β-Hydroxybutyric acid, also known as 3-hydroxybutyric acid or BHB, is an organic compound and a beta hydroxy acid with the chemical formula CH3CH(OH)CH2CO2H; its conjugate base is β-hydroxybutyrate, also known as 3-hydroxybutyrate. β-Hydroxybutyric acid is a chiral compound with two enantiomers: D-β-hydroxybutyric acid and L-β-hydroxybutyric acid. Its oxidized and polymeric derivatives occur widely in nature. In humans, D-β-hydroxybutyric acid is one of two primary endogenous agonists of hydroxycarboxylic acid receptor 2 (HCA2), a Gi/o-coupled G protein-coupled receptor (GPCR).
Free fatty acid receptors (FFARs) are G-protein coupled receptors (GPRs). GPRs are a large family of receptors. They reside on their parent cells' surface membranes, bind any one of a specific set of ligands that they recognize, and thereby are activated to elicit certain types of responses in their parent cells. Humans express more than 800 different types of GPCRs. FFARs are GPCR that bind and thereby become activated by particular fatty acids. In general, these binding/activating fatty acids are straight-chain fatty acids consisting of a carboxylic acid residue, i.e., -COOH, attached to aliphatic chains, i.e. carbon atom chains of varying lengths with each carbon being bound to 1, 2 or 3 hydrogens. For example, propionic acid is a short-chain fatty acid consisting of 3 carbons (C's), CH3-CH2-COOH, and docosahexaenoic acid is a very long-chain polyunsaturated fatty acid consisting of 22 C's and six double bonds : CH3-CH2-CH1=CH1-CH2-CH1=CH1-CH2-CH1=CH1-CH2-CH1=CH1-CH2-CH1=CH1-CH2-CH1=CH1-CH2-CH2-COOH.
C-C chemokine receptor type 1 is a protein that in humans is encoded by the CCR1 gene.
Sphingosine-1-phosphate receptor 3 also known as S1PR3 is a human gene which encodes a G protein-coupled receptor which binds the lipid signaling molecule sphingosine 1-phosphate (S1P). Hence this receptor is also known as S1P3.
Probable G-protein coupled receptor 12 is a protein that in humans is encoded by the GPR12 gene.
G protein-coupled receptor 32, also known as GPR32 or the RvD1 receptor, is a human receptor (biochemistry) belonging to the rhodopsin-like subfamily of G protein-coupled receptors.
Putative G-protein coupled receptor 42 is a protein that in humans is encoded by the GPR42 gene. The human GPR gene is located at the same site as the human FFAR1, FFAR, and FFAR3 genes, i.e., on the long arm of chromosome 19 at position 23.33. This gene appears to be a segmental duplication of the FFAR3 gene. The human GPR42 gene codes for several proteins with a FFAR3-like structure but their expression in various cell types and tissues as well as their activities and functions have not yet been clearly defined in any scientific publication followed by PubMed as of 2023.
Frizzled-7(Fd-7) is a protein that in humans is encoded by the FZD7 gene.
Lysophosphatidic acid receptor 2 also known as LPA2 is a protein that in humans is encoded by the LPAR2 gene. LPA2 is a G protein-coupled receptor that binds the lipid signaling molecule lysophosphatidic acid (LPA).
Hydroxycarboxylic acid receptor 1 (HCA1), formerly known as G protein-coupled receptor 81 (GPR81), is a protein that in humans is encoded by the HCAR1 gene. HCA1, like the other hydroxycarboxylic acid receptors HCA2 and HCA3, is a Gi/o-coupled G protein-coupled receptor (GPCR). The primary endogenous agonist of HCA1 is lactic acid (and its conjugate base, lactate). More recently, 3,5-dihydroxybenzoic acid has been reported to activate HCA1.
Putative P2Y purinoceptor 10 is a protein that, in humans, is encoded by the P2RY10 gene.
Pyroglutamylated RFamide peptide receptor also known as orexigenic neuropeptide QRFP receptor or G-protein coupled receptor 103 (GPR103) is a protein that in humans is encoded by the QRFPR gene.
Hydroxycarboxylic acid receptor 2 (HCA2), also known as GPR109A and niacin receptor 1 (NIACR1), is a protein which in humans is encoded (its formation is directed) by the HCAR2 gene and in rodents by the Hcar2 gene. The human HCAR2 gene is located on the long (i.e., "q") arm of chromosome 12 at position 24.31 (notated as 12q24.31). Like the two other hydroxycarboxylic acid receptors, HCA1 and HCA3, HCA2 is a G protein-coupled receptor (GPCR) located on the surface membrane of cells. HCA2 binds and thereby is activated by D-β-hydroxybutyric acid (hereafter termed β-hydroxybutyric acid), butyric acid, and niacin (also known as nicotinic acid). β-Hydroxybutyric and butyric acids are regarded as the endogenous agents that activate HCA2. Under normal conditions, niacin's blood levels are too low to do so: it is given as a drug in high doses in order to reach levels that activate HCA2.
Relaxin/insulin-like family peptide receptor 4, also known as RXFP4, is a human G-protein coupled receptor.
Oxoeicosanoid receptor 1 (OXER1) also known as G-protein coupled receptor 170 (GPR170) is a protein that in humans is encoded by the OXER1 gene located on human chromosome 2p21; it is the principal receptor for the 5-Hydroxyicosatetraenoic acid family of carboxy fatty acid metabolites derived from arachidonic acid. The receptor has also been termed hGPCR48, HGPCR48, and R527 but OXER1 is now its preferred designation. OXER1 is a G protein-coupled receptor (GPCR) that is structurally related to the hydroxy-carboxylic acid (HCA) family of G protein-coupled receptors whose three members are HCA1 (GPR81), HCA2, and HCA3 ; OXER1 has 30.3%, 30.7%, and 30.7% amino acid sequence identity with these GPCRs, respectively. It is also related to the recently defined receptor, GPR31, for the hydroxyl-carboxy fatty acid 12-HETE.
Lysophosphatidic acid receptor 3 also known as LPA3 is a protein that in humans is encoded by the LPAR3 gene. LPA3 is a G protein-coupled receptor that binds the lipid signaling molecule lysophosphatidic acid (LPA).
Neuronal acetylcholine receptor subunit alpha-2, also known as nAChRα2, is a protein that in humans is encoded by the CHRNA2 gene. The protein encoded by this gene is a subunit of certain nicotinic acetylcholine receptors (nAchR).
GPR182 is a human gene which is an orphan G-protein coupled receptor.
3-Hydroxyoctanoic acid is a beta-hydroxy acid that is naturally produced in humans, other animals, and plants.
The hydroxycarboxylic acid receptor (abbreviated HCA receptor and HCAR) family includes the following human proteins:
