Prolamins of rice, millet, sorghum and maize are
lower in glutamine (Q) and proline (P), but rich in leucine (L) and
alanine (A); avenin of oats is in medium position.
Altogether, the
proportions of the major amino acids, in particular of glutamine (Q)
and proline (P), in prolamins reflect very well their coeliac
toxicity.
The range of molecualr masses shown
in table 1 indicates that the prolamins of wheat, rye and barley
differ from other prolamins by significantly higher values.
Zein 19 000 - 26
000.
3. Classification of prolamin components
Herbert Wieser.
Prolamins in Cereals. Deutsche Forschungsanstalt für
Lebensmittelchemie, Germany.
The prolamin
fraction consist of numerous, in part closely related proteins.
According to their amino acid sequences, amino acid compositions and
molecular masses, they can be classified into different types.
Gliadin,
secalin and hordein have two common types: the
omega-type ( kreikk. ω):
ω -gliadin,
ω-secalin and
ω-
hordein) and the
gamma-type
(kreikk. γ) (γ-gliadins,
γ-secalins and γ-hordein); gliadin additionally contains alfa-type
(kreikk. α) (
α-gliadins).
Omega
(ω)
-Type
prolamins (omega-type)
have high contents of glutamine ( Q, Gln) 48 mol-%), prolamine (P,
Pro) about 25 mol-% and phenylalanine (F, Phe) ( about 8 mol-%),
which together accounts for about 80 % of the total composition
Their molecular masses are in a range from 53 000- 64 000. The
complete amino acid sequences
of these proteins have not yet (1998) been described, with the
exception of a C-hordein, the corresponding gene of which has been
proposed to be silent, but some information on partial sequencess
does exists.
Omega
(ω)-prolamins
consist almost
entirely of repetitive sequences
with only short, non-repetitive N- and C-terminal sequences. The most
dominant repeat appears to be QQPQQPFP
with
numerous modifications of single residues.
Gamma (γ)-
type prolamins of
wheat, rye and barley have molecular masses of about 30 000 and are
homologous to a high degree. Their primary structures are divided
into two completely
different domains.
Their primary structures are divided into two completely different
domains.
The N-terminal domain
( Division I about 140 amino acid residues) has short N-terminal
sequences which are non-repetitive. Then, repetitive sequences
follow; they have motifs similar to those of omega(ω)-gliadins
and are rich in glutamine (Q), proline(P) and phenylalanine (F). (
Table 2).
The C-terminal domain
(divisions III-V, about 160 amino acid residues) possesses a more
usual amino acid composition with less glutamine (Q) and proline(P),
but more charged residues: glutamic acid ( Glu, E), lysine (Lys, K),
arginine (Arg, R) and hydrophobic residues: leucine (Leu,
L), isoleucine (Ile, I), valine ( Val, V). In this domain, cysteine
(Cys, C) residues are also present, forming intramolecular disulphide
bonds (-s-s-). Partially, the sequences show homology with seed
proteins of other plants, e.g. rape. castor
bean.
Alfa( α)
-type gliadins are
unique to the wheat.
Typically these proteins have an
N-terminal
domain
with two different sequence divisions: the N-terminal
division I is
partially non-repetitive (about 32 residues), partially repetitive,
consisting of motifs like QPQPFPPQQPYP,
which are mostly repeated five times. Furthermore, one or two
poly-Gln-sequences (Poly-Q-sequences) (division
II with maximal 18
residues of Gln, (Q) are present. The amino acid composition (Table
2) is similar to that of the gamma (γ)-type,
but with lower proline (P) and higher tyrosine (Y) contents.
The C-terminal domain
of the alfa(α)-type
is homologous in divisions III and V with the (γ)-type,
and contain three intramolecular disulphide bonds (-s-s-).
Avenins,
the prolamins of oats, are homologous with the alfa (α)-
and gamma(γ)-type
prolamins within division III and V of the C-terminal
domain. (Figure1).
Eight cysteine residues (Cys) are present in these divisions, all
forming intramolecular disulphide bonds. Unique to avenins are the
repetitive sequences in divisions IV with repetitive motifs like
QQQVFQPL
or QQQFFQPQM,
which are repeated four times and frequently modified.
The N-terminal domain
is characterised by non-repetitive sequences ( about 16 residues) and
by short repetitive sequences consisting of three repeats of the
motif QQQQPFV
or QQQQMLL.
