Wheat was originated in Southwest Asia, Tigris and Euphrates river valley, in the area known as the Fertile Crescent Sowing of grains from wild grasses, cultivation and repeated harvesting led to domestication of wheat.
Selection of mutant forms with tough ears which remained intact during harvesting, larger grains, and a tendency for the spikelets to stay on the stalk until harvested was the set-off of modern agriculture. Bread wheat (Triticum aestivum) produces one-seeded fruits, which are called grains or kernels.
The pleasant flavor, long shelf-life and unique gluten-forming characteristics of wheat products like pasta, noodles, bread, chapatti etc make them very attractive among other cereals
basic number of chromosomes in Triticum and related species are x=7. The wild species are diploids (2n = 2x = 14T. Nowadays the hexaploid wheat which is mostly being cultivated in the world is Triticum aestivum, AABBDD, (2n=6x=42)
COMPONENT OF WHEAT
Starch is the most important reserve polysaccharide of many cereals In wheat, starch is the most abundant component present in the grain endosperm. It consists of the glucose polymers, amylose and amylopectin. Amylose is essentially linear, consisting of (1→4)-α-linked d-glucopyranosyl units with molecular weight (MW) in the range of 105−106 and a degree of polymerization (DP) of 500–6000 glucose residues.
Carbohydrates can be divided into two main types: available and unavailable. Available carbohydrates are those digested and absorbed by humans, which include (non-resistant) starch and soluble sugars.
In contrast, unavailable carbohydrates (dietary fiber) are not digested by the endogenous secretion of the human digestive tract Dietary fiber is mainly composed of resistant starch (RS), cellulose and other complex polysaccharides, such as arabinoxylans, β-glucans, pectins and arabinogalactans, together with lignin
In wheat grain, lipids are present in small amounts (2%). Essential fatty acids (palmitic and linoleic acids), fat-soluble vitamins and phytosterols are important components of wheat grain lipids – Based on solubility under specific extraction conditions lipids are classified as starch lipids, both free and bound, and non-starch lipids (NSL).
Approximately 2/3–3/4 of the total wheat flour lipids is comprised by NSL that are predominantly triglycerides and other non-polar (NP) lipids such as digalactosyl diglycerides (DGDG).
NP lipids are mainly present in the free NSL fraction, while glyco- and phospholipids are mainly associated with proteins and present in the bound NSL fraction. The fatty acid pattern of the flour lipids is dominated by linoleic acid (C18:2) with lower amounts of palmitic.
Flour lipids have a positive effect on formation of dough and loaf volume during the process of bread making. Loaf volume is negatively affected by the free fatty acids in NP lipids while glycolipids in polar lipids have positive impact on the loaf volume.
Furthermore, loaf volume is affected positively and negatively by polar and NP lipids respectively The volume and softness of steamed Bread and morphology of short-dough biscuits is also influenced by lipids.
The word protein means primary substance, according to Mulder and Berzelius who proposed the name in 1838 . The scientific study of wheat grain proteins extends back for over 250 years, with the isolation of wheat gluten first being described in 1745. Osborne classified wheat protein according to the basis of solubility and functionality in 1908.
Proteins were divided into three major types: simple, conjugated and derived. Osborne concluded that the proteins present in plant tissues were “simple” and comprised of four major types: albumins (soluble in water and dilute buffers), globulins (soluble in salt solutions), prolamins (soluble in 70 – 90% ethanol) and glutelins.
The gluten proteins of wheat classically fall into two of these groups, with the alcohol-soluble gliadins and the alcohol-insoluble glutenins. Glutenins are known being the biggest polymers in nature In 1970 the glutelin fraction was divided into two fractions according to solubility in dilute acetic acid. The fraction which was insoluble in dilute acetic acid corresponded to the fifth fraction according to the Osborne fractions.
