteria for assessing the potential applicability of the enzymatic transformation of plant protopectin complex. Study objects and methods. The research featured the polymerization degree of the homogalacturonan regions of the protopectin complex and produced a system of assessment criteria for the enzymatic fragmentation potential of the protopectin complex. The theoretical calculations were based on the values of the mass fractions of rhamnosyl and galacturonide residues in plant cell walls. The result was a new polymerization degree analytical function. Results and discussion. The ratio of the mass fractions of rhamnosyl and galacturonide residues in the water-insoluble plant tissue served as a dimensionless criterion of applicability. The rational condition for the dimensionless criterion of applicability was based on the fundamental constraint for homogalacturonan regions in the protopectin complex. It was expressed by a fundamental inequation. The rational area for determining the numerical values of the applicability criterion was presented as . The functional dependence was reduced to a two-dimensional criteria space as “width of rhamnosyl branches vs. the criterion of applicability”, where each pectin-containing raw material was given a single uniquely defined position. The boundary conditions for the criteria space were determined analytically. Conclusion. The new approach offers an assessment of the enzymatic fragmentation potential of the plant protopectin complex by homoenzyme preparations. The approach is in fact the second stage of the decision tree in the science-based technology for pectin and its products.
Protopectin complex, rhamnogalacturonan-I, homogalacturonan, transformation, criterion assessment
INTRODUCTION
The biopolymer complex of plant tissue cell walls
is a complex conglomerate of intertwined branched
supramolecular networks of the protopectin complex
and the hemicellulose. The complex is permeated with
cellulose microfibrils and protein extensin (Fig. 1) [1, 2].
All its components are linked to each other by ester,
salt, combined, and hydrogen bonds. Each component
possesses valuable physicochemical properties with a
good potential for food industry [3–7].
Pectins have the most attractive and numerous
functional properties among all the carbohydrates of
plant cell walls [5, 8]. They owe these useful properties
due to their molecular structure. In their native form,
pectins have a water-insoluble supramolecular structure
called the protopectin complex. The structure is an
extended and highly branched linear and lateral network
of polymer fragments (Fig. 2). Lateral branches also
have a complex structure and can be interconnected with
salt and borate bonds [9–18].
Contemporary science knows eight types of
fragments of the protopectin complex: homo-galacturonan,
rhamnogalacturonan-I, rhamnogalacturonan-
II, xylogalacturonan, apiogalacturonan, and
arabinogalacturonan [19].
Homogalacturonans are linear polymeric fragments
of α-D(+)-galacturonic acid residues, linked by
(1 → 4)-glycosidic bonds (Fig. 3) [19, 20]. Each residue
contains a carboxyl group, which naturally may exist
in a free, esterified, or amidated state. Free carboxyl
groups are capable of dissociation, while acquiring a
partial negative charge. Carboxyl groups esterified with
methanol demonstrate inactivated charge formation.
Amidated carboxyl groups, due to the donor-acceptor
bond of the lone-pair electrons, accept cation H+ and
acquire a partial positive charge.
In positions C1 and C2, hydroxyl groups can form
glycosidic bonds with the residues of xylose, ribose,
arabinose, and galactose, as well as ester bonds with
carboxylic acids and aromatic compounds. The state and
total amount of carboxyl groups in the pectin molecule
fragment define the physicochemical properties
of pectins, while the degree and the nature of the
substitution of hydroxyl groups define the inhibition
degree.
The practical use of pectins depends on the chemical
structure of homogalacturonans.
Ramnogalacturonan-I is the second most common
fragment of pectins. Its content can reach 45% in
sugar beet pectin [5, 19, 20]. These fragments include
sequences from the residue of α-L-rhamnose and
α-D(+)-galacturonic acid, linked by a (1 → 4)-glycosidic
bond. In the rhamnosyl residue, the pair can be linked
with other pairs or with the end of the homogalacturonan
by a rhamnosyl-uronic (1 → 2)-glycosidic bond. In the
uronic residue, the pair can be linked with other pair
by a rhamnosyl-uronic (1 → 2)-glycosidic bond or
with the end of homogalacturonan by a uronic-uronic
(1 → 4)-glycosidic bond. As a result, rhamnosyl residues
of rhamnogalacturonan-I are the branching zones of the
pectin molecule, where free functional groups can form
glycosidic bonds with either residues of neutral sugars,
or their polymer sequences, i.e. arabinans, galactans,
arabinogalactans, and galactoarabinans-I and II (Fig. 4).
