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Lingual lipase
Lingual lipase is a member of a family of digestive enzymes called lipases, EC 3.1.1.3, that use the catalytic triad of aspartate, histidine, and serine to hydrolyze long-chain triglycerides into partial glycerides and free fatty acids. The enzyme, released into the mouth along with the saliva, catalyzes the first reaction in the digestion of dietary lipid, with diglycerides being the primary reaction product. However, due to the unique characteristics of lingual lipase, including a pH optimum 4.5 -5.4 and its ability to catalyze reactions without bile salts, the lipolytic activity continues through to the stomach. Enzyme release is signaled by autonomic nervous system after ingestion, at which time the serous glands under the circumvallate and foliate lingual papillae on the surface of the tongue (see Figure 1)
secrete lingual lipase to the grooves of the circumvallate and foliate papillae. The hydrolysis of the dietary fats is essential for fat absorption by the small intestine, as long chain triacyglycerides cannot be absorbed, and as much as 30% of fat is hydrolyzed within 1 to 20 minutes of ingestion by lingual lipase alone.〔
== Proposed mechanism ==
Lingual lipase uses a catalytic triad consisting of aspartatic acid-203 (Asp), histidine-257 (His), and serine-144 (Ser), to initiate the hydrolysis of a triglyceride into a diacylglyceride and a free fatty acid. First, there is a series of deprotonations that make the serine a better nucleophile. The lone pair on the oxygen of the serine then undergoes a nucleophilic addition to either the first or the third carbonyl of the triacylglycerol. Next, the electrons that had moved to form the carbonyl transfer back down to reform the carbonyl. Then the diacyclglycerol leaving group is protonated by His-257. Following another round of deprotonations, the lone pair on the oxygen of water undergoes a nucleophilic addition to the carbonyl that reformed in the previous step. The electrons that had moved up from the carbonyl come back down to reform it and kick off the Ser, which again induces the chain of deprotonation. The final products of the reaction are the conserved catalytic triad, a diacylglycerol and a free fatty acid. Monoacylglyceride is also present in a lower concentration and is produced following a second round of hydrolysis by the same mechanism. It acts on tryglicerides to help breakdown food as a part of saliva composition.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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