gluten e suas proteinas(1)

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Food Microbiology 24 (2007) 115–119
www.elsevier.com/locate/fm
Chemistry of gluten proteins
Herbert Wieser
German Research Centre of Food Chemistry and Hans-Dieter-Belitz-Institute for Cereal Grain Research, D-85748 Garching, Germany
Available online 7 September 2006
Abstract
Gluten proteins play a key role in determining the unique baking quality of wheat by conferring water absorption capacity, cohesivity,
viscosity and elasticity on dough. Gluten proteins can be divided into two main fractions according to their solubility in aqueous
alcohols: the soluble gliadins and the insoluble glutenins. Both fractions consist of numerous, partially closely related protein
components characterized by high glutamine and proline contents. Gliadins are mainly monomeric proteins with molecular weights
(MWs) around 28,000–55,000 and can be classified according to their different primary structures into the a/b-, g- and o-type. Disulphide
bonds are either absent or present as intrachain crosslinks. The glutenin fraction comprises aggregated proteins linked by interchain
disulphide bonds; they have a varying size ranging from about 500,000 to more than 10 million. After reduction of disulphide bonds, the
resulting glutenin subunits show a solubility in aqueous alcohols similar to gliadins. Based on primary structure, glutenin subunits have
been divided into the high-molecular-weight (HMW) subunits (MW ¼ 67,000–88,000) and low-molecular-weight (LMW) subunits
(MW ¼ 32,000–35,000). Each gluten protein type consists or two or three different structural domains; one of them contains unique
repetitive sequences rich in glutamine and proline. Native glutenins are composed of a backbone formed by HMW subunit polymers and
of LMW subunit polymers branched off from HMW subunits. Non-covalent bonds such as hydrogen bonds, ionic bonds and
hydrophobic bonds are important for the aggregation of gliadins and glutenins and implicate structure and physical properties of dough.
r 2006 Elsevier Ltd. All rights reserved.
Keywords: Gluten; Gliadins; Glutenins; Proline and glutamine
1. Gluten
Gluten can be defined as the rubbery mass that remains
when wheat dough is washed to remove starch granules
and water-soluble constituents. Depending on the thor-
oughness of washing, the dry solid contain 75–85% protein
and 5–10% lipids; most of the remainder is starch and non-
starch carbohydrates. In practice, the term ‘gluten’ refers to
the proteins, because they play a key role in determining
the unique baking quality of wheat by conferring water
absorption capacity, cohesivity, viscosity and elasticity on
dough. Gluten contains hundreds of protein components
which are present either as monomers or, linked by
interchain disulphide bonds, as oligo- and polymers
(Wrigley and Bietz, 1988). They are unique in terms of
their amino acid compositions, which are characterized by
high contents of glutamine and proline and by low contents
of amino acids with charged side groups. The molecular
e front matter r 2006 Elsevier Ltd. All rights reserved.
.2006.07.004
ess: h.wieser@lrz.tum.de.
weights (MWs) of native proteins range from around
30,000 to more than 10 million. Traditionally, gluten
proteins have been divided into roughly equal fractions
according to their solubility in alcohol–water solutions of
gluten (e.g. 60% ethanol): the solubles gliadins and the
insoluble glutenins. Both fractions are important contri-
butors to the rheological properties of dough, but their
functions are divergent. Hydrated gliadins have little
elasticity and are less cohesive than glutenins; they
contribute mainly to the viscosity and extensibility of the
dough system. In contrast, hydrated glutenins are both
cohesive and elastic and are responsible for dough strength
and elasticity. To simplify matters, gluten is a ‘two-
component glue’, in which gliadins can be understood as
a ‘plasticizer’ or ‘solvent’ for glutenins. A proper mixture of
both fractions is essential to impart the viscoelastic
properties of dough and the quality of the end product.