The basis of the protopectin complex of plant
tissue cell walls is a network of regions formed
by linear sequences of homogalacturonans and
rhamnogalacturonans-I. Of course, this assumption
excludes two types of lateral branches: the rhamnosilfree
lateral branches (rhamnogalacturonan-II),
which may contain residues of L-rhamnose
and/or α-D(+)-galacturonic acid with proportion
of ≤ 2–3%, and branches formed by neutral sugars
and their oligo- and polymers [16, 19]. Molecular
properties of homogalacturonan fragments define the
physicochemical properties of plant pectin. Therefore,
enzymatic fragmentation is the most effective method
for the protopectin complex. It is a selective hydrolytic
cleavage of rhamnosyl-uronide (1 → 2) and (1 → 4)
glycosidic bonds.
Figure 1 Primary cell wall of higher plants [1]
However, the physicochemical properties of pectin
also depend on the polymerization degree of the
fragmentation products [21]. The maximal possible
degree of polymerization depends on the polymerization
degree of the native homogalacturonan fragments in
the protopectin complex. In each specific case, the
experimental determination of this indicator is a difficult
resource- and time-consuming task.
Therefore, a criteria assessment would be the
optimal approach to evaluate the potential efficiency
of the directed enzymatic fragmentation of a particular
plant protopectin complex. Such assessment can also
define the boundary conditions that determine the
degree of the targeted physicochemical properties of
the fermentolysis products. This approach could also
determine the conditions for processing any plant tissue
or its derivatives. The approach consists of some stepby-
step stages. The first stage was a system of criteria
for assessing the transformation potential of a plant
biopolymer complex [22].
As a next stage, the present research objective was to
develop a system of criteria for assessing the enzymatic
transformation potential of a plant biopolymer complex
as in the case of pectin substances. The research
included the following tasks:
– developing the abovementioned assessment criteria
system, based on the use of zoned criteria space;
– developing a system of boundary conditions for the
classification of plant raw materials according to the
applicability of the enzymatic transformation of its
protopectin complex.
STUDY OBJECTS AND METHODS
The protopectin complex of the plant tissue consists
of three main types of fragments: homogalacturonan,
rhamnogalacturonan-I, and rhamnogalacturonan-II. The
latter type was disregarded as its mass fraction in the
protopectin complex is ≤ 2%.
Rhamnogaracturonan-I has linear polynalacturonan
sites. As a result, the homogalanic component of the
protopectin complex can be considered as part of
rhamnogalacturonan-I fragments.
A pectin molecule can be classified as
rhamnogalacturonan-I only if, in addition to the
homogalacturonan component, it contains at least
one branch formed by at least one rhamnosyl residue.
Consequently, a polymer molecule has at least two
homogalacturonan regions with at least one terminal
link (rhamnosyl residue) each.
Linear and homogalacturonan regions of the
molecular network alternate in the protopectin complex
in a particular order. This order presumably depends
on the taxonomy of the raw material and the function
Figure 4 Fragment of rhamnogalacturonan-I of pectin molecule [19]. Lateral branches: A – arabinan, B – galactan,
C – arabinogalactan, D – galactoarabinan
Figure 3 Homogalacturonan fragment of pectin molecule [20]
of the plant parts. The structural features of the
fragments of rhamnogalacturonan-I are such that the
natural boundaries of the homogalacturonan regions
are L-rhamnose residues connected to the terminal
uronid links (1 → 2) and (1 → 4) by glycosidic bonds.
The fragment can be roughly described by the following
sequence: “terminal link of homogalacturonan –
rhamnose residue (the branching starts) – branching
site – rhamnose residue (the branching ends) –
homogalacturonan region – … – section of
homogalacturonan – rhamnose residue (the branching
starts) – branching site – rhamnose residue (the
branching ends) – terminal link of homogalacturonan”.
In the simplest case, the rhamnogalacturonan-I
fragment has only one branching site ( 1 r b = ). Depending
on its structure, the rhamnogalacturonan-I can include
only one rhamnosyl residue ( 1 Rh z = ). In a more complex
case, the rhamnogalacturonan-I may contain several
rhamnosyl residues (zRh = q, where q = 1, 2, 3, ...), which
alternate with galacturonid residues (Fig. 5).
The number of branching sites may also depend, to
some extent, on the plant species and the functional type
of the plant tissue.
Figure 5 features no fragments of rhamnogalacturonan-
I as their lateral branches are represented
mainly by the nonuronic component.
