Digestion of Fats

 Digestion of Fats

Dietary Fats

The most common fats of the diet are the neutral fats, also known as triglycerides, each molecule of which is composed of a glycerol nucleus and three fatty acid side chains. Neutral fat is a major constituent in food of animal origin but much, much less so in food of plant origin. In the usual diet are also small quantities of phospholipids, cholesterol, and cholesterol esters. The phospholipids and cholesterol esters contain fatty acid and therefore can be considered fats. Cholesterol, however, is a sterol compound that contains no fatty acid, but it does exhibit some of the physical and chemical characteristics of fats; plus, it is derived from fats and is metabolized similarly to fats. Therefore, cholesterol is considered, from a dietary point of view, a fat.

Digestion of Fats in mouth and stomach

Lingual lipase is a type of a family of digestive  enzymes called triacylglycerol lipases that uses the catalytic triad of aspartate, histidine, and serine to hydrolyze medium and 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 secrete lingual lipase to the grooves of the circumvallate and foliate papillae, co localized with fat taste receptors.

The hydrolysis of the dietary fats is essential for fat absorption by the small intestine as long chain Triacylglycerides cannot be absorbed, and as much as 30% of fat is hydrolyzed within 1 to 20 minutes of ingestion by lingual lipase alone. Lingual lipase, together with gastric lipase, comprises the two acidic lipases.

Mechanism

Lingual lipase uses a catalytic triad consisting of aspartic acid, histidine and serine, 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 diacylglycerol leaving group is protonated by Histidine.

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 deprotonations. 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 triglycerides to help breakdown food as a part of saliva composition.

Digestion of Fats in the Intestine

A small amount of triglycerides is digested in the stomach by lingual lipase that is secreted by lingual glands in the mouth and swallowed with the saliva. This amount of digestion is less than 10 percent and generally unimportant. Instead, essentially all fat digestion occurs in the small intestine as follows.

Emulsification by Bile Acids and Lecithin

The first step in fat digestion is physically to break the fat globules into small sizes so that the water-soluble digestive enzymes can act on the globule surfaces. This process is called emulsification of the fat, and it begins by agitation in the stomach to mix the fat with the products of stomach digestion. Then, most of the emulsification occurs in the duodenum under the influence of bile, the secretion from the liver that does not contain any digestive enzymes.

However, bile does contain a large quantity of bile salts, as well as the phospholipid lecithin.

Both of these, but especially the lecithin, are extremely important for emulsification of the fat. The polar parts (the points where ionization occurs in water) of the bile salts and lecithin molecules are highly soluble in water, whereas most of the remaining portions of their molecules are highly soluble in fat. Therefore, the fat-soluble portions of these liver secretions dissolve in the surface layer of the fat globules, with the polar portions projecting.

The polar projections, in turn, are soluble in the surrounding watery fluids, which greatly decreases the interfacial tension of the fat and makes it soluble as well. When the interfacial tension of a globule of non-miscible fluid is low, this non-miscible fluid, on agitation, can be broken up into many tiny particles far more easily than it can when the interfacial tension is great.

Consequently, a major function of the bile salts and lecithin, especially the lecithin, in the bile is to make the fat globules readily fragmentable by agitation with the water in the small bowel.

Each time the diameters of the fat globules are significantly decreased as a result of agitation in the small intestine, the total surface area of the fat increases many fold. Because the average diameter of the fat particles in the intestine after emulsification has occurred is less than 1 micrometer, this represents an increase of as much as 1000-fold in total surface areas of the fats caused by the emulsification process.

The lipase enzymes are water soluble compounds and can attack the fat globules only on their surfaces. Consequently, this detergent function of bile salts and lecithin is very important for digestion of fats.

Digestion of Triglycerides by Pancreatic Lipase

The most important enzyme for digestion of the triglycerides is pancreatic lipase, present in enormous quantities in pancreatic juice, enough to digest within 1 minute all triglycerides that it can reach. In addition, the enterocytes of the small intestine contain additional lipase, known as enteric lipase, but this is usually not needed.

The hydrolysis of triglycerides is a highly reversible process; therefore, accumulation of monoglycerides and free fatty acids in the vicinity of digesting fats quickly blocks further digestion. But the bile salts play the additional important role of removing the monoglycerides and free fatty acids from the vicinity of the digesting fat globules almost as rapidly as these end products of digestion are formed. This occurs in the following way-

Bile salts, when in high enough concentration in water, have the propensity to form micelles, which are small spherical, cylindrical globules 3 to 6 nanometers in diameter composed of 20 to 40 molecules of bile salt. These develop because each bile salt molecule is composed of a sterol nucleus that is highly fat-soluble and a polar group that is highly water-soluble.

The sterol nucleus encompasses the fat digestate, forming a small fat globule in the middle of a resulting micelle, with polar groups of bile salts projecting outward to cover the surface of the micelle. Because these polar groups are negatively charged, they allow the entire micelle globule to dissolve in the water of the digestive fluids and to remain in stable solution until the fat is absorbed into the blood.

The bile salt micelles also act as a transport medium to carry the monoglycerides and free fatty acids, both of which would otherwise be relatively insoluble, to the brush borders of the intestinal epithelial cells. There the monoglycerides and free fatty acids are absorbed into the blood, as discussed later, but the bile salts themselves are released back into the chyme to be used again and again for this ferrying process.

Digestion of Cholesterol Esters and Phospholipids

Most cholesterol in the diet is in the form of cholesterol esters, which are combinations of free cholesterol and one molecule of fatty acid. Phospholipids also contain fatty acid within their molecules.

Both the cholesterol esters and the phospholipids are hydrolyzed by two other lipases in the pancreatic secretion that free the fatty acids-

·        enzyme cholesterol ester hydrolase to hydrolyze the cholesterol

·        ester phospholipase A2 to hydrolyze the phospholipid

The bile salt micelles play the same role in ferrying free cholesterol and phospholipid molecule digestates that they play in ferrying Monoglycerides and free fatty acids. Indeed, essentially no cholesterol is absorbed without this function of the micelles.