Gluten is the generic term used for the storage protein in grains. The specific protein found in wheat, rye and barley is the toxic substance that causes an immune response in individuals with celiac disease.
The storage protein gluten is composed of different protein fractions, which are made up of different peptides in each cereal. Depending on the protein fraction, toxic peptides occur during degradation in the body, which lead to changes in the intestine of coeliac disease patients.
Gluten is the generic term used for the family of storage proteins found in wheat rye and barley . The protein is found in the endosperm of the grain seed. The protein content is approximately 5 - 10% by weight of the dry grain matter and thus around 80% of the grain protein. Insert diagram of plant kingdoms that identifies the tribes of grains and therefore identifies those that are gluten containing and those that are not.
Composition of gluten
The proteins that are toxic to individuals with celiac disease are rich in the amino acids proline and glutamine. This group of toxic proteins are collectively called prolamins. Each grain has its own unique prolamin fractions; gliadin (wheat), secalin (rye), and hordein (barley). The prolamin fraction consists of two sub groups gliadins, which are alcohol soluble, and glutenins which are not soluble in alcohol.
Each grain has its own specific sequence of gliadins and glutenins which consist of hundreds of specific components and amino acid sequences. The specific protein components and amino acid sequences make it possible to identify the grain and even the cultivated strain of the grain. The amino acid sequencing is the specific grains own fingerprint. These strands of amino acids with their specific make up and sequencing are the building blocks of proteins in general and grain proteins specifically.
Digestion of Proteins
In normal digestion these long strands of protein are broken down by digestive enzymes. The enzymes cleave or break off groups of amino acids called peptides. The majority of these peptides can be further broken down, absorbed through the intestine and then transported and used in the body. However, gluten cannot be broken down by the digestive enzymes. There are three specific gluten peptides (alpha gliadin) that can trigger a T-cell response. Of the three there is one particular gliadin peptide with a specific 33 amino acid sequence that is responsible for the aggressive immune response in the gut.
Prolamins are rich in proline and glutamine, two amino acids that are difficult to digest. Neither proline nor glutamine are classified as essential amino acids. It is the high proline and glutamine content in gluten that prevents the proteins from being completely broken down by the digestive enzymes. The long-term result of this is that toxic oligopeptides, proteins with up to ten amino acids, are present in the small intestine. Proline is degraded using an oxidase, which converts proline into glutamic acid using glutamate-gamma-semialdehyde. The glutamic acid in turn must be converted into glutamine, so that it can be transported to the brain, where it is necessary for protein synthesis.
Examples of toxic oligopeptides
Peptides are identified by their specific amino acid sequences. Letters of the alphabet are used to code or label the specific amino acids.
- Peptides with the amino-acid sequence LGQQQPFPPQQPY are referred to as "toxic peptides" and cause cell clumping, programmed cell death, new arrangement of the structural protein actin and lead to morphological changes in the intestine of celiac patients without immunogenicity for T-cells.
- Peptides with the amino-acid sequence QLQPFPQPQLPY are referred to as "T-cell immunogenic peptides", do not lead to any change in the intestine of celiac disease patients.
Holmes, G, Catassi C, Fasano A. Fast Facts: Celiac Disease 2009 Health Press Oxford UK