The primary structures of the
non-toxic prolamins
oryzin, kafirin and zein
are, as far as is known, completely different from those of toxic
prolamins.
Altogether, the N-terminal
domains and- in
particular, their
repetitive sequences
are unique to toxic
prolamins
and mainly characterised by high contents of glutamine (Q),
proline(P) and aromatic amino acids ( F,Y,W) (Table 2). Their
secondary stricture is characterized by the frequent occurrence of
beta-turn conformations. Most in -vivo and in-vitro testing of
prolamin (gliadin) peptides demonstrated that regions
of the N-terminal domains
are involved in activating coeliac disease.
(eräs löytämäni moderni linkki:
http://www.mdpi.com/2072-6643/8/10/644/htm)
N-
terminal |
|
|
|
|
|
|
IV |
V |
C-
terminal |
4. Relation between prolamins and glutelins
Glutelins are
the second major protein fractions in
cereal endosperm. They comprise aggregated proteins linked by
disulphide bonds (-s-s-). After reduction of disulphide bonds, the
resulting subunits are as soluble in aqueous alcohol as prolamins.
Glutelin subunits of
wheat, rye and barley can be classified into high- molecule-weight
(HMW) and low-molecule-weight(LMW) subunits.
The HMW subunits
of three cereals are homologous to a high degree. They have
molecular masses in a range of 95 000- 136 000 (determined by
SDS-PAGE) and contain three different domains: a non-repetitive
N-terminal domain with about 100 residues, a central
repetitive domain with about 400-700 residues and a
non-repetitive C-terminal domain with about 40 residues.
Both N-terminal and
C-terminal have relatively well-balanced amino-acid compositions with
most or all of the cysteine residues.
The central
domain is rich in glutamine (Q), glycine (G) and proline (P) and
contains repetitive hexapeptides like QQPGQG as a backbone,
which are repeated about 50 to 70 times and frequently modified and
interspersed by motifs like YYPTSP an QPG. A remarkable
sequence homology to prolamins cannot be detected.
In contrast, LMW
subunits of glutelins are partially or highly homologous to the
corresponding prolamin.
LMW subunits of
glutenin ( glutelin of wheat) ar homologous to alfa-and
gamma-type gliadin within division III and V of the C-terminal
domain. ( Figure 1). Division IV is less homologous, elongated
about 20 residues, and contains one cysteine residue, which forms an
intermolecular disulphide bond with other gluten proteins.
The N-terminal
domain (division I) differs significantly from those of alfa- and
gamma-type prolamins and is characterised by short non-repetitive and
long repetitive sequences. The typical repeat motifs contain a series
of glutamines (Q) (two to six residues) followed by units
such as PPFS. A small portion of omega-, alfa- and gamma-type
gliadins is also present in the glutelin fraction. By substitution
of a single residue by cysteine, they have an odd number of cysteines
and are covalently bound to the glutenin aggregate; consequently,
they are not extractable with aqueous alcohols. This could be the
reason why glutenin has been described as weakly toxic.
LMW subunits of
rye glutelin comprise a group of proteins with a molecular mass
of about 75 000 and are named 75 K gamma-secalin, because they
are closely related to the corresponding gamam-type prolamins (40
K-gamma (γ)-secalin). It
has been proposed that 75 K γ-secalins
were formed from 40 K
(γ)-secalins
by elongation of the glutamine (Q)- and proline (P)-rich repetetive
sequences and by insertion of at least one cysteine residue.
LMW
subunits of barley glutelin
(B-hordeins)
have C-terminal domains highly homologous with those of LMW subunits
of wheat. The N-terminal domain consists exclusively of repetetive
sequences related to those of gamma(γ)-hordeins.
Summarising,
one portion of the
glutelin subunits , namely the HMW subunits, are totally different
from prolamins, whereas the other portion, the LMW subunits, are
closely related to corresponding prolamins in the case of rye and
abrley. The LMW subunits of wheat, however, have respective sequences
distinctly different from those of the gliadins.
(Tabel
1)
Taulukko
2 ei ole kirjoitettu näkyviin.
Kuva
1 ei ole kirjoitettu näkyviin.
Yllä
oleva tekemäni kaava vain selventänee N- ja C- terminaalin ja niiden divisioiden jaon
periaatteen.
