Basic function of the alimentary tract:
providing the nutrients needed by the body
- energy substrates
- essential macronutrients :essential AA
essential FA
- essential micronutrients : vitamines
trace elements
- water, electrolytes
Digestion:
Physiological point of view:
1. movement of food through the alimentary tract
2. secretion of the digestive juices
3. absorption
Biochemical point of view:
To digest food means to brake down large polymers to
monomers before they can be absorbed.
Our program:
1. general considerations about absorption and intestinal
epithelial transport
2. digestion and absorption of proteins
3. digestion and absorption of carbohydrates
4. digestion and absorption of triacylglycerols and cholesterol
5. structure, function and metabolism of lipoproteins
6. bile acid metabolism
7. absorption of vitamines , metabolism of iron
8. review of functions of individual parts of alimentary tract
Epithelial transport and principles of
absorption
Basic mechanisms of absorption:
- diffusion (movement along electrochemical
gradient). May be passive or facilited.
- secondary active transport (coupled with Na)
Energy gives NaK-ATPase of contraluminal
surface.
- both ways: Na-cotransport through luminal
membrane of enterocyte and diffusion through the
contraluminal one
Water is absorbed by the process of osmosis – the chyme remains
isoosmotic. Water follows absorbed substances into the blood.
Absorption of sodium ions:
!crucial for energization of nutrient uptake and water absorption!
- active component: NaK-ATPase of contraluminar surface
- passive component: various ways how to move sodium from the
lumen of intestine to the cytoplasm of enterocyte:
1. coupled transport with monosaccharides, AA, bile acids
2. Na/H exchange (plus Cl/HCO exchange)
3. Electrogenic Na chanels (lower portion of large intestine,
regulated by aldosterone)
Chloride ions follow sodium along electrical gradient (passive
diffusion). Cl/HCO exchanger also exists.
Digestion and absorbtion of proteins
Whole proteins and oligopeptides are not absorbed (with the
exception of a short period after birth).
Enzymes participating in splitting a peptide linkage:
1. endopeptidases = proteases: attack internal bonds and
liberate large peptide fragments, di- and tripeptides
2. exopeptidases: cleave off one AA from the –COOH
(karboxypeptidases) or –NH3 (aminopeptidases) terminus
Gastric digestion – pepsin
Pepsin is capable of digesting essentialy all the types of protein,
major products of its action are large peptide fragments.
Secreted as zymogen (pepsinogen), autoactivationautocatalysis.
Favorable pH ranges around 2-3  active form present in the
stomach only.
Prefers peptide bonds formed by aromatic AA (Phe, Tyr)
The role of gastric HCl: 1. denaturation of proteins
2. make pH optimal for the action of pepsin
Enzymes of pancreatic juice
ENZYME
Trypsin
Chymotrypsin
Elastase
Carboxypeptidase A
Carboxypeptidase B
PROENZYME
Trypsinogen
ACTIVATOR
AA prefered
Enteropeptidase Arg, Lys
, trypsin
Chymotrypsino- Trypsin
Tyr, Trp, Phe,
gen
Met, Leu
Proelastase
Trypsin
Ala, Gly, Ser
Procarboxypepti Trypsin
Val, Leu, Ile
dase A
Procarboxypepti Trypsin
Arg, Lys
dase B
Other names:
Serine proteases: in the active site of the enzyme is serine group: trypsin,
chymotrypsin, elastase
Zn-peptidases: metalloenzymes: pancreatic carboxypeptidases A and B
(Digestion and absorption of proteins – continued)
Intestinal phase of protein digestion
The combined action of pancreatic and gastric endopeptidases
and pancreatic exopeptidases (carboxypeptidase A, B) results in the
formation of mixture of free AA (40%) and small polypeptides (60%)
Enzymes of the small intestinal surface
ENZYME
Aminopeptidase A
Aminopeptidase N
Dipeptidyl
aminopeptidase
Leucine
aminopeptidase
Endopeptidase 24.11
Enteropeptidase
SUBSTRATE
Acidic AA
Neutral AA NH2terminus
Pro, Ala
Neutral AA
of oligopeptide
Protein(!), internal
hydrophobic AA
Trypsinogen
COMMENTARY
Present as active
form, need not
activator!