Though cysteine belongs to the minor amino acids of
gluten proteins (E2%), it is extremely important for the
structure and functionality of gluten (Grosch and Wieser,
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dx.doi.org/10.1016/j.fm.2006.07.004
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H. Wieser / Food Microbiology 24 (2007) 115–119116
1999; Wieser, 2003). Most cysteines are present in an
oxidized state and form either intrachain disulphide bonds
within a protein or interchain disulphide bonds between
proteins. These bonds are the main target for most redox
reactions that occur during kernel maturation, milling,
dough preparation and baking (Wieser, 2003). Additional
covalent bonds formed during breakmaking are tyrosine–
tyrosine crosslinks between gluten proteins (Tilley et al.,
2001) and tyrosine–dehydroferulic acid crosslinks between
gluten proteins and arabinoxylans (Piber and Koehler,
2005). The covalent structure of the gluten network is
superimposed by non-covalent bonds (hydrogen bonds,
ionic bonds, hydrophobic bonds). Though this class of
chemical bonds is less energetic than covalent bonds, they
are clearly implicated in gluten protein aggregation and
dough structure (Wieser et al., 2006). Evidence for the
presence of hydrogen bonds in gluten proteins are the
dough weakening effect of hydrogen bond breaking agents
(e.g. urea) and the dough strengthening effect of heavy
water compared with that of ordinary water. The
importance of ionic bonds can be demonstrated by the
strengthening effect of NaCl or of bipolar ions such as
amino acids or dicarboxylic acids. Hydrophobic bonds
contribute significantly to the stabilization of gluten
structure. They are different from other bonds, because
their energy increases with increasing temperature; this can
provide additional stability during the baking process.
2. Gliadins
Most gliadins are present as monomers; they were
initially classified into four groups on the basis of mobility
at low pH in gel electrophoresis (a-, b-, g-, o-gliadins in
order of decreasing mobility). Later studies on amino acid
sequences, however, have shown that the electrophoretic
mobility does not always reflect the protein relationships
and that a- and b-gliadins fall into one group (a/b-type).
Modern methods such as two-dimensional electrophoresis
or reversed-phase high-performance liquid chromatogra-
phy (RP-HPLC) allow the separation of the gliadin
fraction into more than hundred components. Based on
the analysis of complete or partial amino acid sequences,
amino acid compositions and MWs, they can be grouped
Table 1
Characterisation of gluten protein types
Type MW� 10�3 Proportionsa (%) Part
Gln
o5-Gliadins 49–55 3–6 56
o1,2-Gliadins 39–44 4–7 44
a/b-Gliadins 28–35 28–33 37
g-Gliadins 31–35 23–31 35
x-HMW-GS 83–88 4–9 37
y-HMW-GS 67–74 3–4 36
LMW-GS 32–39 19–25 38
aAccording to total gluten proteins.
into four different types: o5-, o1,2-, a/b- and g-gliadins
(Table 1) (Wieser, 1996). Within each type, structural
differences are small due to substitution, deletion and
insertion of single amino acid residues. o-Gliadins are
characterized by the highest contents of glutamine, proline
and phenylalanine which together account for around 80%
of the total composition. o5-Gliadins have higher MWs
(E50,000) than o1,2-gliadins (E40,000). Most o-gliadins
lack cysteine, so that there is no possibility of disulphide
crosslinks. These proteins consist almost entirely of
repetitive sequences rich in glutamine and proline (e.g.
PQQPFPQQ). a/b- and g-gliadins have overlapping MWs
(E28,000–35,000) and proportions of glutamine and pro-
line much lower than those of o-gliadins (Table 1). They
differ significantly in the contents of a few amino acids, e.g.
tyrosine. Each of both types has two clearly different N-
and C-terminal domains. The N-terminal domain (40–50%
of total proteins) consists mostly of repetitivesequences
rich in glutamine, proline, phenylalanine and tyrosine and
is unique for each type (sequence Sections I and II, Fig. 1).
