Digestion & Absorption Of Carbohydrates

Question 1

What are the two types of bonds that sugars may be attached to a non-carbohydrate group?

Answer 1

N-glycosidic link = NH2 groups

O-glycosidic link = OH group

Question 2

What are the main types of glycosidic bonds?

Answer 2

B (1-4) glycosidic bond
- *lactose has this

A (1-4) glycosidic bond
- *maltose has this

A (1-2) glycosidic bond
- *sucrose has this

A (1-6) glycosidic bond

numbering is based off of connecting carbon on the 2nd carb

Question 3

What is Benedict’s reagent?

Answer 3

A reagent used to test urine for undigested and reducing sugars
- positive test means underlying pathology in the GI system (sugar isnt being digested properly)

works for all monosaccharides (fructose, glucose, galactose) but not all disaccharides

Question 4

What sugars work with Benedict’s reagent?

Answer 4

If the OH of the anomeric carbon of cyclized sugar is NOT linked to another compound by a glycosidic bond

Question 5

Glycogen facts

Answer 5

Is a highly branched polymer of glucose

• contains a(1-4) and a(1-6) linkages
• major energy storage in animal liver/muscle cells

Question 6

Amylose, amylopectin and cellulose facts

Answer 6

3 forms of starch that are plant polymers glucose in plants

Amylose: unbranded a(1-4) glycosidic

Amylopectin: branched a(1-4) and (1-6) glycosidic

Cellulose: unbranched b(1-4) glycosidic linkages
humans can’t digest this one

Question 7

Is fructose or glucose used more in the brain?

Answer 7

Fructose

Question 8

Salivary-(a)amylase

Answer 8

Role = breaks down larger insoluble carb molecules into smaller soluble ones

Substrates = a(1-4) bonds
- starch/lactose/sucrose/glycogen

Specificity = hydrolysis of a(1-4) bonds only

Produces = short branched/unbranched oligosaccharides (dextrin) that possess a(1-6) bonds

Optimum pH = 7.0*

Question 9

Pancreatic-(a)amylase

Answer 9

Role = continues to break down carb molecules into specificity soluble ones

Specificity = hydrolysis of a(1-4) bonds

Substrates = any carbs with a(1-4) bonds
- starch, glycogen, maltose, smaller dextrin

Products = shorter branched and unbranched dextrin and disaccharides

pH = 7.0*

if levels of amylase are in the plasma = pancreatitis

Question 10

Enzymes used in the final digestion of carbohydrates in the mucosal cells

Answer 10

1) sucrase/isomaltase (SI)
- cleaves a(1-2) bonds in sucrose
- cleaves a(1-6) bonds in isomaltose

2) maltose-glucoamylase (MGA)
- cleaves a(1-4) bonds in maltose/maltotriose
- a(1-4) bonds in dextrins

3) lactase
- cleaves B(1-4) bonds in lactose
- also splits mild amounts of B(1,4) in cellulose (not enough to digest it)
- *very high levels in infants, gradually decreases with age

4) trehalase
- cleaves a(1-1) in trehalase from mushrooms and fungi

all of these enzymes are intestinal brush-border transmembrane proteins

Question 11

How are monosaccharides absorbed?

Answer 11

By enterocytes

Question 12

Why cant glucose diffuse directly into cells?

Answer 12

Too large and too polar

Question 13

Mechanisms for monosaccharide absorption

Answer 13

1) Na-independent facilitative glucose transporters (GLUT)
- moves down gradient across membrane with no energy being expended
- facilitated diffusion**
- glucose/fructose (GLUT-5) and lactose can use this
- seen in all tissues**

2) Na- dependent monosaccharide cotransporter system (SGLT)
- uses active transport**
- transports only glucose and sodium into cells against the gradient
- seen only in small intestines and renal PCTs**

Question 14

Where are the different subtypes of GLUT transporters located?