Differs in function!
Free amino acids , di- and tripeptides are absorbed, the cleavaging
to amino acids is finished within the cytosole of the enterocyte!
Carrier – mediated transport of free amino acids
 secondary active transport: against concentration gradient ( cotransport with Na) in the luminal membrane, Na independent in the
contraluminal membrane
 transporters are stereospecific, L- forms of AA are absorbed only
 There are six transport systems:
1. for acidic AA: Asp, Glu
2. for basic AA: Lys, Arg (and cystine)
3. for neutral AA with short or polar side chain: Ser, Thr, Ala
4. for neutral AA with aromatic or hydrophobic side chain: Phe,
Tyr, Met, Val, Leu, Ile
5. for imino AA: Pro, Hyp
6. for - AA: taurin, -Ala
Di- and tripeptides are absorbed via specific transport system and
hydrolysed to free AA within the cytosol of enterocyte.
Digestion and absorption of carbohydrates
Dietary carbohydrates involve mono-, di- and polysaccharides.
Essentially all the carbohydrates are absorbed in the form of
monosaccharides: di- and polysaccharides recquire hydrolysis.
1. - amylase
 the most important enzyme for starch and glycogen
cleavaging.
 present in saliva (ptyalin) and pancreatic juice
 specifity for internal -1,4 bonds (-1,6 bonds are not
attacked, neither -1,4 bonds that serve as branch point)
 hydratation of starch (during heating) is essential for efficient action
of this enzyme
 substrates: -1,4 glucose polymers (starch and glycogen). Other
polysaccharides cannot be hydrolysed nor absorbed!
 products: maltose, maltotriose, - limit dextrines (oligosaccharide
with average 8 Glc units and one or more -1,6 branching)
2. Surface enzymes: oligo- and disaccharidases
- enzymes present on the luminal plazma membrane
of the enterocyte
- for final hydrolysis of di- and oligosaccharides ( from
the diet or made by the action of -amylase)
- clip off one monosaccharide unit from non-reducing
end of oligosaccharide ( or cleave disaccharide)
SPECIFITY
SUBSTRATE
ENZYME
(bond to hydrolyze)
(common name)
Amylose
Glucoamylase
 Glc-(14) Glc (of
oligo- or polysaccharide)
 Glc-(14) Glc
 Glc-( 16) Glc
 Glc-(11) Glc
 Glc-(11) Fru
 Gal-(14) Glc
Maltose, maltotriose
Isomaltose,- dextrine
Trehalose
Succrose
Lactose
Maltase
Isomaltase
Trehalase
Succrase
Lactase
Carbohydrates that are not hydrolyzed by the action of -amylase
and/or surface enzymes cannot be absorbed  are anaerobically
metabolised by the intestinal bacteria  can cause diarrhea .
Absorbtion of monosaccharides
D-glucose and D-galactose are absorbed by Na – coupled carrier –
mediated transport, D-fructose by facilitated diffusion. Other
monosaccharides (e.g. pentoses, L-forms etc.) are absorbed only in very
small quantities by passive diffusion.
Transport of glucose and galactose
- SGLT – 1 : Na – dependent transporter on the
luminal surface of enterocyte
- GLUT-2 : transporter on the contraluminal plazma
membrane of enterocyte ( Na – independent,
facilitated diffusion)
This way of moving glucose through a cell is very similar in kidney
(uptake of glucose in proximal tubular cell)
Transport of fructose
is slightly different from that of most other monosaccharides. Because
fructose becomes rapidly phosphorylated within the epitelial cell and than
converse to glucose, cytosolic concentration of fructose remains low.
Therefore, fructose is transported by facilitated diffusion, using a
specific carrier protein.