Grain proteins of wheat can also be divided into structural/metabolic (non-gluten) and storage proteins (gluten) (Structural/metabolic proteins consist of albumin, globulin and amphiphilic proteins. Non-membrane amphiphilic proteins have been reported to have large effects on grain hardness and dough rheological properties Wheat storage proteins are collectively known as prolamins because of their high content of the amino acids, proline and glutamine.
Another system of classification divided prolamins into three groups: sulphur-rich, sulphur-poor and high molecular weight glutenin subunits (HMW-GS). Sulphur rich prolamins include β-, γ-gliadins, B- and C- low molecular weight glutenins (LMW-GS). Sulphur poor prolamins contain ω-gliadins and D- LMW-GS.
The individual polypeptides of wheat storage proteins are synthesized on ribosomes on the rough endoplasmic reticulum (RER) and pass via the usual translocation machinery into the lumen, with the loss of an N-terminal signal peptide.
Once within the lumen it is probable that protein folding and disulphide bond formation occurs with no further post-translational modifications taking place (i.e. no glycosylation or proteolysis, The genes which are responsible for encoding gliadins and glutenin subunits are located to several complex loci on the homologous chromosomes 1 and 6.
Each of these homologous chromosomes consists of several tightly linked genes. Genes responsible for coding HMW-GS occur on the long arm of chromosomes 1A, 1B and 1D, while the genes responsible for coding LMW-GS, ω- and γ- gliadins are located on the short arm of chromosomes 6A, 6B and 6D
Albumins and globulins
The non-prolamin proteins; albumins and globulins of wheat, comprises 15-20% of total wheat flour proteins. Albumins are soluble in water and globulins are soluble in salts The molecular weights (MW) of albumins and globulins are mostly lower than 25,000, although a significant proportion of the proteins has MW between 60,000 and 70,000.
Albumins and globulins are considered to have nutritionally better amino acid compositions because of their higher lysine and methionine contents as compared to the rest of the proteins in the wheat grain.
Alpha- amylase/trypsin, serpins and purothionins are predominant albumins and globulins. These predominant albumins and globulins serve as nutrient reserves for the germinating embryo. Secondly they also help in protecting embryo from insects and pathogens before germination
The rubbery mass that is left when wheat flour is washed with water to remove starch, non-starchy polysaccharides, and water-soluble constituents, is called gluten. Gluten is comprised of 80–85% protein and 5% lipids; most of the remainder is starch and non-starch carbohydrates Wheat storage proteins play a crucial role in forming the strong, cohesive dough that will retain gas and produce light baked products.
These properties make wheat alone suitable for the preparation of a great diversity of food products- breads, noodles, pasta, cookies, cakes, pastries and many other foods Gluten proteins are present in the mature wheat grain endosperm where they form a continuous matrix around the starch granules. Gluten contains hundreds of protein.
Components which are present either as monomers or, linked by inter- and intra- chain disulphide bonds (cysteine oxidized form), as oligo- polymers In the amino acid composition of gluten, cysteine residues are in minor amount (≈2%), although this little amount is very crucial for the structure and functionality of gluten. Cysteine (oxidized form) normally form inter-chain disulphide bonds within and between Proteins
Gliadins are monomeric proteins that consist of single chain polypeptides and constitute from 30 to 40% of total flour protein content. Gliadins are polymorphic mixture of proteins soluble in 70% alcohol. The bonds which are formed in the gliadins are intra-chain cysteine disulphide bridges resulting in less or more globular monomeric nature of gliadins Gliadins are rich in proline and glutamine and have a low level of charged amino acids.
The molecular weights of gliadins is 30–80 kDa and they are classified into four groups of α, β, γ and ω on the basis of molecular mobility at low pH in acid polyacrylamide gel electrophoresis grouped gliadins into four different classes ω5-, ω1 and 2-, α/β– and γ-gliadins This division was based on sequences and composition of amino acids, and molecular weights of different classes of gliadins. α, β and γ gliadins contain inter-chain disulfide bonds, while ω-gliadins lack cysteine.