The repetitive units of a/b-gliadins are dodecapeptides such
as QPQPFPQQPYP which are usually repeated five times
and modified by the substitution of single residues. The
typical unit of g-gliadins is QPQQPFP, which is repeated
up to 16 times and interspersed by additional residues.
Within the C-terminal domains, a/b- and g-gliadins are
homologous (sequence Sections III–V, Fig. 1). They
present sequences which are non-repetitive, have less
glutamine and proline than the N-terminal domain and
possess are more usual composition. With a few exceptions,
a/b-gliadins contain six and g-gliadins eight cysteines
located in the C-termional domain; they form three and
four homologous intrachain crosslinks, respectively
(Grosch and Wieser, 1999) (Fig. 1). Studies on the
secondary structure have indicated that the N-terminal
domains of a/b- and g-gliadins are characterized by b-turn
conformation, similar to o-gliadins (Tatham and Shewry,
1985). The non-repetitive C-terminal domain contains
considerable proportions of a-helix and b-sheet structures.
Though the distribution of total gliadins among the
different types is strongly dependent on wheat variety
(genotype) and growing conditions (soil, climate, fertiliza-
tion), it can be generalized that a/b- and g-gliadins are
ial amino acid composition (%)
Pro Phe Tyr Gly
20 9 1 1
26 8 1 1
16 4 3 2
17 5 1 3
13 0 6 19
11 0 5 18
13 4 1 3
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Fig. 1. Schematic architecture and disulphide structures of gluten proteins
(adapted from Grosch and Wieser, 1999).
H. Wieser / Food Microbiology 24 (2007) 115–119 117
major components, whereas the o-gliadins occur in much
lower proportions (Wieser and Kieffer, 2001) (Table 1). A
minor portion of gliadins have an odd number of cysteines
due to point mutations and are linked together or to
glutenins. They appear either in alcohol-soluble oligomers
in the gliadin fraction or in the alcohol-insoluble glutenin
polymers. This form of gliadins is proposed to act as
terminator of glutenin polymerization (see below). The
oligomeric fraction has been called high-molecular-weight
(HMW) gliadin, aggregated gliadin or ethanol-soluble
glutenin (Shewry et al., 1983; Huebner and Bietz, 1993).
It contains a/b-, g-gliadins and low-molecular-weight
(LMW) subunits linked by interchain disulphide bonds;
the MWs range from around 100,000–500,000.
3. Glutenins
The glutenin fraction comprises aggregated proteins
linked by interchain disulphide bonds; they have varying
size ranging from about 500,000 to more than 10 million
(Wieser et al., 2006). Thus, a part of glutenins belongs to
the largest proteins in nature. The MW distribution of
glutenins has been recognized as one of the main
determinants of dough properties and baking performance.
The largest polymers termed ‘glutenin macropolymer’
(GMP) make the greatest contribution to dough properties
and their amount in wheat flour (E20–40mg/g) is strongly
correlated with dough strength and loaf volume.
After reduction of disulphide bonds, the resulting
glutenin subunits show a solubility in aqueous alcohols
similar to gliadins. The predominant protein type are
LMW glutenin subunits (LMW-GS), their proportion
amounts to E20% according to total gluten proteins
(Wieser and Kieffer, 2001). LMW-GS are related to
a/b- and g-gliadins in MW and amino acid composition
(Table 1). Similarly, they contain two different domains:
The N-terminal domain (sequence Section I, Fig. 1)
consists of glutamine- and proline-rich repetitive units
such as QQQPPFS and the C-terminal domain (sequence
Sections III–V, Fig. 1) is homologous to that of a/b- and g-
gliadins within Sections III and V. LMW-GS contain eight
cysteines (Grosch and Wieser, 1999; Wieser, 2003); six
residues are in positions homologous to a/b- and g-gliadins,
and therefore, are proposed to be linked by intrachain
disulphide bonds (Fig. 1). Two additional cysteine residues
unique to LMW-GS are located in Sections I and IV. They
are not able to form an intrachain bond, probably for steric
reasons. Consequently, interchain disulphide bonds with
cysteines of different gluten proteins are generated.
