Digestion & Absorption Of Carbohydrates Flashcards
What are the two types of bonds that sugars may be attached to a non-carbohydrate group?
N-glycosidic link = NH2 groups
O-glycosidic link = OH group
What are the main types of glycosidic bonds?
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
What is Benedict’s reagent?
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
What sugars work with Benedict’s reagent?
If the OH of the anomeric carbon of cyclized sugar is NOT linked to another compound by a glycosidic bond
Glycogen facts
Is a highly branched polymer of glucose
- contains a(1-4) and a(1-6) linkages
- major energy storage in animal liver/muscle cells
Amylose, amylopectin and cellulose facts
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
Is fructose or glucose used more in the brain?
Fructose
Salivary-(a)amylase
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*
Pancreatic-(a)amylase
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
Enzymes used in the final digestion of carbohydrates in the mucosal cells
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
How are monosaccharides absorbed?
By enterocytes
Why cant glucose diffuse directly into cells?
Too large and too polar
Mechanisms for monosaccharide absorption
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**
Where are the different subtypes of GLUT transporters located?
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
Mechanism of GLUT
Are ATP-independent channels that when glucose binds to it, reconfigures itself and allows glucose into the tissues
- uses facilitated diffusion as transport mechanism
SGLT mechanisms
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
types of Dietary fibers
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
What happens in the large intestine during dissacharide deficiencies?
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
Lactase deficiency
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
Sucrase-isomaltase complex deficiency
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
What are causes of secondary lactase deficiency?
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
How are carbohydrates named?
Number of carbons
- have two functional groups*
- aldehyde Or ketone
- alcohol
D vs L configurations of carbohydrates
The last chiral carbon in the chain
- OH group on right = D
- OH group on left = L
both D and L are enantiomers
What are complex carbohydrates?
Carbs that are attached to:
- purines/pyrimidines
- proteins
- lipids
