HRR: digestion/absorption of carbs and protein Flashcards
What are the forms of starch?
Amylose and amylopectin
Describe amylose.
Linear storage polysaccharide
What linkages are found in amylose?
a-1,4
Describe amylopectin.
Branched storage polysaccharide
What linkages are found in amylopectin?
a-1,4 and a-1,6
Describe glycogen.
Heavily branched storage polysaccharide
Where is glycogen found?
Muscle and liver
What linkages are found in glycogen?
a-1,4 and a-1,6
What is the difference between amylopectin and glycogen?
Glycogen is way more branched
Describe cellulose.
Storage polysaccharide found in plants
What linkages are in cellulose?
B-1,4
Describe sucrose.
Disaccharide consisting of glucose and fructose
Describe lactose.
Disaccharide consisting of glucose and galactose
Describe trehalose.
Disaccharide consisting of glucose with a-1,1 links
Where is there the most absorption of monosaccharides?
Duodenum and upper jejunum
Where does carb digestion start?
In the mouth with salivary a-amylase
Describe the digestion of carbs.
- Partial digestion by salivary amylase 2. Further digestion via pancreatic a-amylase in the small intestine 3. Conversion of disaccharides and oligosaccharides by brush border enzymes in the small intestine 4. Absorption into enterocytes and export into the bloodstream
What is the intraluminal hydrolysis phase?
The process of breaking down complex carbohydrates to oligosaccharides and disaccharides. The process is mediated by salivary and pancreatic a-amylase that hydrolyze a-1,4 links
What can amylase NOT cleave?
Terminal a-1,4 links, a-1,6 links, and a-1,4 links immediately next to a-1,6
Pancreatic amylase is stimulated by…
Cholecystokinin
What is the membrane digestion phase?
Digestion of partially broken-down carbohydrates in the brush border of the small intestine via hydrolases
What are the 3 big disaccharidases found in the brush border of the small intestine?
Glucoamylase, sucrase-isomaltase, and lactase
What links can glucoamylase break?
Terminal a-1,4 links; can break down maltose, maltotriose, and limit dextrin
What are the final products of glucoamylase?
Glucose and shortened limit dextrin
What links can sucrase-isomaltase break?
Terminal a-1,4, a-1,6
What are the final products of sucrase-isomaltase?
Glucose
What links can sucrase itself break?
a-1,2
What is the final product of sucrase breaking down sucrose?
Glucose and fructose
What links can lactase break?
B-1,4
What are the final products of lactase?
Galactose and glucose
Describe the SGLT1 transporter.
Takes galactose and glucose out of lumen and into enterocytes, but requires Na to be brought in as well
How do we balance osmolarity from SGLT1 transporter?
An Na-K pump on the basolateral membrane removes sodium from the enterocyte and brings in potassium to correct salt balance and create a gradient for Na to allow SGLT1 to continue functioning
Describe the GLUT5 transporter.
Takes fructose out of lumen and into enterocytes in the small intestine
Describe the GLUT2 transporter.
Allows glucose, galactose, and fructose to exit enterocytes and enter the interstitial space
Describe lactose intolerance.
Primary deficiency of lactase production or secondary to intestinal injury from something like infection
What causes symptoms from lactose intolerance?
The lactose is converted by bacteria into lactic acid, methane, and hydrogen gas. The osmotic effect of these in the lumen of the bowel causes bloating, nausea, abdominal pain, etc due to drawing in more water
Where does digestion of protein begin?
In the stomach by pepsin
Describe the digestion of proteins.
- Digestion begins in the stomach by pepsin 2. Pancreatic proteases continue to break down proteins in the small intestine 3. Oligopeptides get digested by brush-border enzymes into di and tripeptides and amino acids 4. Di and tripeptides are absorbed into enterocytes 5. Di and tripeptides are converted into amino acids within enterocytes, then exported into the blood
How does pepsin work?
Cleaves bonds at the carboxyl ends of aromatic or acidic amino acid residues
What does enterokinase do?
Convert trypsinogen to trypsin
Describe the function of trypsin.
Once trypsin is activated by enterokinase, it sets off a cascade and activates the other pancreatic proenzymes, including more trypsin
What are endopeptidases?
Those that prefer cleaving in the central parts of the molecule vs terminal links. They’re very specific in terms of where and what they like to cut
What are the endopeptidases?
Trypsin, chymotrypsin, elastase, pepsin
What are exopeptidases?
They like to cut towards the outside/terminal ends of molecules
What are the exopeptidases?
Aminopeptidases and carboxypeptidases
How are amino acids brought into enterocytes?
A lot of them require sodium-dependent transporters, but some are sodium-independent
How are di and tripeptides taken into enterocytes?
Proton-dependent pepT1 transporter
How do amino acids exit enterocytes?
Sodium-independent amino acid transporters
Describe the role of NHE3 in enterocytes.
Di and tripeptides need a proton to be taken into the cell via pepT1. In order to maintain a proton gradient, NHE3 puts protons back into the lumen in exchange for sodium. The sodium will be balanced out by Na-K pump on basolateral membrane
Describe the B0AT1 transporter.
It delivers neutral amino acids into enterocytes, such as tryptophan and phenylalanine
What is hartnup disease?
Defective B0AT1 transporter causes an inability to absorb neutral amino acids like tryptophan. An inability to bring in tryptophan means we do not have the precursor for niacinamide and cannot make NAD and NADP. This leads to pellagra
What are the signs of pellagra?
Dermatitis, dementia, diarrhea
