Gliadin (a type of

prolamin Prolamins are a group of plant storage proteins having a high proline amino acid content. They are found in plants, mainly in the seeds of cereal grains such as wheat ( gliadin), barley ( hordein), rye ( secalin), corn ( zein), sorghum ( kafiri ...

) is a class of

proteins Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, re ...

present in

wheat Wheat is a group of wild and crop domestication, domesticated Poaceae, grasses of the genus ''Triticum'' (). They are Agriculture, cultivated for their cereal grains, which are staple foods around the world. Well-known Taxonomy of wheat, whe ...

and several other

cereal A cereal is a grass cultivated for its edible grain. Cereals are the world's largest crops, and are therefore staple foods. They include rice, wheat, rye, oats, barley, millet, and maize ( Corn). Edible grains from other plant families, ...

s within the grass genus ''

Triticum Wheat is a group of wild and domesticated grasses of the genus ''Triticum'' (). They are cultivated for their cereal grains, which are staple foods around the world. Well-known wheat species and hybrids include the most widely grown comm ...

''. Gliadins, which are a component of

gluten Gluten is a structural protein naturally found in certain Cereal, cereal grains. The term ''gluten'' usually refers to the elastic network of a wheat grain's proteins, gliadin and glutenin primarily, that forms readily with the addition of water ...

, are essential for giving bread the ability to rise properly during baking. Gliadins and

glutenin Glutenin (a type of glutelin) is a major protein within wheat flour, making up 47% of the total protein content. The glutenins are protein aggregates of high- molecular-mass (HMW) and low-molecular-mass (LMW) subunits with molar masses from ab ...

s are the two main components of the gluten fraction of the wheat

seed In botany, a seed is a plant structure containing an embryo and stored nutrients in a protective coat called a ''testa''. More generally, the term "seed" means anything that can be Sowing, sown, which may include seed and husk or tuber. Seeds ...

. This gluten is found in products such as

wheat flour Wheat flour is a powder made from the grinding of common wheat used for human consumption. Wheat varieties are called "soft" or "weak" if gluten content is low, and are called "hard" or "strong" if they have high gluten content. Hard flour, or ...

. Gluten is split about evenly between the gliadins and glutenins, although there are variations found in different sources. Neither gliadins nor glutenins are

water-soluble In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution. The extent of the solub ...

, but gliadins are soluble in 70% aqueous ethanol. There are three main types of gliadin (α, γ, and ω), to which the body is intolerant in coeliac (or celiac) disease. Diagnosis of this disease has recently been improving. Gliadin can cross the intestinal epithelium.

Breast milk Breast milk (sometimes spelled as breastmilk) or mother's milk is milk produced by the mammary glands in the breasts of women. Breast milk is the primary source of nutrition for newborn infants, comprising fats, proteins, carbohydrates, and a var ...

of healthy human mothers who eat gluten-containing foods presents high levels of non-degraded gliadin.

Types

The α, γ, and ω gliadin types are separated and distinguished based on their amino acid sequences in the N-terminal cysteine domain. * α-/β-gliadins – soluble in low-percentage alcohols. * γ-gliadins – ancestral form of cysteine-rich gliadin with only intrachain disulfide bridges * ω-gliadins – soluble in higher percentages, 30–50% acidic acetonitrile.

Chemistry

The gliadins are

intrinsically disordered proteins In molecular biology, an intrinsically disordered protein (IDP) is a protein that lacks a fixed or ordered protein tertiary structure, three-dimensional structure, typically in the absence of its macromolecular interaction partners, such as other ...

meaning that they have continuously altering shapes making it difficult to study them. The performed image analysis and computer simulations of the proteins show that the average shape of the gliadins follows an elliptical shape. More specifically the protein likely has a tadpole-like structure with a hydrophobic core and a loose disordered tail. Compared to the other gluten proteins like the

s, which form extended networks of polymers due to disulphide bonds, gliadins are monomeric molecules in the cell, even if they in many ways are very similar. Especially the low molecular weight glutenins are similar in the way that they have

cysteine Cysteine (; symbol Cys or C) is a semiessential proteinogenic amino acid with the chemical formula, formula . The thiol side chain in cysteine enables the formation of Disulfide, disulfide bonds, and often participates in enzymatic reactions as ...

s located in matching locations as many of the gliadins. However, the gliadins are unable to form polymers in the cell since its cysteines form intra-chain disulphide bonds at synthesis due to

hydrophobic In chemistry, hydrophobicity is the chemical property of a molecule (called a hydrophobe) that is seemingly repelled from a mass of water. In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, thu ...

interactions. Gliadins are capable to aggregate into larger oligomers and interact with other gluten proteins, due to large hydrophobic sections, poly-Q and repetitive sequences. These sections are likely to aggregate hydrophobically, liquid-liquid phase separate, potentially form β-sheets aggregates or simply entangles by its structural properties.

