Digestion and Absorption of Lipids & Proteins Flashcards
(48 cards)
1
Q
Dietary lipid intake
A
- 25-160g/day
- Mostly TAG
- Phospholipid
- Cholesterol ester and free cholesterol
2
Q
What are the essential lipids
A
- Fat soluble - A, D, E, K
- Essential FAs - linoleic acid, linolenic, arachidonic acid
- < 6 g of fat is eliminated/day (bacterial cells and cell debris)
3
Q
What lipids are soluble in water
How are they transported
A
- Short chain FAs (< 10 C) are soluble in water
- Absorbed by passive diffusion
- Transported down their conc gradient and into portal blood
- Rapid process
4
Q
How are non-soluble lipids transported
A
- Chyme in lumen
- Cell interior
- IS fluid
- Lymph
- Blood
5
Q
Fats that are water soluble
A
- Lipases
- Phospholipases
- Cholesterol esterases
6
Q
Composition of Triglyceride
A
7
Q
Pathway of digestion of Triacylglycerol
A
- Begins in the stomach (lingual lipase has a minor role)
- Gastric lipase
- Pancreatic lipase - yields 2 monoacylglycerol and FAs
- CHOLESTEROL esterases in pancreatic juice - yields free cholesterol and FAs
- PHOSPHOLIPID A in pancreatic juice (zymogen) - activated in SI by trypsin - acts on a variety of phospholipids
8
Q
What are acid stable lipases
What is their role
A
- Lingual and gastric
- Play a major role in neonates
- Digest milk which is a major source of short and medium chain FAs ( < 12 C)
- Major source of calories
- Important in pancreatic insufficiency - cystic fibrosis
9
Q
Physical emulsification of fats
A
- Change in physical nature begins in stomach
- Core body temp helps liquify lipids
- Peristaltic movements aid in emulsion formation
10
Q
Chemical emulsification of fats
A
- Acid-stable salivary and gastric lipases assist
- Initial hydrolysis is slow
- Relatively small lipid-water interface
- Once hydrolysis begins, water-immiscible TAGs are degraded to FAs and they act as surfactants
- FAs confre hydrophilic surface to lipid droplets and break them down into smaller particles, increasing the lipid-water interface, facilitating rapid hydrolysis
- Lipid phase becomes dispersed throughout the aqueous phase as an emulsion
11
Q
Benefits of the emulsification of lipid
A
- Increase SA so that digestive enzymes can work on them
- Detergent properties of bile salts - derivatives of cholesterol
- Mechanical mixing due to peristalsis
12
Q
What is the most abundant bile salt in humans
A
Glycocholate
13
Q
Sequence of events in fat digestion
A
- TAG - pancreatic lipase yields 2 monoacylglycerol and free FAs
- Colipase (secreted as procolipase) binds lipase and anchors it to the aqueous surface
- Cholesteryl ester - cholesteryl esterase - activity greatly increased in presence of bile salts
14
Q
Schematic of digestion of lipid
A
15
Q
Phospholipid degradation
A
PROENZYME OF PHOSPHOLIPASE A
- Activated by trypsin
- Requires bile salts for optimum activity
- Removes 1 FA from C 2 to form lysophospholipid
- Next FA at C 1 can be removed to form glycerylphosphoryl base
16
Q
Control of lipid digestion
A
17
Q
How are lipids absorbed
A
BY ENTEROCYTE
- Free FA, free cholesterol and 2 monoacylglycerol
- With bile salts form mixed micelles
- hydrophobic grps on inside
- Hydrophilic grps on outside
- mix with unstirred water layer
- Short and medium chain FAs DO NOT require formation of micelles for transport
18
Q
How are triacylglycerols and cholesteryl esters re-synthesised
A
- Mixture of lipids migrate to ER
- Remake complex lipids
- FAs are converted to fatty acyl CoA
- Fatty acyl CoA synthetase
- 2 monoacylglycerols converted to triacylglycerol
19
Q
Secretion of lipids from enterocytes
A
- TG and CE are VERY hydrophobic
- Need to be re-packaged
- lipid droplets are surrounded by a thin layer composed of phospholipids, unesterified cholesterol and a protein molecule apolipoprotein B-48
- The protein component, apolipoprotein B-48 is essential for the final release of chylomicrons from the enterocyte - requires protein synthesis
- Sent to lymph
20
Q
2 causes of fat malabsorption
A
PANCREATIC FAILURE
- bulky, fatty, floating stools that contain undigested triglycerides - steatorrhea
- Deficiencies of fat-soluble vitamins can occur
LACK OF BILE SALTS
- Most of the “fat” in the stools consists of unabsorbed FAs, monoglycerides and diglycerides
- Fat malabsorption can be treated effectively
- Pancreatic enzymes and bile salts both available in tablet form
- Used routinely in patients with pancreatic insufficiency or biliary disease
21
Q
Fat malabsorption
A
22
Q
Total daily protein load
A
- 70 - 100g of dietary protein
- 30 200g of endogenous proteins
23
Q
How much protein is lost in digestion
A
6-12g - digestion is very efficient
24
Q
Very little absorption of…..