Digestion and absorbtion of lipids
Lipids are defined by their poor water solubility. In aqueous phase they
tend to aggregate to large complexes and become not easily
accessible to the digestive enzymes and to the cell surface for
absorbtion. Therefore, physical changes (solubilization) must be
connected to chemical changes (hydrolysis) during digestion and
absorbtion.
Stomach fase of lipid digestion – acid-stable lipase
- the enzyme is secerned by Ebner‘s glands at the back of the
tongue and small amounts in the stomach
- initial hydrolysis of non-emulgated TAG
- products of reaction – MAG and FFA – spontaneously adsorb
to water/lipid interface and increase the surface area
(dispersion into smaller droplets)
Pancreatic lipase – the most important enzyme for TAG
hydrolysis
- specifity: for esters in the position 1 and 3
- products: FFA and MAG
- secerned as active form, but strongly inhibited by the bile
acids
- activated: by the action of colipase
Colipase binds to both the surface of lipid droplets and to the
lipase, thereby anchoring and activating of the enzyme.
Colipase is secreted as proenzyme and is activated by the
action of trypsin.
The function of bile acids
- biological detergents secerned by the liver
- in sufficent concentration (2-5 mM) form
aggregates called micelles
schema micely
Bile salt micelles can solubilize other lipids (MAG, FFA, PL,C)
forming so-called mixed micelles. The products of TAG
hydrolysis are continuously transferred from the emulsion
droplets to the micelles. When the mixed micelle is close to the
cell surface, absorbtion can occure. MAG and FFA are
absorbed by the proces of passive diffusion. Reesterification of
MAG and FFA within the enterocyte is responsible for keeping
the concentration gradient.
Digestion of other lipids:
- lipid esterase: non-specific, attacks MAG,
CE, etc.
- cholesterolesterhydrolase
- phospholipase A2
Resythesis of TAG, formation of chylomicrons
Fatty acid with short and medium chain (12 C) pass through the
enterocyte directly into the portal blood. Long-chain fatty acids are
resynthesizet into TAG. Resynthesized TAG with phospolipids and Apo
B48 and Apo AI form chylomicrons. They are released into the lymph.
Schema Devlin1082hore
Structure and metabolism of lipoproteins
Basic function: transport of water insoluble material – TAG
and cholesterol in the aqueous environment – blood.
General structure:
Schéma lipopr.
Types of apoproteins:
Function of Apo
Type of Apo
Apo AI
Cofactors
Apo AII
Apo CII
Ligands for
Apo AI
lipoprotein receptors Apo B100.E
Apo E
Structural
apoproteins
Apo AI
Apo B48
Apo B100
Commentary
For LCAT
For hepatic lipase
For LPL
For HDL receptors
For IDL and LDL
For chylomicron
remnants
HDL
Chylomicrones
VLDL, IDL, LDL
Review of functions of various parts of
alimentary tract
Enzymes are necessary for digestion. Where do they act?
1. the lumen of alimentary tract – enzymes from glands
-
-
-
salivary glands: amylase starts cleavaging starch and
glycogene
stomach: pepsin= endopeptidase splitting almost any
types of ingested proteins
gastric lipase is responsible for digesting
30% of ingested lipids
pancreas: proteolytic enzymes: trypsin,
chymotrypsin, carboxyypeptidase
-amylase
pancreatic lipase and cholesterolesterase
other: ribonuklease, deoxyribonuklease
liver secretes bile (no enzymes!): bile acids
2. luminal plazma membrane of enterocyte – brush border.
Substrates for these enzymes are the nutrient oligomers and
dimers that result from the pancreatic digestion of food.
- disacharidases: maltase, sucrase/isomaltase,
glucoamylase, lactase,
-glucosidase
- dipeptidases and aminopeptidases: many types
very specific for hydrolyzing individual types of
peptide linkage
- other: enteropeptidase, alcaline phosphatase
3.cytoplasm of enterocyte:
- hydrolysis of absorbed di- and tripeptides
(60% of ingested proteins is absorbed this way)
- triglyceride resynthesis, formation of
chylomicrones