text: Butyric acid (pK 4.82) is fully ionized at physiological pH, so its anion is the material that is mainly relevant in biological systems.
  It is one of two primary endogenous agonists of human hydroxycarboxylic acid receptor 2 (, also known as GPR109A), a  G protein-coupled receptor (GPCR),
  Like other short-chain fatty acids (SCFAs), butyrate is an agonist at the free fatty acid receptors FFAR2 and FFAR3, which function as nutrient sensors that facilitate the homeostatic control of energy balance; however, among the group of SCFAs, only butyrate is an agonist of HCA. It is also an HDAC inhibitor (specifically, HDAC1, HDAC2, HDAC3, and HDAC8), a drug that inhibits the function of histone deacetylase enzymes, thereby favoring an acetylated state of histones in cells. Histone acetylation loosens the structure of chromatin by reducing the electrostatic attraction between histones and DNA. In general, it is thought that transcription factors will be unable to access regions where histones are tightly associated with DNA (i.e., non-acetylated, e.g., heterochromatin). Therefore, butyric acid is thought to enhance the transcriptional activity at promoters, which are typically silenced or downregulated due to histone deacetylase activity.