Digestion of dietary carbohydrate

The 1-4 linkages of extracellular amylopectin starch, a glucose polymer containing linear segments formed by alpha-1,4 linkages and a smaller number of alpha-1,6 linkages forming branch points, are digested by the endoglucosidase activity of alpha-amylases, yielding maltose, maltotriose, and longer maltosides from the alpha-1,4 linear segments and alpha-limit dextrins from the branch points. Alpha-limit dextrins are glucose (G) oligomers linked by 1-4 and 1-6 bonds. 1-6 branch points make up about 5% of all amylopectin glucose bonds - the exact fraction depends on the source of the starch. Mass spectro-scopic analysis of alpha-limit dextrin shows it to be a mixture of maltosides and isomaltosides containing two to forty G residues, but the most common contain fewer than seven. Maltose (G2) is the shortest 1-4 maltoside produced by alpha-amylase. Isomaltose (G2) is the shortest 1-6 isomaltoside. 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. The 1-6 linkages in extracellular limit dextrins are hydrolyzed by sucrase-isomaltase to yield maltose, maltotriose, longer maltosides, and glucose (Conklin et al. 1975; Nichols et al. 1998). In the body, this enzyme is found on the external face of enterocytes in microvilli of the small intestine (Hauri et al. 1985), and acts on limit dextrins generated by the hydrolysis of amylopectin starch. The alpha-1,4 linkages of extracellular maltose are hydrolyzed to yield glucose in a reaction catalyzed by maltase-glucoamylase (Nichols et al. 1998; Semenza et al. 2001). In the body, this enzyme is found as a dimer on the external face of enterocytes in microvilli of the small intestine (Hauri et al. 1985), and acts on maltose derived directly from the diet and from the hydrolysis of starch. Preceded of of limit to (amylopectin) amylase, of linear starch (amylose) extracellular amylase The alpha-1,4 linkages of extracellular maltose are hydrolyzed to yield glucose in a reaction catalyzed by sucrase-maltase (Nichols et al. 1998). In the body, this enzyme is found as a dimer on the external face of enterocytes in microvilli of the small intestine (Hauri et al. 1985), and acts on maltose derived directly from the diet and from the hydrolysis of starch. Preceded of of to yield amylase, Digestion of linear starch


Digestion of linear starch (amylose) by extracellular amylase ↗ Location: Digestion of dietary carbohydrate
Stable identifier: R-HSA-188979

Type: transition
Compartments: extracellular region Extracellular amylose starch, linear polymers of glucose joined by alpha-1,4 linkages, is digested by the endoglucosidase activity of alpha-amylases, yielding maltose, maltotriose, and longer maltosides. 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).
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).

Literature references
Hauri, HP., Fransen, JA., Bienz, D., Marxer, A., Sterchi, EE. (1985). Expression and intracellular transport of microvillus membrane hydrolases in human intestinal epithelial cells. Extracellular trehalose, a disaccharide, is cleaved by trehalase associated with the plasma membrane to yield two molecules of glucose. Trehalase has been purified to homogeneity from rabbit intestine and shown to be a monomer attached to the plasma membrane by a GPI anchor (Galand 1984;Ruf et al. 1990).
A human cDNA encoding a closely homologous protein has been cloned and its protein product has been shown to have trehalase activity in vitro (Ishihara et al. 1997). The human enzyme has not been characterized further, and so both the posttranslational modifications of the human enzyme and its activity in vivo have been inferred from the properties of the well studied rabbit enzyme.