Protein Digestion & Absorption Flashcards
What is the importance of proteins?
involved in all these things:
- enzymes & hormones
- structure proteins like contractile proteins (myosin, actin) or fibrous proteins (collagen, elastin, keratin)
- immunoprotein (immunoglobulins)
- transport proteins such as albumin, transthyretin, hemeproteins, transferrin,
- glycoproteins and lipoproteins
What is the importance of dietary protein?
- synthesis of new proteins for growth and maintenance
- replacement of exisitng proteins (worn-out)
- synthesis of glucose, ketones, fatty acids (i.e. energy source)
- production of non-protein nitrogen (NPN) containing compounds (ex: urea, ammonia, etc)
What are the two classes of protein sources?
- Endogenous (70g/day)
- Exogenous = from our diets (>100g/day)
What are some exogenous sources of protein?
- animal products (meat, poultry, eggs and dairy products)
- plant products (grains, legumes and vegetables)
what are some endogenous sources of protein?
- mucosal cells by desquamation
- digestive enzymes
- glycoproteins
where does protein digestion take place?
- stomach
- lumen of small intestine
- brushborder of small intestine
- enterocytes
What happens to protein in the stomach??
In the stomach, HCL and pepsin denature the proteins (making them lose their 4th, 3rd and 2nd structures). Then pepsin can more easily access the peptide bonds and break them down into smaller polypeptides.
**some larger polypeptides remain and are broken down in the small intestine
Protein digestion requires two groups of enzymes, what are they? What do they do?
- Endopeptidase: split up polypeptides at interior bonds (inside chain)
- Exopeptidase: cleave off amino acid at extremity of polypeptide chain (ends of chain)
What is the end product of endopeptidases and exopeptidases?
endopeptidases = smaller polypetides, tripeptides or dipeptides
exopeptidases= amino acids
What are some examples of endopeptidases?
- trypsin
- chymotrypsin
- elastase
- pepsin
what are some examples of exopeptidases?
- carboxypeptidase (A or B)
- aminopeptidase
What stimulates secretion of HCL in the stomach ?
- Gastrin
- gastrin releasing peptide (GRP)
- acetylcholine
which structures (1st,2nd,3rd,4th) of a protein does HCL break?
HCL denatures 4th, 3rd, 2ndary structures of protein (unfolding)
Pepsinogen + HCL = _______
Pepsin
what are the endproducts of pepsin on protein ?
- polypeptides
- oligopeptides (2-20aa)
- free amino acids
what do gastric chief cells secrete?
pepsinogen
does pepsin function in a pH of 3.5 or higher?
NO!
what stimulates the intestinal endocrine cells to produce CCK?
food mixed with chyme entering the duodenum
What stimulates the pancreas acinar cells?
CCK and the vagus nerve
what are zymogens? what produces them? give three examples.
Zymogens are inactive enzymes produced by the pancreas acinar cells ex: trypsinogen, chymotrypsinogen, procarboxypeptidase
how are chymotrypsin and carboxypeptidases formed?
- first trypsinogen is activated to trypsin by an enteropeptidase (enterokinase) secreted by the duodenal enterocytes
- then trypsin triggers the activation of chymotrypsinogen into chymotrypsin and procarboxypeptidase into carboxypeptidases
what are the proteolytic proenzymes secreted by the pancreas in response to CCK?
- trypsinogen
- chymotrypsinogen
- pro-carboxypeptidases A &B
- pro-elastase
- collagenase
What peptide bonds will trypsin cleave?
Trypsin cleaves peptide bonds with arginine and cysteine at the C terminal (peptide bonds adjacent to basic AA)
what other enzymes are activated by trypsin?
Procolipase, Phospholipase A2
what peptide bonds will chymotrypsin cleave?
chymotrypsin breaks peptide bonds adjacent to aromatic AA
what peptide bonds will carboxypeptidase A and carboxypeptidase B cleave?
carboxypeptidase break peptide bonds adjacent to C-teminal neutral (A) and basic amino acid (B)
what are the end products of carboxypeptidase A on polypeptides with free carboxyl groups?
they are broken down to lower peptides, aromatic amino acids and neutral amino acids
what are the end products of carboxypeptidase B on polypeptides with free carboxyl groups?
they are broken down to lower peptides and basic amino acids
what peptide bond does elastase cleave?
elastase cleaves the peptide bond at alpheric C-terminal
what are the end products of elastase and collagenase on polypeptides?
oligopeptides and tripeptides
what forms of proteins are transported into the enterocytes of the small intestine from the brush border?
- free amino acids
- tripeptides
- dipeptides
what are the enzymes found in the brush border membrane called? what do they hydrolyze?
Oligopeptidases hydrolyze larger oligopeptides
what are large oligopeptides broken down into? where are they broken down?
tripeptides, dipeptides and amino acids
broken down in the brush border membrane by oligopeptidases
what enzymes are found in the enterocytes?
- peptidases : aminopeptidases (tri and di), dipeptidylaminopeptidases (di), and tripeptidases (tri)
Peptides are absorbed into the brushborder ______ than free amino acids
FASTER
Pep T1 is found on the ______
brushborder
Transport of peptides is associated with movement of what ion?
H+ movement
How are amino acids transported across the basolateral membrane?
actively in the upper small intestine (Duodenum) Na+ dependent or independent
How are peptides transported both across the apical membrane and the basolateral membrane of the enterocytes?
- peptides are transported into the cell with H+ (Pep T1)
- the H+ are pumped back in exchange of a Na+ ion
- A pump is required to pump Na+ out of the cells for K+ across the basolateral membrane
what factors affect amino acid absorption?
- hydrocarbon mass of the side chain (higher mass is fastest)
- electrical charge (neutral=faster than acidic or basic)
- essential AA = faster than nonessential
absorption of amino acids is ______
active transport (Na+ dependent/independent)
How are amino acids used by the intestinal cells?
- use for synthesis of apoproteins for lipoprotein formation
- use for synthesis of digestive enzymes
- synthesis of hormones
- synthesis of N-contraining compounds
- Glutamine used extensively for energy sources, catabolize glutamine to glutamate and NH3
- synthesis of proline from glutamine
What are the major steps of protein digestion?
- gastric hydrolysis of peptide linkages
- digestion of protein to smaller peptides by pancreatic enzymes
- hydrolysis of peptide linkages into oligopeptides by brush border peptidases and transport of amino acids and di and tri peptides across the membrane of the enterocytes
- further digestion of tri and di-peptides by cytoplasmic peptidases in the enterocytes
- metabolism of amino acids within the enterocytes
- transport of amino acids across the basolateral membrane of the enterocyte into the interstitial membrane
Which of the following activities does NOT occur in the brush border membrane region of enterocytes?
A. Endocytosis of polypeptides
B. Transport of free amino acids
C. sucrose digestion
D. glutamine metabolism
A. Endocytosis of polypeptides does not occur in the brush border membrane.
carboxypeptidase B will act on polypeptides that have been split by ________
**Trypsin **
** carboxypeptidase A** will act on polypeptides that have been cleaved by _____and ______
chymotrypsin and elastase
How are amino acids transported across the apical membrane into the enterocytes?
Active transport
