Ghrelin O-acyltransferase

The structure of ghrelin O-acyltransferase has not been fully elucidated or tested experimentally. Using biochemical mapping tools, researchers showed that the ghrelin O-acyltransferase enzyme consists of eleven transmembrane helical domains and one reentrant loop. The C-terminus of ghrelin O-acyltransferase is located on the cytosolic side of the endoplasmic reticulum (ER) while the N-terminus resides in the lumen of the membrane. The topology models of ghrelin O-acyltransferase are similar to those of acetyl-coenzyme A acetyltransferase 1 and glycerol uptake protein 1. This is consistent with ghrelin O-acyltransferase being a member of the membrane bound O-acyl transferase family. Ghrelin O-acyltransferase contains conserved asparagine (Asn307) and histidine (His338) residues that are found across the membrane bound O-acyl transferase family. These amino acids are positioned on opposite side of the endoplasmic reticulum membrane. His338 resides in the lumen and is necessary for the observed catalytic activity of the enzyme. == History ==

The first reported discovery of ghrelin O-acyl transferase was in February 2008 by two separate research groups at Eli Lilly and the University of Texas at Austin (UT). Preclinical research has shown that this drug prevented weight gain with positive pharmacological and safety profiles. Additionally, ghrelin O-acyl transferase inhibition has been used in clinical trials in the United Kingdom in studying appetite regulation of post-surgical bariatric patients. The study measured, among other things, the effect of decreased ghrelin levels on gut hormone, adipokine, and cytokine levels. == Chemical properties ==

Structure-activity studies have demonstrated that the ‘GSSF’ N-terminal sequence of ghrelin is essential to ghrelin O-acyl transferase substrate recognition. While the enzyme will accept sequences with a threonine reside in place of serine, catalytic activity is reduced in these cases. Ghrelin O-acyl transferase is also poorly stereospecific and is able to recognize and acylate both stereoisomers of serine. The enzymatic activity of GOAT is also altered when treated with cysteine-modifying substances. Interestingly, the enzyme is relatively insensitive to pH changes in the range of 6–9. Kinetic studies also suggest that GOAT activity is consistent with Michaelis-Menten kinetics in the acylation of ghrelin. One paper proposes that bioavailability of the GOAT substrates represent the rate-limiting step of the acyl transfer reaction and not the chemical process itself. Biochemical assays Several biochemical assays have been developed to study the activity of the enzyme under in vitro conditions. The most common is the use of an amino acid and radioactive isotope tags to monitor the formation of reaction products. Also widespread is the use of indirect methods to observe ghrelin O-acyl transferase function such as the detection of acylated ghrelin products, which indicate enzyme activity. These assays are based on high-throughput ELISA based techniques. == Metabolic function ==

In humans, the ghrelin O-acyl transferase function enzyme post-translationally modifies an inactive form of ghrelin, known as proghrelin, on an N-terminus serine residue. This function is essential in forming an active form of ghrelin, which can then be secreted into the bloodstream. Ghrelin O-acyltransferase also plays a role in preventing hypoglycemia when there is prolonged negative energy balance in the body. GOAT plays a crucial role in the ability to maintain glucose levels during fasting and starvation. Additionally, the enzyme has a key role in modulating the secretion and sensitivity of insulin. These interactions are used in regulating glucose metabolism in the body. Functional genes for MBOAT4 are not present in some groups of animals. Among reptiles, snakes, chameleons and toadhead agamas have either lost the gene or it has become so mutated that it can no longer give rise to the protein. These animals can go for long periods without eating and commonly have their meals spaced weeks, months or even years apart. It appears that this gene loss has happened independently in several evolutionary separated groups of animals. They also lack ghrelin. Lack of ghrelin and MBOAT4 may be relevant to the way that these reptiles can live at a low energy demand for months or years. Their fat stores are used at a low level. The presence of many mutations in genes for aspects of mitochondrial function in these animals may also be involved in their reduced energy demand. Regulation Ghrelin O-acyltransferase is highly regulated by energy balance and is upregulated when energy is restricted, such as when fasting. There have also been recent advances in small molecule, drug-like inhibition of the ghrelin O-acyl transferase enzyme. One study has shown that a class of triterpenoid related molecules has significant inhibitory activity towards the enzyme. These analyses suggest that there is an essential cystine residue in the active site of ghrelin O-acyl transferase that has a role in the enzyme's catalytic function or inhibitor site binding. This class of inhibitors have been shown to alter ghrelin signaling and reportedly result in weight loss and increased glucose tolerance. == See also ==