Digestion, Absorption and Transport of Carbohydrates Flashcards
What is the enzyme involved in salivary digestion?
Salivary α-amylase (α-1,4 endoglucosidase)
What does salivary α-amylase (α-1,4 endoglucosidase) hydrolyze?
The interior α-1,4 bonds between glucose residues within amylopectin, amylose, and glycogen, converting the large polysaccharides to smaller entities.
What are the products after salivary α-amylase hydrolyzes amylose?
Maltose (2 C’s), Maltotriose (3 C’s), and oligosaccharides (4-9 C’s and is the most common result)
What isomeric form are naturally occurring sugars?
D-isomers
What isomeric form are enzymes?
L-isomers
Does the α configuration have the -OH group up or down?
Downwards (while beta is opposite)
What is sucrose composed of?
It is composed of glucose linked to fructose by α,β-1,2 bonds. (Not reducing sugar because both anomeric carbons are linked)
What is lactose composed of?
It is composed of galactose linked to glucose by β-1,4 bonds. (It is reducing because the anomeric carbon on glucose is free)
What is trehalose composed of?
It is composed of two glucose molecules linked by their anomeric carbons, αα -1,1). (Not reducing)
What are the products after salivary α-amylase hydrolyzes amylopectin?
α-dextrin, maltotriose, and maltose
What carbohydrates are hydrolyzed in the stomach?
Amylase is denatured and there may be some limited acid (nonenzymatic) hydrolysis of sucrose to yield fructose and glucose.
What does cholecystokinin do?
It acts on pancreas to release digestive enzymes including pancreatic α-amylase.
What does secretin do?
It causes pancreas to release bicarbonate to neutralize pH.
What are final products of pancreatic α-amylase?
Same as salivary amylase. Maltose, maltotriose and branched dextrins.
Where is lactose broken down and what breaks it down?
The brush-border membrane of epithelial cells in the intestinal villi and by the lactase (β-galactosidase) which is specific for the β-1,4 bond between galactose and glucose in lactose
Where is trehalose broken down and what breaks it down?
The brush-border membrane of epithelial cells in the intestinal villi and by trehalase which hydrolyzes the (α, α -1, 1bond) to yield two molecules off glucose.
Where are oligosaccharides, maltose, and maltotriose broken down and what breaks it down?
The brush-border membrane of epithelial cells in the intestinal villi and the glucoamylase complex cleaves the α-1,4 linkages between glucose units in oligosaccharides, starting from the nonreducing end of the chains.
Where is sucrose broken down and what breaks it down”
The brush-border membrane of epithelial cells in the intestinal villi and sucrase-isomaltase. The sucrase site hydrolyzes sucrose to glucose and fructose plus maltase activity, while the isomaltase site hydrolyzes the α-1,6 bonds between glucose residues plus maltase activity.
How is starch linked together?
Starch is a polymer of α-D-glucose units linked together through α-1,4 and α-1,6 glycosidic bonds. All starches are mixtures of amylose and amylopectin.
How is glycogen linked together?
Glycogen, the carbohydrate storage form in animals, is a polymer of α-D-glucose units linked together through α-1,4 and α-1,6 glycosidic bonds.
How is cellulose linked together?
Cellulose is an unbranched polymer of β-D-glucose units linked together through β-1,4 bonds.
What are the active transporters for the intestinal epithelial cells?
SGLT-1 is the protein that allows glucose and galactose to be transported into intestinal epithelial cells by an active, energy-dependent process that requires a concurrent uptake of sodium ions. (SGLT-2 is for the kidneys)
What are the GLUT transporters?
GLUTs are all transmembrane proteins that alternate between conformations in order to bind and release a monosaccharide on opposite sides of the membrane.
What are the GLUT-1 details?
It’s expressed in cell types with barrier functions. They are found in RBC’s and the blood-brain barrier.
What are the GLUT-2 details?
They’re found in the liver, Pancreatic β-cells, and contraluminal membrane of intestinal epithelial cells. They accept all 3 monosaccharides and help them pass out of cell after SGLT-1 allows them in.
What are the GLUT-3 details?
Major transporter in the CNS and is found in brain (neurons)
What are the GLUT-4 details?
Found in adipose tissue, skeletal and heart muscle. It is insulin-sensitive transporter.
What are the GLUT-5 details?
Found in intestinal epithelial cells and sperm. It’s a fructose transporter.
What is the time table for blood glucose levels?
Within 30-60 minutes after a meal, the blood glucose levels reach a peak and then decrease to fasting levels in two hours. The disposition of blood glucose is dependent on its utilization in biochemical processes.
