Introduction

Figure 1. This image shows the location of the DGAT protein within the Endoplasmic Reticulum Membrane

DGAT, or Diacylglycerol Acyltransferase, is a polytopic endoplasmic reticulum membrane protein embedded within the membrane of the ER. DGAT is highly expressed in epithelial cells of the small intenstine of homo sapiens. It can also be found in the liver, where it helps synthesize fats for storage, and the female mammary glands, where it produces fat in the milk. ^[1]

DGAT makes triglycerides from a diglyceride in plasma (Figure 2). In order to do this, DGAT uses two substrates: a fatty acyl-CoA and a diacylglycerol substrate. The basic mechanism consists of a lone pair on a hydroxyl group of glycerol attacking the carbon of the thioester bond of CoA. This results in the breakage of the thioester bond, and the attached acyl group attaches to the glycerol, creating a triglyceride.

Figure 2. General schematic of DGAT's function.

Structure

DGAT consists of two domains, one cytoplasmic and one luminal. The cytoplasmic domain interacts with the interior of the cell and relays signals. The luminal domain senses misfolded proteins. The structure of DGAT consists of two protein chains, one ligand, two polymers, eighteen and zero beta sheets. The transmembrane helices form a large central cavity within the membrane that opens to the bilayer via a wide lateral gate. Through openings on the cytosolic and luminal sides of DGAT, this central cavity is also accessible. The majority of the transmembrane helices present within the structure also form a concave-shaped ridge on either side of the membrane. These aspects of the domain structure are deemed as the 'MBOAT core'. Within this core, a tunnel-like region, similar to a binding pocket, is also present. Access to the active site of DGAT by substrates is done through the lateral gate, which lies on the ER lumen side, within the membrane. This tunnel-like region is referred to as the .^[1]

Active Site

The active site of DGAT is located within the membrane, with the catalytic histidine residue (-represented in white) buried inside the central cavity. This central cavity serves as the catalytic site. The acyl-acceptor lipid substrates access the active site through the lateral gate within the membrane. The active site also contains and several nearby (including Asn378, Gln437, and Gln465) whose side chains are oriented towards the cavity center. These residues interact and create a hydrophilic channel within the active site. The His415 residue is also likely involved in catalysis, making it increasingly significant. In face, single mutations of His415 and Asn378 terminated DGAT activity. This suggests that the central cavity of DGAT within the membrane is the catalytic site. ^[1].