Answer 14

GLUT 1 = blood-“x”- barriers
- x = retina, brain, placenta, testies

GLUT 2 = Liver, Kidney, Pancreatic B-cells, Serosal surface of the intestinal mucosa

GLUT 3 = Brain (neurons)

GLUT 4 = Adipose tissue, Skeletal/cardiac muscle

GLUT 5 = Intestinal epithelium and sperm
- actually a fructose transporter

Question 15

Mechanism of GLUT

Answer 15

Are ATP-independent channels that when glucose binds to it, reconfigures itself and allows glucose into the tissues
- uses facilitated diffusion as transport mechanism

Question 16

SGLT mechanisms

Answer 16

Transports glucose against concentration gradients

• uses the energy from Na+ natural gradient as a co transporter
• is secondary active transport and requires Na+/K+ pumps

Question 17

types of Dietary fibers

Answer 17

Two types:

1) insoluble
- includes cellulose, hemicelluloses and lignin
- there are no enzymes to break down

2) soluble
- includes pectin, gums, mucilages
- humans can break these down

Question 18

What happens in the large intestine during dissacharide deficiencies?

Answer 18

Carbs remain in the large intestine instead of being broken down and resorbed from the small intestine
- results in water staying in the large intestine = diarrhea and bloating

Question 19

Lactase deficiency

Answer 19

Is both genetic and environmental (more environmental)

Lower levels of lactase either by genetic mutations at birth or overtime with age
- makes it harder to break down lactose and causes accumulation of disaccharides in the large intestines

Results in osmotic imbalance and causes water to leave cells into interstitum = diarrhea

Also results in bacteria of the microbiome that usually doesnt break down lactose, requiring to break down lactose by fermentation = cramping/bloating due to excess CO2/H2

Diagnosis = measures H2 gas on breath. Higher than normal amounts = lactose intolerant

Treatment = avoid dairy or supplementary lactase pills

Question 20

Sucrase-isomaltase complex deficiency

Answer 20

Is an autosomal recessive, loss of function disorder that results in sucrose intolerance
- similar to lactose intolerance with symptoms

Diagnosis = tolerance test with sucrose and maltose sugars as well as others. (+) = cant tolerate sucrose and maltose

Treatment = avoid sucrose or take replacement enzyme pills

Question 21

What are causes of secondary lactase deficiency?

Answer 21

secondary lactase deficiency = disease/injuries that cause the disorder

Causes:

• injury to intestinal villi
• kwashiorkor disorder
• colitis
• gastroenteritis
• tropical sprue
• excessive alcohol consumption

results in excess sucrase, maltase, isomaltase and glucoamylase enzymes

Question 22

How are carbohydrates named?

Answer 22

Number of carbons

• have two functional groups*
• aldehyde Or ketone
• alcohol

Question 23

D vs L configurations of carbohydrates

Answer 23

The last chiral carbon in the chain

• OH group on right = D
• OH group on left = L

both D and L are enantiomers

Question 24

What are complex carbohydrates?

Answer 24

Carbs that are attached to:

• purines/pyrimidines
• proteins
• lipids

Question 25

What are reducing sugars?

Answer 25

all monosaccharides, but not all disaccharides are reducing sugars

Is any sugar that is capable of reducing other compounds due to having a free aldehyde/free ketone group
- the aldehyde group always gets oxidized in this case

all reducing sugars can be measured via Benedict’s reagent

Question 26

A-glycosidase vs B-glucosidase

Answer 26

Splits (a)1,4 glycosidic bonds

- amylose, amylopectin, glycogen, maltose

Question 27

Differences between GLUT transporters in neural and non-neural tissues

Answer 27

Neural:

• tight junctions between endothelial cells
• narrow intercellular space
• NO pinocytosis
• continuous basement membrane
• glucose transporters in both membranes

Non-neural:

• NO tight junctions
• wider intercellular gaps
• YES pinocytosis
• discontinuous basement membrane
• glucose can diffuse between cells