Biochemistry - Gastroenterology

1. BiochemistryBiochemistry
MS410MS410
Lecture 1Lecture 1
Nutrition
Dr. Adil KhalilDr. Adil Khalil
1

2. Nutrition, digestion &
absorption

3. • Diet:
• metabolic fuels:
• carbohydrates and lipids
• Protein: for growth and turnover of
tissue proteins
• Fiber: for roughage
• minerals
• vitamins and essential fatty acids

4. • The polysaccharides, triacylglycerols,
and proteins must be hydrolyzed to
their constituent monosaccharides,
fatty acids, and amino acids,
respectively, before absorption and
utilization.

5. • Under nutrition leads to:
• impaired growth,
• defective immune systems
• excessive food consumption
(especially of fat), leading to:
• Obesity
• development of cardiovascular disease

6. • Deficiencies of vitamin A
• iron
• iodine
• result in major health concerns, and
deficiencies of other vitamins and
minerals are a major cause of ill health.

7. DIGESTION & ABSORPTION
OF CARBOHYDRATES
• Mouth
• Intestine
• The digestion of complex
carbohydrates is by hydrolysis to
liberate oligosaccharides, then free
mono- and disaccharides.

8. • The hydrolysis of starch by salivary
and pancreatic amylases catalyze
random hydrolysis of α(1→4) glycoside
bonds, yielding dextrins, then a mixture
of glucose, maltose.

9. • The disaccharidases—maltase, sucrase
lactase are located on the brush border
of the intestinal mucosal cells
(monosach).
• lactose intolerance:
• Lactose remains in the intestinal
lumen, where it is a substrate for
bacterial fermentation to lactate,
resulting in discomfort and diarrhea.

10. • Glucose and galactose are absorbed by
a sodium-dependent process.
• Other monosaccharides are absorbed
by carrier-mediated diffusion.

11. DIGESTION & ABSORPTION
OF LIPIDS

12. • The major lipids in the diet are
• 1. triacylglycerols and
• 2. phospholipids.
• These are hydrophobic molecules and
must be hydrolyzed and emulsified to
very small droplets (micelles) before
they can be absorbed.

13. • The fat-soluble vitamins—A, D, E, and
K—and a variety of other lipids
(including cholesterol) are absorbed
dissolved in the lipid micelles.
Absorption of the fat-soluble vitamins
is impaired on a very low fat
diet.

14. • Hydrolysis of triacylglycerols is
initiated by lingual and gastric lipases
that attack the sn-3 ester bond, forming
1,2-diacylglycerols and free fatty acids,
aiding emulsification.

15. • Pancreatic lipase is secreted into the
small intestine It is specific for the
primary ester links—ie, positions 1 and
3 in triacylglycerols—resulting in 2-
monoacylglycerols and free fatty acids
as the major end-products of luminal
triacylglycerol digestion

16. • Monoacylglycerols are hydrolyzed with
difficulty to glycerol and free fatty
acids, so that less than 25% of ingested
triacylglycerol is completely
hydrolyzed to glycerol and fatty acids

17. • Bile salts, formed in the liver and
secreted in the bile ( emulsification )
• micelles

18. • DIGESTION & ABSORPTION OF
PROTEINS
• Several Groups of Enzymes Catalyze
the Digestion of Proteins

19. • There are two main classes of
proteolytic digestive enzymes:
• proteases: 1. Endopeptidases
hydrolyze peptide bonds between
specific amino acids throughout the
molecule. They are the first enzymes
to act, yielding a larger number of
smaller
fragments, eg, pepsin in the gastric
juice and trypsin, chymotrypsin, and
elastase secreted into the small
intestine by the pancreas.

20. • 2. Exopeptidases,
Carboxypeptidases,
• aminopeptidases

21. • Dipeptides, which are not substrates
for exopeptidases, are hydrolyzed in
the brush border of intestinal
mucosal cells by dipeptidases.

22. • The proteases are secreted as inactive
zymogens:
• Pepsinogen is activated to pepsin
trypsinogen, the precursor of trypsin
• chymotrypsinogen to chymotrypsin,
proelastase to elastase,
procarboxypeptidase to
carboxypeptidase,
• and proaminopeptidase to
aminopeptidase.

23. • The end product is a mixture of free
amino acids, di- and tripeptides, and
oligopeptides, all of which are
absorbed.
• Free amino acids are absorbed across
the intestinal mucosa by sodium-
dependent active transport.
• Dipeptides and tripeptides enter the
brush border of the intestinal mucosal
cells, where they are hydrolyzed to free
amino acids.