Biochemistry

Gliadins are

s and are separated on the basis of electrophoretic mobility and

isoelectric focusing Isoelectric focusing (IEF), also known as electrofocusing, is a technique for separating different charged molecules by differences in their isoelectric point (pI). It is a type of zone electrophoresis usually performed on proteins in a gel tha ...

. Gliadin peptides cross the intestinal barrier by active transport.

Metabolism

Gliadins are known for their role, along with glutenin, in the formation of

. They are slightly

soluble In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution. The extent of the solubi ...

ethanol Ethanol (also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound with the chemical formula . It is an Alcohol (chemistry), alcohol, with its formula also written as , or EtOH, where Et is the ps ...

and contain only intramolecular

disulfide In chemistry, a disulfide (or disulphide in British English) is a compound containing a functional group or the anion. The linkage is also called an SS-bond or sometimes a disulfide bridge and usually derived from two thiol groups. In inorg ...

links. They also cause some of the best examples of food-derived

pathogenesis In pathology, pathogenesis is the process by which a disease or disorder develops. It can include factors which contribute not only to the onset of the disease or disorder, but also to its progression and maintenance. The word comes . Descript ...

. People with

celiac disease Coeliac disease (British English) or celiac disease (American English) is a long-term autoimmune disorder, primarily affecting the small intestine. Patients develop intolerance to gluten, which is present in foods such as wheat, rye, spel ...

(also known as gluten-sensitive enteropathy) are sensitive to α, β, and γ gliadins. Those with wheat-dependent urticaria and baker's asthma are sensitive to ω-gliadins. Gliadin can also serve as a useful delivery method for sensitive

enzymes An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as pro ...

(such as

superoxide dismutase Superoxide dismutase (SOD, ) is an enzyme that alternately catalyzes the dismutation (or partitioning) of the superoxide () anion radical into normal molecular oxygen (O2) and hydrogen peroxide (). Superoxide is produced as a by-product of oxy ...

, which is fused with gliadin to form glisodin). This helps protect them from stomach acids that cause breakdown. For useful description of the gliadins see: * Triticeae glutens * Immunochemistry of gluten

Deamidated gliadin

Deamidated gliadin is produced by acid or enzymatic treatment of gluten. The enzyme tissue transglutaminase converts some of the abundant

glutamine Glutamine (symbol Gln or Q) is an α-amino acid that is used in the biosynthesis of proteins. Its side chain is similar to that of glutamic acid, except the carboxylic acid group is replaced by an amide. It is classified as a charge-neutral ...

s to

glutamic acid Glutamic acid (symbol Glu or E; known as glutamate in its anionic form) is an α- amino acid that is used by almost all living beings in the biosynthesis of proteins. It is a non-essential nutrient for humans, meaning that the human body can ...

. This is done because gliadins are soluble in alcohol and cannot be mixed with other foods (like milk) without changing the food's qualities. Deamidated gliadin is soluble in water. The cellular immunity to deamidated α-/β-gliadin is much greater than α/β-gliadin and can result in symptomatic gluten-sensitive enteropathy.

Celiac disease

Celiac disease Coeliac disease (British English) or celiac disease (American English) is a long-term autoimmune disorder, primarily affecting the small intestine. Patients develop intolerance to gluten, which is present in foods such as wheat, rye, spel ...

(or coeliac disease) is a chronic, immune-mediated intestinal disorder, in which the body becomes intolerant to gliadin, which is a component of

. Individuals with celiac disease exhibit a lifelong intolerance of wheat, barley and rye – all of which contain prolamins. The main problem with this disease is that it often goes unrecognized for many years, in which case it can cause serious damage to several organs, and most cases currently remain unrecognized, undiagnosed and untreated. Gliadin proteins have the ability to provoke an autoimmune

enteropathy Enteropathy refers to any pathology of the intestine. Although enteritis specifically refers to an inflammation of the intestine, and is thus a more specific term than "enteropathy", the two terms are sometimes used interchangeably. __TOC__ Types ...

