BioChem Digestion & Uptake (14)
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What is the energy value for dietary glucose and carbohydrates?
What is the energy value for dietary fat in Cal/gram?
9 Cal/gram: it's the highest value of dietary macromolecules
Tell me the names, the origins, and the areas in which the six enzymes involved in carbohydrate digestion act
- 1) alpha-amylase
- salivary glands
- glu-alpha (1-->4)-glu in starch
- 2) alpha-amylase
- glu-alpha (1-->4)-glu in starch
- 3) maltase
- intenstinal mucosa
- glu-alpha (1-->4)-glu in maltose
- 4) isomaltase
- intestinal mucosa
- glu-alpha (1-->6)-glu in isomaltose and branched dextrins
- 5) sucrase
- intestinal mucosa
- glu-alpha (1-->2) beta-fru in sucrose
- 6) lactase
- intestinal mucosa
- gal-beta (1-->4)-glu in lactose
Where are the enzymes What enzymes work in the intestinal mucosa to digest carbohydrates? (4)
- 1) Maltase
- 2) Isomaltase
- 3) Sucrase
- 4) Lactase
- * specifically they are located in the mucosal brush border of the small intestine & cleave disaccharides into monosaccharides
What are the end products of starch digestion by alpha-amylase?
- called limit dextrins: maltose, maltotriose, & small branched oligosaccharides
- *amylase does NOT generate glucose
Which bond is cleaved by the enzyme alpha-amylase?
Glu-alpha(1,4)-glu in starch
Which bond is cleaved by the enzyme maltase?
Glu-alpha(1,4)-glu in maltose
Which bond is cleaved by the enzyme isomaltase?
Glu-alpha(1,6)-glu in isomaltose and dextrins
Which bond is cleaved by the enzyme sucrase?
Glu-alpha(1,2)-beta-fru in sucrose
Which bond is cleaved by the enzyme lactase?
How is salivary amylase inactivated?
by the low pH in the lumen of the stomach
True or False: sucrase and isomaltase are distinct enzymes encoded by two DIFFERENT genes
- FALSE: Sucrase and isomaltase are distinct enzymes but they are encoded by a SINGLE gene
- an alteration of this gene can therefore affect the activity of both enzymes
- trisaccharide composed of galactose, fructose, and glucose found in beans [& other veggies]
- can only be hydrolyzed by enzyme NOT found in the human digestive
- intestinal bacteria metabolize these sugars giving off H & CO2 gas (beans = musical fruit)
- caused by an insufficiency of lactase therefore a failure to digest lactose
- common in African & Asian descent
- increases w/ age
- can be cause by illness that injures mucosa
- diagnosed by failing to observe glucose increase after lactose challenge; can also observe H2 in breath
What is the biochemical composition of dietary fiber?
they're undigested polysaccharides that play an important role in gastrointestinal physiology
What is the physiological mechanism behind a correlation between high dietary fiber intake & resistance to colon cancer and reduced risk of coronary heart disease?
Dietary fibers, especially soluble ones, may absorb, remove & promote excretion of carcinogenic substances & cholesterol from digested foods in the intestine
List some physiological roles of dietary fiber
- affects gastric emptying
- intestinal absorption of monosaccharides
- intestinal mobility and transit time
- contributes to stool bulk
Why can't humans digest cellulose, a prominent dietary fiber found in the cell wall of many green plants?
- because it is composed of glucose monomers bound in a beta(1-->4) conformation
- humans don't have an enzyme that can cleave glu-beta(1-->4)-glu bonds
non-carbohydrate plant cell wall materials that are found in dietary fiber
Which monosaccharides are absorbed in the small intestine via simple diffusion?
- Xylose & Arabinose
- * these monosaccharides are only produced in small amounts by actions of intestinal bacteria on polysaccharides that are not digested by human enzymes
Which monosaccharides are absorbed in the small intestine via facilitated transport?
- Fructose, mannose & glucose
- * glucose is moved into the cell by BOTH facilitated & active transport
Which monosaccharides are absorbed in the small intestine via active transport?
Glucose & Galactose
Where does the energy to actively transport glucose (and galactose) into mucosal cells of the small intestine AGAINST their concentration gradient come from?
- Transport of glucose and galactose into mucosal cells of the small intestine is coupled to the entry of Na+ into the cell DOWN its concentration gradient
- Na+ gradient is maintained by a Na+/K+ ATPase located on the basolateral membrane of the mucosal cells which pumps excess Na+ out of the cell and K+ in
Glucose and galactose enter the mucosal cells of the small intestine by ________ transport and leave the cells to enter the bloodstream by ________ transport
- enter cells from small intestine via active transport
- leave intestinal mucosa via facilitated transport (uniport probz)
- * the same transporter protein serves for absorption of glucose and galactose --> defect in the gene that encodes this protein leads to malabsorption of both sugars
mucosa of SI
consists of the epithelium itself and also the supporting loose connective tissue (lamina propria)
major metabolite used for energy in intestinal mucosal cells; it reduces the need for mucosal intestinal cells to metabolize glucose for energy
Insulin stimulates glucose uptake in which tissues? (4)
Skeletal & cardiac muscle, fat cells and white blood cells via the GLUT4 facilitative transporter
- a sodium-linked glucose transporter found in the intestinal mucosa & kidney brush border
- (the 'epithelium' part of both organs)
What facilitative glucose transporters are NOT responsive to insulin?