24. • Relatively large peptides may be
absorbed intact, either by uptake into
mucosal epithelial cells or by passing
between epithelial cells.
• Many such peptides are large enough
to stimulate antibody formation— this
is the basis of allergic reactions to
foods

25. DIGESTION & ABSORPTION
OF VITAMINS & MINERALS
• fat-soluble vitamins are absorbed in lipid
micelles
• water-soluble vitamins and most minerals
are absorbed from the small intestine
• 1. by active transport
• 2. by carrier-mediated diffusion
• Vitamin B12: intrinsic factor;
• calcium absorption: vitamin D;
• zinc absorption requires a zinc-binding
ligand secreted by the exocrine pancreas;
• absorption of iron is limited.

26. Calcium Absorption
• vitamin D: Synthesis of the intracellular
calcium binding protein, calmodulin.
• High concentrations of fatty acids in
the intestinal lumen can also reduce
calcium absorption by forming
insoluble calcium salts
• high intake of oxalate can sometimes
cause deficiency, since calcium oxalate
is insoluble

27. Iron Absorption
• Inorganic iron is absorbed only in the
Fe2+ (reduced) state, and for that
reason the presence of reducing
agents
will enhance absorption (vitamin C)
• absorption of iron is impaired by
calcium a glass of milk with a meal
significantly reduces availability

28. • Absorption of iron is strictly regulated
• iron deficiency is a common problem
• overload (hemochromatosis)

29. • Absorption of iron: strictly regulated.
• iron: in intestinal mucosal cells bound
to an intracellular protein, ferritin.
• Once the ferritin in the cell is saturated
with iron, no more can enter.
• Iron can only leave the mucosal cell if
there is transferrin in plasma to bind to.

30. • Once transferrin is saturated with iron,
any that has accumulated in the
mucosal cells will be lost when the
cells are shed.
As a result of this mucosal barrier, only
about 10% of dietary iron is normally
absorbed

31. ENERGY BALANCE
• Food intake in excess of energy
expenditure leads to obesity,
• intake less than expenditure leads to
emaciation and wasting, as in
marasmus and kwashiorkor

32. • Marasmus can occur in both adults and
children and occurs in vulnerable
groups of all populations.
• Kwashiorkor only affects children.
• Decreased: immunity, muscle mass,
absorption (intestinal cell regeneration)

33. • Basal metabolic rate (BMR):
• is the energy expenditure by the body
when at rest Under controlled
conditions of thermal neutrality,
measured at about 12 hours after the
last meal, and depends on weight, age,
and gender
• Total energy expenditure depends on
the basal metabolic rate

34. • The decrease in BMR with increasing
age, even when body weight remains
constant, is due to muscle tissue
replacement by adipose tissue, which
is metabolically much less active.
• Similarly, women have a significantly
lower BMR than men of the same body
weight because women’s bodies have
more adipose tissue than men.

35. • children with kwashiorkor:
• oedema, associated with a decreased
concentration of plasma proteins.
• Enlargement of the liver due to
accumulation of fat

36. Nitrogen balance
• The state of protein nutrition can be
determined by measuring the dietary
intake and output of nitrogenous
compounds from the body.

37. • The output of nitrogen from the body is
mainly in urea and other compounds
in urine.
• undigested protein in feces,
• and significant amounts may also be
lost in sweat and shed skin.

38. • The difference between intake and
output of nitrogenous compounds is
known as nitrogen balance.
• Equilibrium
• Negative
• positive

39. • equilibrium when intake equals output,
and there is no change in the total body
content of protein.
• Positive nitrogen balance:
• Growing children, pregnancy, recovery
from illness.
• Negative: late cancers, malnutrition,
surgery, burn.

40. Minerals
• inorganic substances
• formation of structural components and
energy transfer
• maintain osmotic pressure and fluid
balance
• activity of nervous and muscular systems
• Macrominerals:calcium, phosphorus,
potassium, sodium, chloride, magnesium,
sulfur
• Microminerals: cobalt, copper, iodine,
iron, manganese, selenium, zinc

41. • sodium, potassium, chloride
• calcium, phosphorus magnesium
and zinc
• excess excreted in urine
• over-supplementation
–one mineral prevents absorption of
another
–zinc inhibits calcium and copper
absorption
–calcium inhibits zinc absorption
–manganese interferes with iron
absorption

42. • calcium
– 35% of bone structure
– essential for muscle contraction
– Blood clotting
– Action potential
– Activation of enzymes
– Release of neurotransmitters

43. • phosphorus
– 15% of skeleton
– require for energy transfer reaction -
ATP and ADP
– calcium:phosphorus ratio 1:1
• potassium
– major intracellular cation
– maintain osmotic pressure and acid-
base balance

44. • sodium
– major extracellular cation
– maintain acid base balance and osmotic
regulation
• chloride
– extracellular anion
– acid base balance and osmotic
regulation

45. • magnesium
– .05% of body mass
• 60% for skeleton
– activator of enzyme
• iron
– constituent of hemoglobin
– anemia - iron deficiency

46. • selenium
– component of enzyme glutathione
peroxidase
• cell membrane damage
– Oxidation of polyunsaturated fatty acids