Free fatty acid receptor

GPR84 binds and is activated by medium-chain fatty acids consisting of 9 to 14 carbon atoms such as capric, undecaenoic, and lauric acids. It has been recognized as a possible member of the free fatty acid receptor family in some publications but has not yet been given this designation perhaps because these medium-chain fatty acid activators require very high concentrations (e.g., in the micromolar range) to activate it. This allows that there may be a naturally occurring agent(s) that activates GPR84 at lower concentrations than the cited fatty acids. Consequently, GPR89 remains classified as an orphan receptor, i.e., a receptor whose naturally occurring activator(s) is unclear. GPR109A binds and thereby is activated by the short-chain fatty acids, butyric, β-hydroxybutyric, pentanoic and hexanoic acids and by the intermediate-chain fatty acids heptanoic and octanoic acids. GPR109A is also activated by niacin but only at levels that are in general too low to activate it unless it is given as a drug in high doses. GPR81 (also termed hydroxycarboxylic acid receptor 1, HCAR1, GPR104, GPR81, LACR1, TA-GPCR, TAGPCR, and FKSG80) binds and is activated by the short-chain fatty acids, lactic acid and β-hydroxybutyric acid. A more recent study reported that it is also activated by the compound 3,5-dihydroxybenzoic acid. GPR109B (also known as hydroxycarboxylic acid receptor 3, HCA3, niacin receptor 2, and NIACR2) binds and is activated by the medium-chain fatty acid, 3-hydroxyoctanoate, niacin, and by four compounds viz., hippuric acid, The latter three compounds are produced by Lactobacillus and Bifidobacterium species of bacteria that occupy the gastrointestinal tracts of animals and humans. ==References==