The conditional assumption is that the uronidecontaining
part of rhamnogalacturonan-I is completely
determined by the following variables: HG n is total
homogalacturonan sites, Rh n is total rhamnosyl units
in the branching sites, GalA(b) n is total uronid residues in
the branching sites, Rh z is number of rhamnosyl residues
per branching site, GalA(b) z is number of uronid residues
per branching site, and br n is total branch sites. Table 1
demonstrates the numerical values of the variables in
particular cases of the distribution of homogalacturonan
and branching sites in Fig. 5.
The ratios in Table 1 can be expressed by the
following formulae:
( 1) Rh HG Rh n = n − ⋅ z , (1)
Figure 5 Distribution of homogalacturonan and branching sites in rhamnogalacturonan-I at br = 1–4. Not to scale. а) 2 Rh z = ;
b) 3 Rh z = ; c) 4 Rh z =
Table 1 Particular cases of the distribution of variables that determine the structure of rhamnogalacturonan-I,
at different values of br
Number
of branching
sites, r b
Cases
А B C
HG n Rh n GalA(b) n HG n Rh n GalA(b) n HG n Rh n GalA(b) n
1 2 2 1 2 3 2 2 4 3
2 3 4 2 3 6 4 3 8 6
3 4 6 3 4 9 6 4 12 9
4 5 8 4 5 12 8 5 16 12
… ... … … … … … … … …
br n ( 1) 2 Rh HG n = n − ⋅
( ) ( 1) 1 GalA b HG n = n − ⋅
( )
1
2
Rh
GalA b
n
n
⋅
=
( 1) 3 Rh HG n = n − ⋅
( ) ( 1) 2 GalA b HG n = n − ⋅
( )
2
3
Rh
GalA b
n
n
⋅
=
( 1) 4 Rh HG n = n − ⋅
( ) ( 1) 3 GalA b HG n = n − ⋅
( )
3
4
Rh
GalA b
n
n
⋅
=
a b
c
homogalacturonan region
rhamnosyl residue
α-(D+)-galacturonic acid residue
nGalA(b) = (nHG −1) ⋅ zGalA(b) (2)
The structure of the rhamnogalacturonan-I
fragments suggests that the main structural unit is the
amount of rhamnosyl residues in the branching sites. As
a result, formulae (1) and (2) take the following form:
Rh 1 Rh Rh
HG
Rh Rh
n n z
n
z z
+
= + = , (3)
( )
( )
Rh GalA b
GalA b
Rh
n z
n
z
⋅
= (4)
Based on the data in Table I,
( ) 1 GalA b Rh z = z − (5)
Thus, the final formula (4) is:
( )
( 1) Rh Rh
GalA b
Rh
n z
n
z
⋅ −
= (6)
These dependences give an approximate quantitative
idea of the structure of rhamnogalacturonan-I. For their
practical use, they have to be linked to the real chemical
composition of a particular raw material.
The line of reasoning follows the next path.
Considering that the molecular weight of the
rhamnosyl residue is Rh M (Da) and the mass fraction of
rhamnose in the composition of the natively insoluble
part of the raw material is Rh ω (%), the amount of
rhamnosyl residues in the mass of the natively insoluble
part of the raw material m (g) can be calculated
according to the formula below:
100
Rh
Rh
Rh
m
n
M a
⋅ω
=
⋅ ⋅
(7)
where a is the atomic mass unit (1.66053892×10–24
g/Da).
A combination of formulae (6) and (7) gives the
number of moles of α-D(+)-galacturonic acid residues in
the branch sites:
( )
( 1)
100 ( 1)
100
Rh
Rh
Rh Rh Rh
GalA b
Rh Rh Rh
m z
M a m z
n
z M z a
ω
ω
⋅
⋅ −
⋅ ⋅ ⋅ ⋅ −
= =
⋅ ⋅ ⋅
(8)
Consequently, the mass fraction of α-D(+)-
galacturonic acid residues in the insoluble part of the
raw material in the branching sites is:
( )
( )
100 100 ( 1)
100
GalA GalA b GalA Rh Rh
GalA b
Rh Rh
M n a M a m z
m m M z a
ω
ω
⋅ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅ −
= = ⋅ ⇒
⋅ ⋅ ⋅
×
× 100 100 ( 1)
100
GalA Rh Rh
Rh Rh
a M a m z
m M z a
ω
⋅ ⋅ ⋅ ⋅ ⋅ −
= ⋅ ⇒
⋅ ⋅ ⋅
( 1) GalA Rh Rh
Rh Rh
M z
M z
⋅ω ⋅ −
⇒
⋅
(9)
where GalA M is the molar mass of α-D(+)-galacturonic
acid residue, Da.
