Catechin

Catechin is a specific type of secondary metabolite known as a flavan-3-ol.

These molecules belong to a larger group of plant substances called flavonoids. In the natural world, catechins serve vital antioxidant roles for plants. An antioxidant helps protect living cells from damage. Catechin is also a fundamental building block for larger structures. It helps create condensed tannins called proanthocyanidins.

The chemical structure of catechin is quite complex. It consists of two benzene rings, labeled as the A and B rings. It also contains a dihydropyran heterocycle, which is known as the C ring. The C ring has a hydroxyl group located on carbon 3. The A ring is similar to a resorcinol moiety. The B ring is similar to a catechol moiety. Because the molecule has two chiral centers on carbons 2 and 3, it exists in four different diastereoisomers. These include two isomers in a trans configuration, called catechin, and two in a cis configuration, called epicatechin. The most common isomer is (+)-catechin. Another common version is (−)-epicatechin.

Plants create catechin through a detailed biological process called biosynthesis.

This process begins with a starter unit called 4-hydroxycinnamoyl CoA. This unit is produced from L-phenylalanine through the Shikimate pathway. First, an enzyme called phenylalanine ammonia lyase converts L-phenylalanine into cinnamic acid. Then, cinnamate 4-hydroxylase oxidizes it into 4-hydroxycinnamic acid. Next, the enzyme chalcone synthase catalyzes a condensation reaction. This joins 4-hydroxycinnamoyl CoA with three molecules of malonyl-CoA to form chalcone. Following this, chalcone isomerase transforms chalcone into naringenin. Further oxidation by flavonoid 3′-hydroxylase and flavanone 3-hydroxylase produces eriodictyol and taxifolin. Finally, enzymes like dihydroflavanol 4-reductase and leucoanthocyanidin reductase reduce taxifolin to yield catechin.

Catechin is found in many different plants and foods. It is a ubiquitous constituent of vascular plants. You can find high amounts in tea, cacao, and grapes. According to one database, cocoa contains the highest amount of catechins at 108 mg per 100 g. Prune juice contains 25 mg per 100 ml, and broad bean pods contain 16 mg per 100 g. Açaí oil also contains (+)-catechins at 67 mg/kg. In the mouth, these molecules can create a slightly astringent taste. This feeling is often described as a dryness rather than bitterness.

In the environment, catechins can act as powerful tools for plants. Some plants use a process called allelopathy to influence their neighbors. For example, the spotted knapweed plant releases catechin isomers through its roots. This may act as an antibiotic or herbicide to stop other plants from growing. One theory suggests it causes a wave of reactive oxygen species to kill target root cells. However, most European plants have defenses against this. In North America, the knapweed is an invasive weed because few plants are protected. Catechins also help protect other plants, such as strawberries, from infections. They can even help coffee plants by inhibiting certain diseases.

When humans eat catechins, their bodies must process them through metabolism.

Most dietary catechins are metabolized by the colonic microbiome into gamma-valerolactones and hippuric acids. These are then transformed in the liver through glucuronidation, sulfation, and methylation. Catechins are also absorbed in the jejunum of the gastrointestinal tract. The shape of the molecule, or its stereochemical configuration, affects how well it is absorbed. For instance, uptake is highest for (−)-epicatechin and lowest for (+)-catechin.

While catechins are common in diets, they can sometimes cause health issues. Catechin and its metabolites can bind to red blood cells. This may induce autoantibodies, which can lead to renal failure or haemolytic anaemia. Because of this, a drug called Catergen was withdrawn from the market in 1985. Additionally, catechins from green tea can be hepatotoxic. The European Food Safety Authority recommends not exceeding 800 mg of green tea catechins per day. Some research also suggests that eating seven cups of green tea per day might slightly reduce prostate cancer risk. Scientists are currently researching nanoparticle methods to improve how these molecules are delivered in medicine.