- GLUT 1, 2, 3, 5, & 7
- (*GLUT4 is)
- glucose transporter found in erythrocytes, brain tissue & fetal tissue
- (unlike SLGLUT1 it's NOT sodium-linked)
- main glucose transporter of the liver and beta-cells of the pancreas
- it's NOT responsive to insulin
- (also in intestine & kidney)
main glucose transporter of the brain, kidney & many other organs
main fructose transporter
main glucose transporter of the glucose across the ER membrane
How does insulin stimulate glucose absorption?
- by increasing the number of GLUT4 receptors on the membrane
- Insulin binds to a tyrosine kinase receptor that mobilizes glucose transporters (especially GLUT4) to migrate to the cell membrane from the endoplasmic reticulum
where sugars transported from the intestine into the blood stream go to reach the liver (directly from the portal vein)
How does the liver play a central role in regulating the amount of glucose that enters the circulation?
- 1) by absorbing glucose from the portal system & metabolizing/converting it to glycogen for storage
- 2) by regulated release of glucose (derived from stored glycogen or generated by new synthesis) from liver cells into the blood via hepatic vein
- * absorption of glucose into liver cells [unlike that into intestinal cells] DOES NOT REQUIRE active transport
What is the close to a constant blood glucose concentration maintained by the liver?
4.5 - 5 mM (80 to 90 mg/100 ml of plasma)
- (GI) measures how much a carbohydrate raises blood glucose concentration
- It is also defined on the basis of the area under the curve of the post-meal rise and fall of blood glucose concentration
After a meal rich in carbohydrates, peak blood glucose in normal individuals is reached within _____ minutes, and the concentration returns to baseline by _____ hours. How are these values different in subjects affected by diabetes?
- Peak blood glucose is reached within 30-45 minutes, and returns to baseline = 1.5-2 hours
- people with diabetes have:
- - a higher baseline & peak concentration of blood glucose
- - elevated concentrations of blood glucose that persist longer
What is the glycemic index of glucose? What is the glycemic index of other carbohydrates?
- Glucose GI = 100
- All other carbohydrates have a glycemic index of LESS THAN 100
How do you keep glucose in the cell?
- once glucose is transported into a cell, it is phosphorylated to glucose-6-phosphate
- adding a Pi group to glucose produces glucose-6-phosphate, a molecule that is much more hydrophilic because of the Pi's negative charge
- this traps glucose in the cells and maintains the gradient required for facilitated transport
- enzymes that catalyze the same reaction even though they differ in amino acid sequence, kinetic behavior and/or regulatory properties
- eg. hexokinase and glucokinase
What is the function of hexokinase and glucokinase?
- They're two distinct kinds of kinases that can catalyze the transfer of phosphate
- from ATP to glucose
hexokinase _____ sensitive to changes in blood glucose levels
is not; blood glucose levels are always much higher than the Km of hexokinase
Glucokinase ______ sensitive to changes in blood glucose levels
- it's designed to respond to to blood glucose concentration changes
- it has a Km for glucose higher than the physiological blood glucose concentration
How do hexokinase and glucokinase differ in terms of their tissue distribution?
- Hexokinase is present in all tissues
- Glucokinase is present mainly in the liver & beta-islet cells of the pancreas
How do hexokinase and glucokinase differ in terms of their Km for glucose?
- Hexokinase Km for glucose = 0.2 mM
- Glucokinase Km for glucose = 10-20 mM
- hexokinase has a much LOWER Km than glucokinase for glucose
How do hexokinase and glucokinase differ in terms of regulation?
- glucose-6-phosphate inhibits Hexokinase (it's regulated by product feedback inhibition)
- glucose-6-phosphate doesn't inhibit Glucokinase; instead glucokinase's rate of reaction is dependent on substrate concentration [glucose] + the amount of enzyme present in the cell
How do hexokinase and glucokinase differ in terms of their substrate specificity?
Hexokinase can phosphorylate MANY hexoses
Glucokinase can ONLY phosphorylate glucose
Why is hexokinase not sensitive to changes in blood glucose levels after a meal?
- whether there are high or low blood glucose levels, hexokinase still needs to produce glucose-6-phosphate for glycolysis/cell needs
- therefore it's inhibited only by its product, glucose-6-phosphate, NOT by blood glucose concentrations
What is the main function of hexokinase in all cells and how is it different than the function of glucokinase?
- Hexokinase catalyzes the phosphorylation of hexoses to meet the metabolic needs of cells
- Glucokinase regulates blood glucose levels by increasing glucose uptake in the liver and acting as part of the glucose sensor system in the beta-cells of the pancreas
In the liver, insulin _________ gene transcription and synthesis of glucokinase, while glucagon _________ gene transcription and synthesis of glucokinase
- Insulin stimulates
- Glucagon inhibits
As the blood glucose level rises, the glucokinase-catalyzed reaction ______
- the enzyme shows cooperativity even though it has only one polypeptide chain
- it's most sensitive to changes in substrate concentration near normal blood glucose concentrations
Mature Onset Diabetes of the Young (MODY) is associated with a mutation in a gene that codes for which enzyme?
- It can also be caused by a mutation in genes that code for transcription factors that regulate tx for liver/pancreatic Beta cell genes (specifically those that influence the insulin response to blood glucose)
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