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GIT biochem > Digestion and Absorption of Lipids & Proteins > Flashcards

Digestion and Absorption of Lipids & Proteins Flashcards

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1
Q

Dietary lipid intake

A
  • 25-160g/day
  • Mostly TAG
  • Phospholipid
  • Cholesterol ester and free cholesterol
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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)
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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
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4
Q

How are non-soluble lipids transported

A
  • Chyme in lumen
  • Cell interior
  • IS fluid
  • Lymph
  • Blood
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5
Q

Fats that are water soluble

A
  • Lipases
  • Phospholipases
  • Cholesterol esterases
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6
Q

Composition of Triglyceride

A
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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
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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
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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
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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
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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
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12
Q

What is the most abundant bile salt in humans

A

Glycocholate

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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
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14
Q

Schematic of digestion of lipid

A
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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
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16
Q

Control of lipid digestion

A
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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
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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
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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
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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
Q

What are the proteolytic enzymes

A
  • 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
Q

Where does protein digestion start and end

A

Starts in the stomach

Ends in the intestine

27
Q

Proenzymes (zymogens) -> active enzymes

A
28
Q

Endopeptidase function

3 examples

A
  • Attack internal bonds
  • Initial breakdown
  • Large fragments
  • Trypsin, chymotrypsin and elastase (endopeptidases - all SERINE proteases)
29
Q

Exopeptidase function

A

Cleaves end

  • Carboxypeptidases
  • Aminopeptidases
  • Carboxypeptidases A and caroboxypeptidase B
30
Q

Products of actions of peptidases

A

Free AAs, di and tri-peptides

31
Q

What does gastric juice contain

What does it do

A
  • 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
Q

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

A
  • 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
Q

Protein digestion by pancreatic enzymes

A
  • 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
Q

Specificity of peptidases

A
35
Q

Hormonal control of secretion

A
  • 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
Q

Where are exopeptidases produced and where do they act

A

Produced by intestinal cells, act within the brush border and also within the cell

37
Q

Where are aminopeptidases located

What do they do

A

Located at the brush border

Cleave 1 AA at a time from the amino terminal end

38
Q

What do IC peptidases act on

A

Small peptides that are absorbed by the cell

39
Q

Products of brush border/IC enzymes

A

Free AAs

Di and tri-peptides

40
Q

Absorption of protein after the action of brush border/IC enzymes

A

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
Q

Enzymes responsible for protein digestion

A
42
Q

Cellular processes for absorption of AAs

A
  • Secondary active Na+ dependent transport systems
  • Facilitated diffusion also
43
Q

Absorption of small peptides

A
  • Secondary active transport
  • H+ dependent co-transporter
  • Di and tripeptides are absorbed at a faster rate than AAs
  • Degraded within cell to AAs
44
Q

Where does the AA pool in cells come from

A

Dietary AAs and from the degradation of existing proteins within the cell

45
Q

t1/2 of proteins

A

5 mins - days

46
Q

What are high turnover proteins

A
  • 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
Q

Kwashiorkor

A

Deficiency of protein in the diet - adequate in calories

48
Q

AA transport disorders (2)

A

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

GIT biochem (2 decks)

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