HMW glutenin subunits (HMW-GS) belong to the minor
components within the gluten protein family (E10%). Each
wheat variety contains three to five HMW-GS which can be
grouped into two different types, the x- and the y-type, with
MWs from 83,000–88,000 and 67,000–74,000, respectively
(Table 1). The nomenclature on single HMW-GS is based
on the coding genome (A, B, D), the type (x, y) and the
mobility of SDS-PAGE (nos. 1–12). HMW-GS consist of
three structural domains (Fig. 1): a non-repetitive N-
terminal domain (A) comprising about 80–105 residues, a
repetitive central domain (B) of about 480–700 residues and
a C-terminal domain (C) of 42 residues (Shewry et al., 1992).
Domains A and C are characterized by the frequent
occurrence of charged residues and by the presence of most
or all of cysteines. Domain B contains repetitive hexapep-
tides (unit QQPGQG) as a backbone with inserted
hexapeptides (e.g. YYPTSP) and tripeptides (e.g. QQP or
QPG). The most important difference between the x- and
the y-type lies within the A and B domains. For example, the
y-type has an insertion of 18 residues including two
neighbouring cysteines in domain A, and typical repetitive
units are less frequently repeated and more frequently
modified in domain B of the y-type. Because HMW-GS does
not occur in flour and dough as monomers, it is generally
assumed that they form interchain disulphide bonds. The x-
type except subunit Dx5 has four cysteines, three in domain
A and one in domain C (Shewry and Tatham, 1997) (Fig. 1).
Two residues of domain A are linked by an intrachain, the
other two by interchain disulphide bonds. Subunit Dx5 has
an additional cysteine at the beginning of domain B and it
has been suggested that this might form another interchain
bond. The y-type has five cysteines in domain A and one in
each of domains B and C. At present, interchain bonds have
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H. Wieser / Food Microbiology 24 (2007) 115–119118
only be found for the adjacent cysteines of domain A which
are connected in parallel with the corresponding residue of
another y-type, and for cysteine of domain B which is linked
to a cysteine of LMW-GS.
Various approaches have been combined to give details
of the secondary structure of HMW-GS. Studies of the
repetitive domain B indicated the presence of b-reverse
turns (Shewry et al., 1992). These were predicted to be
overlapping and form a loose spiral which was considered
to contribute decisively to gluten’s elasticity. The non-
repetitive domains A and C were proposed to have
globular structures containing a-helices. Numerous studies
demonstrated that dough properties are strongly influenced
by the quantities of HMW-GS. Among HMW-GS the
contribution of the x-type to dough properties has been
found to be more important than that of the y-type (Wieser
and Kieffer, 2001). Considering single subunits, the
presence of subunit Dx5 (which has an additional cysteine
for an interchain crosslink) and subunit Bx7 (which occurs
in the greatest amounts) has been proposed to be
particularly important for dough quality and loaf volume
(Wieser and Zimmermann, 2000).
4. Disulphide bonds
Disulphide bonds play an important role in determining
the structure and properties of gluten proteins. Monomeric
Fig. 2. A model double unit for the interchain disulphide structures of LMW-G
2006).
a/b- and g-gliadins present three and four intrachain
disulpide bonds, respectively, whereas polymeric LMW-
and HMW-GS include both intra- and interchain bonds
(Fig. 1) (Shewry and Tatham, 1997). The disulphide
structure of native glutenins is not in a stable state, but
undergoes a continuous change from the maturing grain to
the end product (e.g. bread) (Wieser, 2003; Wieser et al.,
2006). The MW distribution of glutenins has beenrecognized as one of the main determinants of dough
quality and is governed by the state of disulphide structure
that depends on genetic factors (e.g. presence of subunit
Dx5, ratio of HMW-GS to LMW-GS, amount of chain
terminators), environmental factors (e.g. sulphur defi-
ciency, heat or water stress), and the redox state (e.g.
