Thionins are a family of small proteins found solely in

higher plants Vascular plants (), also called tracheophytes () or collectively Tracheophyta (), form a large group of land plants ( accepted known species) that have lignified tissues (the xylem) for conducting water and minerals throughout the plant. They ...

. Typically, a thionin consists of 45–48

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...

residues. 6–8 of these are

cysteine Cysteine (symbol Cys or C; ) is a semiessential proteinogenic amino acid with the formula . The thiol side chain in cysteine often participates in enzymatic reactions as a nucleophile. When present as a deprotonated catalytic residue, sometime ...

forming 3–4

disulfide bonds In biochemistry, a disulfide (or disulphide in British English) refers to a functional group with the structure . The linkage is also called an SS-bond or sometimes a disulfide bridge and is usually derived by the coupling of two thiol groups. In ...

. They include phoratoxins and viscotoxins. Alpha- and beta- thionins are related to each other. The gamma thionins have a superficially similar structure but are an unrelated class of protein, now called plant defensins.

Activity

The proteins are toxic to animal cells, presumably attacking the cell membrane and rendering it permeable: this results in the inhibition of sugar uptake and allows potassium and phosphate ions, proteins, and nucleotides to leak from cells. Thionins are mainly found in seeds where they may act as a defence against consumption by animals. A barley (''

Hordeum vulgare Barley (''Hordeum vulgare''), a member of the grass family, is a major cereal grain grown in temperate climates globally. It was one of the first cultivated grains, particularly in Eurasia as early as 10,000 years ago. Globally 70% of barley pr ...

'') leaf thionin that is highly toxic to plant pathogens and is involved in the mechanism of plant defence against microbial infections has also been identified. The hydrophobic protein

crambin Crambin is a small seed storage protein from the Abyssinian cabbage. It belongs to thionins Thionins are a family of small proteins found solely in higher plants. Typically, a thionin consists of 45–48 amino acid residues. 6–8 of these are ...

from the Abyssinian kale ('' Crambe abyssinica'') is also a member of the thionin family. Some thionins have

cytotoxic Cytotoxicity is the quality of being toxic to cells. Examples of toxic agents are an immune cell or some types of venom, e.g. from the puff adder (''Bitis arietans'') or brown recluse spider (''Loxosceles reclusa''). Cell physiology Treating cells ...

activity and they are therefore interesting in the development of new

drugs A drug is any chemical substance that causes a change in an organism's physiology or psychology when consumed. Drugs are typically distinguished from food and substances that provide nutritional support. Consumption of drugs can be via inhalat ...

against

cancer Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumors, which do not spread. Possible signs and symptoms include a lump, abnormal b ...

with novel action mechanisms. No thionin has yet been developed into an anti-cancer drug. Thionin is also a minor protein found in mustard (Brassica napus L.) seeds.

Databases

A database for antimicrobial peptides, including thionins is available: PhytAMP.;

References

{{Membrane proteins Peripheral membrane proteins Plant toxins Antimicrobial peptides

Activity

Databases

See also

References