(intestinal disease) caused by an abnormal immune response in genetically susceptible individuals. Specific amino acid sequences within the gliadin proteins are responsible for this activity. It occurs as a result of CD4⁺ T cell recognition of deaminated gliadin polypeptide chains within the intestinal epithelium. CD8⁺ T cells then enter the epithelium and express NK receptors specific for gliadin and transglutaminase causing intraepithelial T cells to kill enterocytes by mediating apoptosis. Celiac disease with "non-classic symptoms" is the most common clinical type and occurs in older children (over 2 years old), adolescents and adults. It is characterized by milder or even absent gastrointestinal symptoms and a wide spectrum of non-intestinal manifestations that can involve any organ of the body, and very frequently may be completely asymptomatic both in children (at least in 43% of the cases) and adults. Untreated celiac disease may cause

malabsorption Malabsorption is a state arising from abnormality in absorption of food nutrients across the gastrointestinal (GI) tract. Impairment can be of single or multiple nutrients depending on the abnormality. This may lead to malnutrition and a varie ...

, reduced quality of life,

iron deficiency Iron deficiency, or sideropenia, is the state in which a body lacks enough iron to supply its needs. Iron is present in all cells in the human body and has several vital functions, such as carrying oxygen to the tissues from the lungs as a key ...

osteoporosis Osteoporosis is a systemic skeletal disorder characterized by low bone mass, micro-architectural deterioration of bone tissue leading to more porous bone, and consequent increase in Bone fracture, fracture risk. It is the most common reason f ...

, an increased risk of intestinal

lymphoma Lymphoma is a group of blood and lymph tumors that develop from lymphocytes (a type of white blood cell). The name typically refers to just the cancerous versions rather than all such tumours. Signs and symptoms may include enlarged lymph node ...

s and greater mortality. It is associated with some autoimmune diseases, such as

diabetes mellitus type 1 Type 1 diabetes (T1D), formerly known as juvenile diabetes, is an autoimmune disease that occurs when the body's immune system destroys pancreatic cells (beta cells). In healthy persons, beta cells produce insulin. Insulin is a hormone required ...

thyroiditis Thyroiditis is the inflammation of the thyroid gland. The thyroid gland is located on the front of the neck below the laryngeal prominence, and makes hormones that control metabolism. Thyroiditis is a group of disorders that all cause thyroi ...

, gluten ataxia,

psoriasis Psoriasis is a long-lasting, noncontagious autoimmune disease characterized by patches of abnormal skin. These areas are red, pink, or purple, dry, itchy, and scaly. Psoriasis varies in severity from small localized patches to complete b ...

vitiligo Vitiligo (, ) is a chronic autoimmune disorder that causes patches of skin to lose pigment or color. The cause of vitiligo is unknown, but it may be related to immune system changes, genetic factors, stress, or sun exposure, and susceptibili ...

autoimmune hepatitis Autoimmune hepatitis, formerly known as lupoid hepatitis, plasma cell hepatitis, or autoimmune chronic active hepatitis, is a chronic, autoimmune disease of the liver that occurs when the body's immune system attacks liver cells, causing the liv ...

dermatitis herpetiformis Dermatitis herpetiformis (DH) is a chronic autoimmune blistering skin condition, characterised by intensely itchy blisters filled with a watery fluid. DH is a cutaneous manifestation of coeliac disease, although the exact causal mechanism is not ...

primary sclerosing cholangitis Primary sclerosing cholangitis (PSC) is a long-term progressive disease of the liver and gallbladder characterized by inflammation and scarring of the bile ducts, which normally allow bile to drain from the gallbladder. Affected individuals may ...

, and more. The only available treatment for celiac disease is a strict

gluten-free diet A gluten-free diet (GFD) is a nutritional plan that strictly excludes gluten, which is a mixture of prolamin proteins found in wheat (and all of its species and hybrids, such as spelt, kamut, and triticale), as well as barley, rye, and oats ...

in which the affected person does not ingest any gluten-containing products. There have been searches for an affordable and much better treatment, but the only treatment remains to abstain from ingesting any gluten.

References

External links

* {{MeshName, Gliadin Gluten Glycoproteins Seed storage proteins