2. Three Stages of Metabolism

4. 3 Stages of Metabolism
• First stage:
• It is the breakdown & degradation of
polysaccharides, proteins & lipids into
their simplest units either through
intracellular catabolism or during
digestion

5. Carbohydrates Proteins Lipids
Glucose Amino acids Free Fatty acids
1st Stage of Metabolism

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7. 1
2
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9. Digestive juices

10. Disaccharidase
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11. Dipeptide
Dipeptidase
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12. Lipase < TARGET="display">

13. CoA.
CoA
of Kreb’s cycle.
& glycerol.

14. Carbohydrates Proteins Lipids
1st
Stage
Glucose Amino acids Free Fatty acids
2nd
Pyruvate Acetyl CoA
Stage
ATP
(substrate level phosphorylation)
1st and 2nd Stages of Metabolism

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+ H2 O +

16. Carbohydrates Proteins Lipids
1st
Stage
Glucose Amino acids Free Fatty acids
2nd
Pyruvate Acetyl CoA
Stage
ATP
(Substrate level phosphorylation)
NADH/FADH2
3rd
Stage
Oxidative phosphorylation (CO2 + H2O) + ATP
Stages of Metabolism

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18. The Sun is Energy for Life
• Phototrophs use light to drive synthesis
of organic molecules (C6H12O6)
• Heterotrophs use these organic
molecules as building blocks
• CO2, O2, and H2O are recycled

20. Metabolism
Metabolism includes:
1. Digestion
2. Absorption
3. Catabolism
4. Anabolism
5. Excretion

21. Metabolism
• Catabolism: degradative
pathways, Usually energy-
yielding!
• Anabolism: biosynthetic
pathways, energy-requiring!

22. Different Types of Metabolic Pathways
a) Linear metabolic pathway:
The product of each step is the substrate
for the next step
b) Cyclic pathway forms a closed loop:
In the citric acid cycle, an acetyl group is
metabolized (oxidized) via reactions that
regenerate the intermediates of the cycle
c) Spiral pathway:
The same set of enzymes catalyze a
progressive lengthening of the acyl chain

23. Different Types of Metabolic Pathways
Linear pathway Cyclic pathway Spiral pathway
Fatty acid synthesis
Conversion of 3-phospho- Kreb’sycle
glycerate into serine
Enz 1
Enz 2
Enz 3

24. Comparing Pathways
• Anabolic & catabolic pathways involving the
same product are not the same
• Some steps may be common to both
• Metabolic pathways are multi-step pathways
• This also allows regulation mechanisms to turn
on pathway and other turned off

25. Single-step versus Multi-
step Pathways
(capturing chemical energy)
A • The uncontrolled combustion
of glucose releases a large
C amount of energy all at once
E
• A multi-step enzyme –
catalyzed pathway – releases
the same amount of energy
F
but conserves much of it in a
manageable form

26. Single-step versus Multi-
step Pathways
(capturing chemical energy)
A
1. Multiple points where
C metabolites can enter & leave
2. Multiple points for regulation
E
3. Substrates & products can be
involved in more than one
F
metabolic reaction

27. Regulation of Metabolic Pathways
Starting Substrate Final End Product
Final End Product
Starting Substrate

28. Regulation of Metabolic Pathways
Irreversible Reactions

29. Regulation of Metabolic Pathways
Feedback Inhibition
• Feedback inhibition occurs when a product (usual the
final product) of a pathway controls the rate of its own
synthesis through inhibition of an early step, usually the
first step.

30. Regulation of Metabolic Pathways
Feed-forward Activation
• Feed-forward activation occurs when a metabolite
produced early in a pathway activates an enzyme that
catalyzes a reaction further down the pathway.

31. Regulation of Metabolic Pathways
Coordinated Regulation
+
• Can serve to deplete intermediates &
accelerate their re-supply
• The existence of multiple control points is to be expected

32. Prof. Dr. Mamdouh M. El-Shishtawy

33. 1. Digestion
Digestion of Carbohydrates
• It is the breakdown &
degradation of polysaccharides &
oligosaccharides into their
simplest units (Monosaccharides)

34. Carbohydrates
Digestible Do not need
digestion
Non-Digestible
Starch
Monosaccharides
Glycogen
Cellulose Pentoses
Lactose Hexoses
Sucrose

35. The
Digestive
System

36. PH 6.4 – 6.9. Activated
by Cl-, digests starch to Acidic PH of stomach (1-2) is
dextrins, maltose & unsuitable for salivary
isomaltose amylase
4.
-
(Activated by Cl ).
3. Brush border (Intestinal juice,
Succus Entericus) containing:
Maltase, Lactase & Sucrase

37. Carbohydrate Digestion in Small Intestine
• Salivary amylase stops working in acidic
stomach (if pH  4.5)
• 50% of dietary starch digested before it
reaches small intestine
• Brush border enzymes act upon
oligosaccharides, maltose, sucrose & lactose
• lactose indigestible after age 4 in most
humans (due to lack of lactase)

38. Carbohydrate Digestion
in Small Intestine

39. & Isomaltose

40. 2. Absorption
Monosaccharides Absorption
• Sodium-glucose transport proteins (SGLT)
in membrane help absorption of glucose &
galactose
• Fructose absorbed by facilitated diffusion
then converted to glucose inside the cell

41. Pentoses absorbed
by Passive Diffusion
According
concentration
gradient
Hexoses absorbed
by Active Transport
Against
concentration
gradient
Absorption of Pentoses & Hexoses

42. Types of Transport Carriers

44. Monosaccharides Absorption
Liver

45. Digestion of Lactose
(Lactase)

46. Lactose Intolerance
• Some individuals [90% of certain races (adult blacks &
Orientals)] have a defect in lactase enzyme
• Undigested maltose (osmotically active compound)
passes to the bowel (large intestine), acted upon by
bacteria of large intestine producing short chain fatty
acids & CO2 gas, leading to:
1. Abdominal distention (cramping)
2. Abdominal pain
3. Nausea
4. Bloating
5. Watery diarrhea

47. Lactase deficiency

48. Fate of Absorbed Glucose
• Glucose enter Liver & Brain by Passive
diffusion
• Glucose enter other tissues (skeletal muscle,
adipose tissue, etc…) by Active transport
• Active transport is enhanced by Insulin

50. Utilization of Glucose
Anabolic
Reactions Catabolic
Reactions
1) Storage in the form of
Glycogen (Glycogenesis)
2) Storage in the form of TAG 1) Glycolysis
(Lipogenesis) 2) Kreb’s Cycle
3) Synthesis of sugar alcohols
and amino sugars 3) Pentose Shunt
4) Interconversion between 4) Formation of
monosaccharides Uronic acids