GI- Biochem- Digestion and absorption of carbohydrates- Wells Flashcards
Study this diagram
Which is the sugar molecule with the aldehyde group and which is the one with the keto group?
What are the generic names of sugars with the following number of carbons:
This is an example of what?
enantiomers (mirror images)
What are - carbohydrates are attached to non-carbohydrate structures, such as:
o Purines and pyrimidines (nitrogenous bases in nucleic acids)
o Aromatic rings, (in steroids and bilirubin)
o Proteins (in glycoproteins)
o Lipids (glycolipids)
complex carbohydrates
Carbohydrates play a role in energy and in in the cell
signaling
What are the two types of bonds by which a sugar may be attached to a non-carbohydrate group:
o –NH2 group = N-glycosidic bond (N-linked)
o –OH group = O-glycosidic bond (O-linked)
Bonds that link sugars are called bonds:
o β(1-4) glycosidic o α(1-4) glycosidic o α(1-2) glycosidic o α(1-6) glycosidic
glycosidic
The disaccharide lactose is made up of what two monosacharides and with what type of glycosidic bond?
Lactose = galactose + glucose: β(1-4)
The disaccharide sucrose is made up of what two monosacharides and with what type of glycosidic bond?
Sucrose = glucose + fructose: α(1-2)
The disaccharide maltose is made up of what two monosacharides and with what type of glycosidic bond?
Maltose = glucose + glucose: α(1-4)
What is needed to split the bond of a disaccharide sugar molecule?
H2O and glycosidase
What is a reducing sugar?
a sugar that can open and donate H
All monosaccharides, but not all disaccharides, are reducing sugars.
If the OH-group on the carbon of a cyclized sugar is not/is linked to another compound by a glycosidic bond, the ring can open.
o The sugar can act as a reducing agent (reducing sugar).
o All monosaccharides, but not all disaccharides, are reducing sugars.
anomeric
is not
Reducing sugars can react with agents (for example, the Benedict reagent)
chromogenic
The Benedict’s reagent test can detect a sugar in urine: positive result is indicative of an underlying pathology.
reducing
highly branched polymer of glucose , α 1-4 and α 1-6 linkages, major energy storage in animal (liver and muscle cells)
glycogen
What 2 forms of starch (gluclose) are found in plants and what type of glycosidic linkages do they have:
o Amylose – unbranched; α 1-4 glycosidic linkages
o Amylopectin – branched; α 1-4 and α 1-6 glycosidic linkages.
What is - unbranched glucose polymer in plants; β 1-4 glycosidic linkages (humans can’t digest).
cellulose
What type of glycosidic linkage does cellulose have?
β 1-4 glycosidic linkages
High fructose corn syrup has a faster/slower digestion than gluclose.
faster
Gluclose/fructose
bypasses a key regulatory step which promotes faster digestion, has less effect on insulin, stimulates greater brain pathway activity
fructose (high fructose corn syrup)
Which fructose enzyme deficiency is asymptomatic?
Fructokinase deficiency
What is an autosomal recessive disorder caused by a deficiency of aldose B, the enzyme that cleaves frunctos-1-phosphate and can cause poor feeding as a baby, vomiting, irritability, hypoglycemia, jaundice, hemorrhage, hepatomegaly, hyperuricemia and potentially kidney failure?
Fructose 1-phosphate aldolase (aldolase B) deficiency
Which fructose deficiency is characterized by episodic acute crisis of lactic acidosis and ketotic hypoglycemia which leads to hyperventilation, apneic spells, seizures, and/or coma?
Deficiency of fructose-1,6-biphosphatase
Why is high fructose corn syrup sweeter than sucrose (table sugar)?
Due to the higher ration of fructose to gluclose
Dietary carbohydrates come primarily from which source?
plants
Describe carbohydrate digestion from the top down?
Which enzyme – breaks large insoluble carb molecules into smaller soluble ones in the mouth, specifically α 1-4 bonds?
Salivary α-amylase:
What are the carbs with α 1-4 bonds that can be broken down in the mouth by α-amylase?
starch, glycogen, maltose
What are the products of α-amylase?
– short branched and unbranched oligosaccharides (dextrins)
What pH is optimum for α-amylase and how long does it take to be deactivated in the stomach?
pH optimum, 7.0 - inactivated by the acidic pH in stomach within 20 min
α-amylase is unable to break down branched oligosaccharides with which type of glycosidic bond?
a 1-> 6
What happens to the action of salivary amylase in the stomach?
low pH stops the action of salivary amylase
After the broken down carbohydrates move from the stomach into the small intestine, what enzyme continues to break down carb molecules into smaller soluble ones?
Pancreatic α-amylase
Pancreatic α-amylase breaks down bonds ONLY through hydrolysis.