The conditional assumption is that all the residues
of α-D(+)-galacturonic acid in the insoluble part belong
exclusively to the protopectin complex and are present
only in the composition of homogalacturonan fragments
and branch points of rhamnogalacturonan-I. Then,
the mass fraction of α-D(+)-galacturonic acid residues
in homogalacturonan fragments can be calculated as
follows:
( ) ( )
( 1) GalA Rh Rh
GalA HG GalA GalA b GalA
Rh Rh
M z
M z
ω
ω ω ω ω
⋅ ⋅ −
= − = −
⋅
( ) ( )
( 1) GalA Rh Rh
GalA HG GalA GalA b GalA
Rh Rh
M z
M z
ω
ω ω ω ω
⋅ ⋅ −
= − = −
⋅ (10)
As the plant tissue grows, the protopectin complex of
cell walls and intercellular spaces changes continuously.
As a result, the structure of the complex becomes
heterogeneous. Assuming that all homogalacturonan
regions of the protopectin complex are a native
component of rhamnogalacturonan fragments, the
whole protopectin complex can be represented as
consisting almost entirely of rhamnogalacturonan-I
fragments. The length of the homogalacturonan regions
differs in different parts of the protopectin complex.
Consequently, a particular homogalacturonan molecular
mass is in fact a certain mean value. The molecular
weight of any arbitrarily taken (i-th) homogalacturonan
region of the protopectin complex is related to its
polymerization degree by the following ratio:
HG(i) GalA i M = M ⋅ k (11)
where i k is the polymerization degree of the i-th
homogalacturonan region.
Consequently, the formula for the average molecular
weight of homogalacturonan sites is as follows:
( )
1 1 1
( )
N N N
HG i GalA i i
i i i
HG av GalA GalA av
M M k k
M M M k
N N N
= = =
⋅
= = = ⋅ = ⋅
Σ Σ Σ
( )
1 1 1
( )
N N N
HG i GalA i i
i i i
HG av GalA GalA av
M M k k
M M M k
N N N
= = =
⋅
= = = ⋅ = ⋅
Σ Σ Σ (12)
where av k – average polymerization degree of
homogalacturonan regions and N – total homogalacturonan
regions amount.
The mass fraction of the homogalacturonan
component in the insoluble part can be expressed as
follows:
( ) 100 HG av HG
HG
M n a
m
ω
⋅ ⋅ ⋅
= (13)
A combination of formulae (3) and (13) gives the
following result:
( )
( )
100 ( ) 100
Rh Rh
HG av
Rh HG av Rh Rh
HG
Rh
M n z a z M n z a
m z m
ω
+
⋅ ⋅ ⋅
⋅ + ⋅ ⋅
= = ⋅
( )
( )
100 ( ) 100
Rh Rh
HG av
Rh HG av Rh Rh
HG
Rh
M n z a z M n z a
m z m
ω
+
⋅ ⋅ ⋅
⋅ + ⋅ ⋅
= = ⇒
⋅
( ) 100
100
Rh
HG av Rh
Rh
Rh
M m z a
M a
z m
⋅ω
⋅ + ⋅ ⋅ ⋅ ⋅ ⇒ ⇒
⋅
( )
100 Rh Rh
HG av Rh
Rh Rh
M M a z
m
M z
ω
⋅ ⋅ ⋅ ⋅ +
⇒
⋅
(14)
However, the following inequation occurs at
m ≥ 10−6g and 103 Rh z ≤ :
100 10
Rh Rh 10 M a z
m
− ⋅ ⋅ ⋅
<<
which makes it possible to disregard the sum of
100 Rh Rh M a z
m
⋅ ⋅ ⋅ as insignificant, in which case formula (14)
can be simplified as follows:
HG(av) Rh
HG
Rh Rh
M
M z
ω
ω
⋅
≈
⋅
(15)
The mass fraction of homogalacturonan fragments
and the mass fraction of α-D(+)-galacturonic acid
residues that make up the homogalacturonan fragments
are the same, which leads to the following identical
equation:
HG(av) Rh GalA Rh ( Rh 1)
GalA
Rh Rh Rh Rh
M M z
M z M z
ω ω
ω
⋅ ⋅ ⋅ −
≅ −
⋅ ⋅
(16)
Added to formula (12), the equation assumes the
following form:
( 1) GalA av Rh GalA Rh Rh
GalA
Rh Rh Rh Rh
M k M z
M z M z
ω ω
ω
⋅ ⋅ ⋅ ⋅ −
≅ −
⋅ ⋅
(17)
Applying formula (17) to kav makes it possible
to calculate the average polymerization degree of
homogalacturonan regions in the protopectin complex:
( 1)
Rh GalA Rh GalA Rh Rh Rh GalA 1 1
av Rh
GalA Rh GalA Rh
M z M z M
k z
M M
ω ω ω
ω ω
⋅ ⋅ − ⋅ ⋅ − ⋅
= = − ⋅ + ⋅ ⋅
( 1)
Rh GalA Rh GalA Rh Rh Rh GalA 1 1
av Rh
GalA Rh GalA Rh
M z M z M
k z
M M
ω ω ω
ω ω
⋅ ⋅ − ⋅ ⋅ − ⋅
= = − ⋅ + ⋅ ⋅
(18)
Thus, the mass fractions of galacturonide and
rhamnosyl residues in the plant cell can help to
determine the average polymerization degree of
the homogalacturonan regions in the protopectin
complex.
