Crystal structure of human gastric lipase (PDB 1hlg).
|Locus||Chr. 10 q23|
Gastric lipase is an acidic lipase secreted by the gastric chief cells in the fundic mucosa in the stomach. It has a pH optimum of 3–6. Gastric lipase, together with lingual lipase, comprise the two acidic lipases. These lipases, unlike alkaline lipases (such as pancreatic lipase), do not require bile acid or colipase of optimal enzymatic activity. Acidic lipases make up 30% of lipid hydrolysis occurring during digestion in the human adult, with gastric lipase contributing the most of the two acidic lipases. In neonates, acidic lipases are much more important, providing up to 50% of total lipolytic activity.
Gastric lipase hydrolyzes the ester bonds of triglycerides in the stomach. Fatty acids and diacylglycerols are produced from this reaction. The long chain free fatty acids have the ability to prevent gastric lipase from hydrolyzing more triglycerides. In this case, gastric acid will be responsible for less than 30% of lipid hydrolysis. These enzymes are found in the cytoplasm and cell membranes of gastric cells. Gastric lipase is not the primary lipase needed for the majority of triglyceride hydrolysis. Outside of the stomach, gastric lipase can hydrolyze triacylglycerol in the duodenum with the help of other lipases and bile secretion. It is an essential enzyme for hydrolyzing milk fat globule membranes. For a newborn with an underdeveloped pancreas, LIPF plays a more important role in lipid digestion compared to an adult with a fully functioning pancreas. There is typically an increase in production of LIPF when the pancreas is unable to operate at its optimal potential. Low levels of LIPF are typically seen in the tumors of gastric cancer patients.
Gastric lipase can partially compensate for the decrease in production of pancreatic lipase associated with pancreatic dysfunction, giving some means for the body to digest lipids. A limitation of acidic lipases is that they remove only one fatty acid from each triacylglycerol. The free fatty acid can readily cross the epithelial membrane lining the gastrointestinal tract, but the diacylglycerol cannot be transported across. This leaves the acidic lipases less efficient than alkaline lipases.
Gastric lipase is a polypeptide of 371 residues in length. The structure of gastric lipase was determined using X-ray diffraction with a resolution of 3.00 Å, and is composed of 41% helices and 14% beta sheets. Gastric lipase belongs to the α/β-hydrolase-fold family. It possesses a classical catalytic triad (Ser-153, His-353, Asp-324) and an oxyanion hole (backbone NH groups of Gln-154 and Leu-67) analogous to serine proteases.
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