Namely, any carb with bonds - starch, glycogen, maltose and smaller dextrins
α 1-4
α 1-4
What are the products of Pancreatic α-amylase and what is the optimum pH?
shorter branched and unbranched oligosaccharides (dextrins), and disaccharides.
o pH optimum, 7.0
plasma levels of either pancreatic amylase or total amylase are used as a diagnostic marker for
pancreatitis
Which enzyme is being described?
1 protein - 2 enzymatic activities:
o cleaves α 1-2 bonds in sucrose
o α 1-6 bonds (branches) in isomaltose
Sucrase/isomaltase (SI)
Which enzyme can break down the branched chains α 1-6 bonds (branches) in isomaltose in the small intestine?
Sucrase/isomaltase (SI)
All enzymes in the intestine that break down carbohydrates are
proteins of the brush-border on luminal surface of the intestinal mucosal cells
transmembrane
Which digestive enzyme of the small intestine is being described?
o 1 protein - 2 enzymatic activities:
o cleaves α 1-4 bonds in maltose/maltotriose (maltase activity)
o α 1-4 bonds in dextrins
Maltase-glucoamylase (MGA)
Which digestive enzyme of carbs in the small intestine is being described?
o β 1-4 bonds in lactose (milk sugar) o high expression in infants and gradual decrease with age.
lactase
What is the glycosidic bond in lactose?
β 1-4
What digestive enzyme of the small intestine can breakdown α 1-1 bonds in trehalose (mushrooms and fungi)?
trehalase
What kind of glycosidic bonds are found in mushrooms and fungi?
α 1-1 bonds
Glucose CANNOT diffuse directly into cells. Why?
It is large and charged (many OH groups), it is hydrophillic and cannot move through the membrane
◦ Na+-Independent Facilitative Glucose Transporters
◦ Na+-Dependent monosaccharide cotransporter system (SGLT)
◦ Galactose and Fructose Absorption through Glucose Transporters
all facilitate what?
absorption of monosacharides inside the enterocyte
What gluclose transporter is being described?
◦ Move down gradient across membrane (14 GLUT proteins)
◦ No energy used.
◦ Na+-Independent Facilitative Glucose Transporters
The monosaccharide cotransporter system (SGLT) is Na+-Dependent/Na+-Independent?
Na+-Dependent
How is galactose absorbed through the cell?
– same mechanism as glucose (GLUT and SGLT)
How is fructose absorbed into enteroytes that line the lumen of gut in the small intestine?
GLUT5 (associated with neurons) facilitates movement in both directions across membrane.
◦ Absorption is faster as part of sucrose than as fructose monosaccharide.
Absorption of sucrose/fructose is faster?
sucrose
GLUT proteins span the
o ATP-independent/dependent
o Upon glucose binding it changes , which allows transport across the membrane
membrane
independent
conformation
There are how many isoforms of GLUT?
14
GLUT isoforms:
o Tissue
o Specific regulation
o Specificity for
specific
substrate (GLUT-5, fructose)
Which GLUT isoform is being described:
In Human erythrocyte Blood
–brain barrier Blood
- retinal barrier Blood
- placental barrier
Blood-testis barrier
Expressed in cell types with barrier functions; a high-affinity glucose transport system
GLUT 1
Which GLUT isoform is being described:
In the-
Liver
Kidney
Pancreatic β-cell
Serosal surface of intestinal mucosa cells
A high-capacity, low-affinity transporter
May be used as the pancreas glucose sensor
GLUT 2
Which GLUT isoform is being described:
located in
Brain (neurons)
Major transporter in the central nervous system; a high-affinity system
GLUT 3
Which GLUT isoform is being described:
located in-
Adipose tissue
Skeletal muscle
Heart muscle
Insulin-sensitive transporter; in the presence of insulin, the number of transporters increases on the cell surface; a high-affinity system
GLUT 4
Which GLUT isoform is being described:
located in:
Intestinal epithelium
Spermatozoa
This is actually a fructose transporter
GLUT 5
What is the path of neural GLUT carbohydrate uptake in neural vs. non-neural tissues?
Which tissue has tight junctions?
Which tissues sometimes has wide intracellular gaps?
Which tissue lacks pinocytosis?
Which tissue type has a continuous and which has a discontinuous basement membrane?
Which tissue type has gluclose transporters in both membranes?
Which tissue type can allow diffusion between cells and into interstitial fluid?
Which glucose transport mechanism
o transports glucose against its concentration gradient by co - transporting Na + down its gradient
o Requires energy supplied by an Na +, K + -ATPase that pumps Na + outside the cell to create the Na + gradient.