RESULTS AND DISCUSSION
Let the dimentionless criterion ν is uniquely
determined on the basis of chemical analysis of the
native water-insoluble plant tissue component:
Rh
GalA
ω
ν
ω
= (19)
As a result, formula (18) looks as follows:
Rh 1 1
av Rh
GalA
M
k z
M ν
= − ⋅ + ⋅
(20)
In (20), constituent Rh
GalA
M
M is constant. Subsequently,
formula (20) is a mathematical description of functional
dependence kav = f (ν , zRh ) (Fig. 6). Thus, analytically
obtained ωRh and ωGalA can define the weighted average
degree of polymerization of homogalacturonan regions
of pectins.
Figure 6 Weighted average polymerization degree of homogalacturonan sites of the rhamnogalacturonan fraction in pectins:
functional dependence
260
Kondratenko V.V. et al. Foods and Raw Materials, 2021, vol. 9, no. 2, pp. 254–261
Figure 7 Zoned criteria space of molecular characteristics of homogalacturonan fractions in pectins
In a same time, homogalacturonan regions in the
rhamnogalacturonan fraction of pectin are possible only
at kkaavv≥≥11.
As a result, the rational condition for criterion ν is:
( 1)
Rh Rh
GalA av Rh
M z
M k z
ν
⋅
≤
− +
(21)
Provided that there are homogalacturonan regions in
the rhamnogalacturonan fraction of pectin substances,
the range for determining the numerical values of this
criterion can be represented as 0; Rh
GalA
M
M
ν
∈
. The functional
dependence can be reduced to a criterion space in
coordinates ν and zRh, where kav is boundary zoning
conditions (Fig. 7).
Within this criterion space, zone I is the absence
of homogalacturonan regions in pectins. Zone II is
the presence of regions with the weighted average
polymerization degree of homogalacturonan region in
the range of 1–5; zone III – 5–10; and zone IV – ≥ 10.
Homogalacturonan regions with kav > 10 are of
their own practical importance. Therefore, the use
of homoenzyme preparations for fragmentation of
the native protopectin complex makes sense only for
plant tissues in zone IV. In other cases, the use of
homogalacturonan-specific enzyme preparations for
protopectin complex fragmentation has no sense.
The new criteria-based approach makes it possible
to unambiguously define the effectiveness of targeted
enzymatic fragmentation of the plant protopectin
complex within the boundary conditions that determine
the degree of the targeted physicochemical properties
of the final product. This approach is universal and
represents the second stage of the decision tree started
in [22] as a science-based technology for pectin
production.
CONCLUSION
The research produced a criteria space to assess
the potential effectiveness of the homoenzymatic
transformation of a plant biopolymer complex as in the
case of pectin substances. The method was based on a
two-dimensional criteria space, zoned according to the
key factor, i.e. the targeted polymerization degree of
homogalacturonan fragments in the native protopectin
complex.
We found that the compliance with the first criteria
zone (at kav ≥ 10) determined the feasibility of using
homogalacturonan-specific enzyme preparations to
isolate of homogalacturonan (targeted) regions of the
plant protopectin complex. The compliance with the
second criteria zone (at 1 ≤ kav < 10) determined the
expediency of non-enzymatic fragmentation of the
protopectin complex. The compliance with the third
zone (at kav < 1) meant that the fragmentation of the
protopectin complex would neither increase the mass
fraction of pectin substances in the medium, nor release
pectins.
The new criteria approach is an integral part of the
technologies for obtaining pectin and its products with
targeted physical and chemical properties.
CONTRIBUTION
All authors contributed equally to the manuscript and
are equally responsible for any possible plagiarism.
CONFLICT OF INTEREST
The authors state that there is no conflict of interests
related to the publication of this article.
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