EXCEPT
A
Very little absorption of intact protein
EXCEPTION - newborns can take up maternal antibodies
25
What are the proteolytic enzymes
* Pepsin
* Trypsin
* Chymotrypsin
* Elastase
* Carboxypeptidase
All produced as ZYMOGENS and activated by cleavage after they enter the GI lumen
(Pro and ogen in the name)
26
Where does protein digestion start and end
Starts in the stomach
Ends in the intestine
27
Proenzymes (zymogens) -\> active enzymes
28
Endopeptidase function
3 examples
* Attack internal bonds
* Initial breakdown
* Large fragments
* Trypsin, chymotrypsin and elastase (endopeptidases - all SERINE proteases)
29
Exopeptidase function
Cleaves end
* Carboxypeptidases
* Aminopeptidases
* Carboxypeptidases A and caroboxypeptidase B
30
Products of actions of peptidases
Free AAs, di and tri-peptides
31
What does gastric juice contain
What does it do
* Contains HCl (pH \< 2) and pepsin
* Acid kills micro-organisms
* Denatures protein - partially unfolds them - better substrates for proteases
* Pepsin is stable and active at acid pH
32
Structural change when pepsinogen is converted to pepsin
How is it activated
What does the liberated peptide act as
What are the products of digestion
* Removal of 46 AAs at NH2 terminus
* Autoactivation at pH \< 5 or by pepsinogen itself
* Liberated peptide acts as autoinhibitor above pH 2
* Large peptide fragments (main product), smaller peptide fragments, free AAs
33
Protein digestion by pancreatic enzymes
* Pancreatic juice contains bicarbonate - raises pH - pancreatic enzymes active in this pH
* Pancreatic proteases secreted as zymogens
* Active forms can digest each other
* All activated within a short period of time - activation of trypsinogen
34
Specificity of peptidases
35
Hormonal control of secretion
* Partially digested protein (as well as lipid) in upper SI causes release of CCK
* Acts on gallbladder and pancreas to increase secretions
* Decreases gastric motility
* Low pH of chyme induces release of SECRETIN =\> release of bicarbonate
36
Where are exopeptidases produced and where do they act
Produced by intestinal cells, act within the brush border and also within the cell
37
Where are aminopeptidases located
What do they do
Located at the brush border
Cleave 1 AA at a time from the amino terminal end
38
What do IC peptidases act on
Small peptides that are absorbed by the cell
39
Products of brush border/IC enzymes
Free AAs
Di and tri-peptides
40
Absorption of protein after the action of brush border/IC enzymes
AA and peptide transport systems
Di and tri-peptides are hydrolysed within intestinal epithelial cell
**Only free AAs are found in the portal vein**
41
Enzymes responsible for protein digestion
42
Cellular processes for absorption of AAs
* Secondary active Na+ dependent transport systems
* Facilitated diffusion also
43
Absorption of small peptides
* Secondary active transport
* H+ dependent co-transporter
* Di and tripeptides are absorbed at a faster rate than AAs
* Degraded within cell to AAs
44
Where does the AA pool in cells come from
Dietary AAs and from the degradation of existing proteins within the cell
45
t1/2 of proteins
5 mins - days
46
What are high turnover proteins
* Hb
* Muscle proteins (particularly during fasting)
* Digestive enzymes
* Cells of GI tract - approx 25% of the cells lost and replaced daily
* Enzymes induced in response to physiological demand - high turnover
* Proteins damaged by oxidation
47
Kwashiorkor
Deficiency of protein in the diet - adequate in calories
48
AA transport disorders (2)
HARTNUP DISEASE
Rare autosomal recessive disorder - defect in transport of neutral AAs (particularly tryptophan) across intestinal and renal epithelial cells
CYSTINURIA
Mutations in identified genes have been found
* Build up of cystine in urine
* Defect in reabsorption in kidneys