Residues and do not form disulphide bonds . The α/β– and γ– gliadins are the major components, whereas the ω-gliadins occur in much lower proportions of wheat varieties. Hydrated gliadins have little elasticity and are less cohesive than glutenins; they contribute mainly to the viscosity and extensibility of dough system Gliadins may associate with one another or the glutenins through hydrophobic interactions and hydrogen bonds.
Glutenins are the polymeric proteins of wheat gluten and they are extractable in dilute acetic acid Glutenins and gliadins have very similar amino acid composition, thus glutenin have high levels of glutamine and proline and low levels of charged amino acids.
In wheat flour dough, baking performance strongly depend on the molecular weight distribution of glutenins. Glutenins appear to be largely responsible for gluten elasticity .
In addition, glutenins have the ability to form the largest and most complex protein polymers in nature with MWs of more than 10 millions, making wheat glutenins outstanding proteins in the plant kingdom
Two classes of glutenin subunits, HMW-GS and LMW-GS are present in wheat and they are released at the reduction of disulphide bonds with reducing agents and determined when analyzed by electrophoresis. The glutenin subunits released has also been further classified into four subgroups A B, C and D) on the basis of electrophoretic mobility on SDS-PAGE.
The subgroup A is determined to be HMW-GS and subgroups B, C and D are referred to as LMW-GS .It was also observed that high-molecular-weight gliadins, once reduced and separated by SDS-PAGE, had mobilities similar to those of B and C subunits of LMW-GS HMW-GS constitute no more than 10% of total flour protein; although they may be the. Most important determinants of bread-making quality because of their importance in forming the glutenin polymer.
LMW-GS were first identified by gel filtration of extracts of wheat flour as high-molecular-weight gliadins linked by disulphide bonds, distinguishing them from monomeric gliadins Six HMW-GS genes ares present in hexaploid wheat and because of gene silencing only three of these genes are always expressed in all cultivars. After running wheat protein samples on 2D gel electrophoresis at least fifteen to twenty different LMW-GS proteins are found in wheat (hexaploid).
HMW-GS have molecular weights between 65 and 90 kDa, based on derived amino acid sequence, and 80–130 kDa on SDS-PAGE. They are encoded at complex loci on the long arms of chromosomes 1A, 1B and 1D of hexaploid wheat, the Glu-A1, Glu-B1 and Glu-D1 loci, respectively Molecular analyses have shown that each locus consists of two genes encoding subunits designated x-type, (higher molecular weight) and y-type (lower molecular weight).
Similar to HMW-GS subunits, LMW-GS subunits are linked together through inter and intra chain disulphide bridges B and C subgroups contain about 60% of total LMW-GS. The subgroups B and C of LMW-GS have molecular weights 42–51 kDa and 30–40 kDa respectively. The amino acid sequences of LMW-GS in C subgroup are similar to amino acid sequence of γ- and α-gliadins.
Highly acidic LMW-GS, having molecular weight 58 kDa, are present in subgroup D. These LMW-GS are derived from modified ω-gliadins The LMW-GS have also been classified into LMW-m subunits which have methionine.
As the first amino acid in the sequence, and LMW-s subunits which have serine as the first amino acid in the sequence The set of genes at Glu-A3, Glu-B3 and Glu-D3 loci on chromosomes 1AS, 1BS and 1DS help in encoding some of the C group LMW-GS and many of the B group LMW-GS.
All cultivars contain 1Bx, 1Dx and 1Dy subunits, while other cultivars also contain a 1By and/or 1Ax and 1Ay subunit (Payne and Lawrence, 1983). The silencing of some of the genes could be caused by the presence of a transposon-like insertion in the coding region.
The full amino acid sequences of a number of subunits have been determined, including x-type and y-type proteins encoded by all three genomes, by sequencing of genomic DNA Allelic variation in the subunits encoded by active genes results in proteins subunits with different mobility on gel electrophoresis
JAFFAR IQBAL: MSC (HONS) (AGRONOMY)
FAIZAN ANJUM: PHD (AGRONOMY)
M. AHMAD MEHMOOD MSC (HONS )AGRONOMY