SGLT system
Where are the 3 main locations of the SGLT carbohydrate transporters?
o Epithelial cells of the intestines
o Renal tubules
o Choroid plexus (part of the blood -brain barrier)
A patient has a genetic defect that causes intestinal epithelial cells to produce disaccharides of much lower activity than normal. Compared to a normal person, after eating a bowl of oatmeal and milk sweetened with table sugar, this patient will exhibit higher levels of which of the following?
A) maltose, sucrose, and lactose in stool
◦ B) Starch in stool ◦
C) Galactose and fructose in stool ◦
D) Glycogen in muscles ◦
E) Insulin in the blood
A) maltose, sucrose, and lactose in stool
Salivary amylase function in the mouth remains to digest starches to glucose, but maltose and disaccharides pass to the small intestine and into the stool.
Do lactose and cellulose have similar nutrition value?
No, because even though they both have β-1,4 bonds, lactose is a disaccharide and cellulose is a polysaccharide with many bonds and animals lack the enzyme to break down this bond whereas lactase will break down the lactose glycosidic bond.
What is the disorder?
-> carbs into large intestine = osmotic diarrhea, bloating.
carbs include hemicelluloses, components of pectin, galactose, and phenyl propane derivatives found in lignin
◦ Disaccharide digestion deficiency
Reduced levels of lactase on the intestinal epithelial cell surface lead to reduced lactose digestion in the intestinal lumen, providing substrate for flora in the large intestine. Metabolism of the lactose by these bacteria leads to the generation of organic acids and gases.
Both environmental and genetic origins
Lactose intolerance
Healthy diets with controlled intake of carbohydrates will be beneficial in managing blood glucose levels in this particular disease?
Both environmental and genetic causes
Type 2 diabetes
What is being described?
Inability to absorb fructose in the small intestine, leading to colonic bacteria metabolism of fructose and the generation of organic acids and gases
Genetic condition
Fructose malabsorption
What environmental condition is being described:
Increased cAMP levels in the intestinal epithelial cells lead to inhibition of ion transport and significant water extrusion from the affected cells, leading to severe diarrhea.
cholera
Lactose deficiency leads to intolerance. How many people are affected?
And what are the treatments?
lactose
75% of the world’s adult population
Treatment: avoidance of milk, substitute with yogurt and some cheeses, lactase-treated products or take supplementary lactase.
Lactose intolerance:
cause an imbalance: water is drawn from mucosal cells into intestinal lumen resulting in diarrhea.
Lactose are substrates for the intestinal microbiota (bacteria) that ferment them to 2 and 3 carbon compounds (acetic and lactic acids) that are also osmotically active and release large volumes of that cause cramping and bloating.
osmotic
gases (CO2, H2)
What is the diagnosis for lactose intolerance?
measurement of H2 gas produced by bacteria in breath – reliable test to determine amount of carbs not absorbed by the body.
Why is it that lactose cannot be absorbed if there is a lactase deficiency?
it is very polar, lots of -OH groups cause that it cannot be absorbed directly
If lactose is not broken down (due to lactase deficiency) and therefore not absorbed, what happens to the sugar?
intestinal microbiota ferment it leading to gas build up, cramping, and bloating
Congenital Sucrase-isomaltase complex deficiency:
o Sucrose intolerance
o Caused by loss of enzyme function o Autosomal recessive disorder – 0.02% in Europeans, more common in people in Greenland and Canada
o Treatment: to avoid or take replacement enzymes
genetic
sucrose
How can Sucrase-isomaltase complex deficiency be diagnosed?
Diagnosis: tolerance test with individual disaccharides.
Intestinal injury can result in lactase deficiency which can be caused by diseases that injure intestinal absorptive cells.
Secondary
villi
Kwashiorkor (protein malnutrition), colitis, gastroenteritis, tropical and non-tropical sprue, and excessive alcohol consumption are all diseases that can lead to what type of injury?
intestinal
Concerning intestinal injury?
◦Digestion of other disaccharides are affected, but the relevant enzymes are present already far in such excess, so there is no pathology.
◦ In excess: sucrase,, isomaltase, and
maltase
glucoamylase
A young man entered his physician’s office complaining of bloating and diarrhea. His eyes were sunken, and the physician noted signs of dehydration. Temperature was normal. He had just finished an ice-cream eating contest at symptom onset. Similar things had happened previously with high dairy intake. These symptoms are most like due to:
◦ A) isomaltase deficiency ◦ B) lactase deficiency ◦ C) pancreatic amylase deficiency ◦ D) salivary amylase deficiency ◦ E) sucrase deficiency
◦ B) lactase deficiency
◦ GLUT and SGLT transporter function are key to cellular distribution.
glucose
Review slide 45 for study guide points