presence of reducing or oxidizing agents) (Wieser et al.,
2006). Disulphide formation starts rapidly after synthesis
of proteins within the lumen of endoplasmatic reticulum as
an integral part of protein folding. It has been proposed
that intrachain links form more rapidly than interchain
links (Kasarda, 1999). To participate in a growing polymer,
proteins need at least two cysteines forming interchain
disulphide bonds. HMW- and LMW-GS usually fulfil this
requirement; they act as ‘chain extender’. The only
crosslink found between HMW- and LMW-GS until now
is a disulphide bond between a cysteine residue in domain
B of y-HMW-GS and a cysteine residue in the C-terminal
domain of LMW-GS. The backbone of HMW-GS
S (�) and HMW-GS (&) of gluten polymers (adapted from Wieser et al.,
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H. Wieser / Food Microbiology 24 (2007) 115–119 119
polymers is established by end-to-end, probably head-to-
tail linkages. At least, four different cysteines lead to
branching of the backbone. LMW-GS form also linear
polymers via cysteines of the N-terminal and C-terminal
domain. Terminators of polymerization were found to be
glutathione or gliadins with an odd number of cysteines.
With respect to quantitative data on the subunits
composition of glutenins (ratio of HMW-GS to LMW-
GS E1:2 and of x- to y-type E2.5:1) and the MW of
subunits, the basis molecular ‘double unit’ of glutenin
polymers might consist of 2 y-type HMW-GS, 4 x-type
HMW-GS and around 30 LMW-GS covalently linked by
interchain disulphide bonds (Wieser et al., 2006) (Fig. 2).
The MW of this double unit amounts to about 1.5 million.
The largest of the glutenins (GMP) might include more
than ten of these double units in which the x/y-type ratio
and the HMW–/LMW-GS ratio increase.
The overriding importance of disulphide bonds can be
demonstrated by the addition of reducing agents weaken-
ing dough and of thiol-blocking or oxidizing agents
strengthening dough (Wieser, 2003). During the whole
process of breadmaking, the state of large glutenin
polymers is characterized by three competitive redox
reactions: (1) the oxidation of free SH groups which
support polymerization; (2) the presence of ‘terminators’
that stop polymerization and (3) SH/SS interchange
reactions between glutenins and thiol compounds such as
glutathione that depolymerize polymers. Oxygen is known
to be essential for the formation of large glutenin polymers
during dough mixing. Oxidizing agents such as potassium
bromate, potassium iodate and L-ascorbic acid (after
oxidation to dehydrascorbid acid) show the same effect
as atmospheric oxygen. The baking process produces
dramatic changes in the glutenin structure and function-
ality. For example, extractability in urea or SDS is strongly
reduced and most of cysteine containing a/b- and g-gliadins
are covalently bound to glutenin polymers. Disulphide
interchange reactions between gliadins and glutenins have
been postulated to be involved in the heat-induced effects
(Wieser, 1998).
5. Conclusions
Gluten proteins are among the most complex protein
networks in nature due to numerous different components
and different size, and due to variability caused by
genotype, growing conditions and technological processes.
They play a key role in determining the unique rheological
dough properties and baking quality of wheat. In spite of
the large number of excellent studies dedicated to
characterizing structure and functionality of gluten pro-
teins, important gaps of knowledge has to be filled by
future research. One of the priorities should be to get more
insight into changes of the disulphide structure beginning
from the synthesis of proteins in the growing plant and
ending in the baked products. Another point should be a
better understanding of the relations between structure and
functionality of gluten proteins, particularly in combina-
tion with additives such as dough improvers. Heterologous
expression and protein engineering could be a valuable aid
in structure–function studies (Tamas and Shewry, 2006).
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	Chemistry of gluten proteins
	Gluten
	Gliadins
	Glutenins
	Disulphide bonds
	Conclusions
	References