Digestion (Homo sapiens)
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Description
Dietary carbohydrates, fats, and proteins must be broken down to their constituent monosaccharides, fatty acids and sterols, and amino acids, respectively, before they can be absorbed in the intestine.
Dietary lipids such as long-chain triacylglycerols and cholesterol esters are hydrolyzed in the stomach and small intestine to yield long-chain fatty acids, monoacylglycerols, glycerol and cholesterol through the action of a variety of lipases, and are then absorbed into enterocytes.
Carbohydrates include starch (amylose and amylopectin) and disaccharides such as sucrose, lactose, maltose and, in small amounts, trehalose. The digestion of starch begins with the action of amylase enzymes secreted in the saliva and small intestine, which convert it to maltotriose, maltose, limit dextrins, and some glucose. Digestion of the limit dextrins and disaccharides, both dietary and starch-derived, to monosaccharides - glucose, galactose, and fructose - is accomplished by enzymes located on the luminal surfaces of enterocytes lining the microvilli of the small intestine.
Dietary protein is hydrolyzed to dipeptides and amino acids by the action of pepsin in the stomach and an array of intestinal hydrolases. All of these enzymes are released in inactive (proenzyme) forms and activated by proteolytic cleavage within the gastrointestinal lumen (Van Beers et al. 1995; Yamada 2015). View original pathway at:Reactome.
Dietary lipids such as long-chain triacylglycerols and cholesterol esters are hydrolyzed in the stomach and small intestine to yield long-chain fatty acids, monoacylglycerols, glycerol and cholesterol through the action of a variety of lipases, and are then absorbed into enterocytes.
Carbohydrates include starch (amylose and amylopectin) and disaccharides such as sucrose, lactose, maltose and, in small amounts, trehalose. The digestion of starch begins with the action of amylase enzymes secreted in the saliva and small intestine, which convert it to maltotriose, maltose, limit dextrins, and some glucose. Digestion of the limit dextrins and disaccharides, both dietary and starch-derived, to monosaccharides - glucose, galactose, and fructose - is accomplished by enzymes located on the luminal surfaces of enterocytes lining the microvilli of the small intestine.
Dietary protein is hydrolyzed to dipeptides and amino acids by the action of pepsin in the stomach and an array of intestinal hydrolases. All of these enzymes are released in inactive (proenzyme) forms and activated by proteolytic cleavage within the gastrointestinal lumen (Van Beers et al. 1995; Yamada 2015). View original pathway at:Reactome.
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X-ray crystallographic studies of amylase 1A and 2A proteins show them to be monomers, complexed with single calcium and chloride ions (Ramasubbu et al. 1996; Brayer et al. 2000). Biochemical characterization of amylase 2A indicates that the enzyme efficiently cleaves poly-glucose chains so as to release maltose - a glucose disaccharide - from the reducing end of the chain (Braun et al. 1993; Brayer et al. 2000).
Trehalase deficiency has been described in two isolated cases in Europe (Bergoz 1971; Madzarovovà -Nohejlova 1973) and at high frequency in a population of Greenland natives (Gudmand-Hoyer et al. 1988) but molecular defects responsible for the deficiency have not yet been described so it is not annotated in Reactome.
The human genome contains five functional alpha-amylase genes, encoding structurally closely related isoenzymes (Gumucio et al. 1988). Three of these genes encode proteins synthesized in the parotid glands and released into the saliva (amylase 1A, B, and C), and the other two encode proteins synthesized in the exocrine pancreas and released into the small intestine (amylase 2A and B). In the human body, starch digestion thus commences in the mouth, mediated by salivary amylases, and is continued in the small intestine, mediated by the pancreatic ones.
X-ray crystallographic studies of amylase 1A and 2A proteins show them to be monomers, complexed with single calcium and chloride ions (Ramasubbu et al. 1996; Brayer et al. 2000). Biochemical characterization of amylase 2A indicates that the enzyme efficiently cleaves poly-glucose chains so as to release maltose - a glucose disaccharide - from the reducing end of the chain (Braun et al. 1993; Brayer et al. 2000).
While alternative splicing gives rise to two CEL isoforms, only the longer one encodes all of the residues that form the active site of the enzyme (Reue et al. 1991). In vitro, monomeric CEL protein is active even in the absence of bile salts. Its activity is greatly increased when it is complexed with two molecules of cholate, chenodeoxycholate, or their glycine or taurine conjugates (Lombardo and Guy 1980), and the predominant form of the enzyme active on lipid micelles in the gut is a dimer of two such complexes (Aubert-Jousset et al. 2004).
CEL is synthesized in pancreatic acinar cells and released into the small intestine. It is also synthesized in the mammary gland and is a constituent of breast milk. The milk CEL is thought to play a role in digestion of milk fat in newborn infants, whose own pancreatic synthesis of the enzyme is low (Lombardo 2001; Bernback et al. 1990).
While alternative splicing gives rise to two CEL isoforms, only the longer one encodes all of the residues that form the active site of the enzyme (Reue et al. 1991). In vitro, monomeric CEL protein is active even in the absence of bile salts. its activity is greatly increased when it is complexed with two molecules of cholate, chenodeoxycholate, or their glycine or taurine conjugates (Lombardo and Guy 1980), and the predominant form of the enzyme active on lipid micelles in the gut is a dimer of two such complexes (Aubert-Jousset et al. 2004).
CEL is synthesized in pancreatic acinar cells and released into the small intestine. It is also synthesized in the mammary gland and is a constituent of breast milk (Lombardo 2001; Bernback et al. 1990).
While alternative splicing gives rise to two CEL isoforms, only the longer one encodes all of the residues that form the active site of the enzyme (Reue et al. 1991). In vitro, monomeric CEL protein is active even in the absence of bile salts. its activity is greatly increased when it is complexed with two molecules of cholate, chenodeoxycholate, or their glycine or taurine conjugates (Lombardo and Guy 1980), and the predominant form of the enzyme active on lipid micelles in the gut is a dimer of two such complexes (Aubert-Jousset et al. 2004).
CEL is synthesized in pancreatic acinar cells and released into the small intestine. It is also synthesized in the mammary gland and is a constituent of breast milk (Lombardo 2001; Bernback et al. 1990).