Glycolysis and the citric acid cycle are the main pathways for glucose metabolism and energy production in cells. Glycolysis breaks down glucose into pyruvate, generating a small amount of ATP. Pyruvate can then enter the citric acid cycle in mitochondria to be further oxidized, with electrons being transferred to oxygen through the electron transport chain. This generates a proton gradient that is used by ATP synthase to produce the majority of ATP through oxidative phosphorylation. Various pathways like gluconeogenesis, the pentose phosphate pathway, and glycogen metabolism also interact with glycolysis and the citric acid cycle to regulate glucose and energy homeostasis in the body.

1. CARBOHYDRATECARBOHYDRATE
METABOLISMMETABOLISM
Week 3-5

2. Major pathways of glucose utilizationMajor pathways of glucose utilization

3. LIVER BLOOD MUSCLE
glycogen glycogen
Fructose & galactose
glucose glucose Glucose
ATP ATP ATP
pyruvate pyruvate pyruvate
Lactate Lactate ATP
CO2 + H2O CO2 + H2O
Lipid
sterol &
cholesterol
glucose has a normal blood level of 70-90 mg/dL
< 70 mg/ 100 ml  hypoglycemia
> 70 mg/ 100 ml  hyperglycemia

4. glicolysisglicolysis
 Glycolysis is a series of reactions that takes place in the
cytoplasm of all prokaryotes and eukaryotes
 a molecule of glucose is degraded in a series of enzyme-
catalyzed reactions to yield two molecules of the three-
carbon compound pyruvate
 Also called Embden Mayerhoff metabolism
 Aerobic (glucose  pyruvate) and anaerobic (glucose
 lactate)
 Fermentation (glucose  ethanol)

5. Three possible catabolic fates of the pyruvate formed inThree possible catabolic fates of the pyruvate formed in
glycolysis.glycolysis.

6. An Overview: Glycolysis Has Two PhasesAn Overview: Glycolysis Has Two Phases

8. Glycolysis is tightly regulated in coordination
with other energy-yielding pathways to assure a
steady supply of ATP.
◦ Hexokinase, PFK-1, and pyruvate kinase are all
subject to allosteric regulation that controls the flow
of carbon through the pathway and maintains
constant levels of metabolic intermediates.

9. Entry of glycogen, starch, disaccharides, and hexoses into the
preparatory stage of glycolysis

10. Fates of Pyruvate under Anaerobic
Conditions: Fermentation
The NADH formed in glycolysis must be
recycled to regenerate NAD, which is required
as an electron acceptor in the first step of the
payoff phase. Under aerobic conditions,
electrons pass from NADH to O2 in
mitochondrial respiration.

11. Pyruvate is the terminal electron acceptor in Lactic AcidPyruvate is the terminal electron acceptor in Lactic Acid
FermentationFermentation
Under anaerobic or
hypoxic conditions,
many organisms
regenerate NAD by
transferring
electrons from
NADH to pyruvate,
forming lactate

12. Ethanol Is the Reduced Product in Ethanol Fermentation
 Other organisms, such
as yeast, regenerate
NAD by reducing
pyruvate to ethanol and
CO2.
 In these anaerobic
processes
(fermentations), there is
no net oxidation or
reduction of the carbons
of glucose.

13. GluconeogenesisGluconeogenesis
 Gluconeogenesis is the
pathway for glucose
synthesis from
noncarbohydrate
precursors

14.  Important for the maintenance of blood glucose levels
during starvation or during vigorous exercise.
 The brain and erythrocytes depend almost entirely on
blood glucose as an energy source.
 Gluconeogenesis occurs mainly in the liver and to a
lesser extent in the kidney.
 Most enzymes of gluconeogenesis are cytosolic, but
pyruvate carboxylase and glucose 6-phosphatase are
located in the mitochondrial matrix and bound to the
smooth endoplasmic reticulum, respectively.

15. The Pathway of GluconeogenesisThe Pathway of Gluconeogenesis
 Seven of the steps in
gluconeogenesis are catalyzed by
the same enzymes used in
glycolysis; these are the reversible
reactions.
 Three irreversible steps in the
glycolytic pathway are bypassed by
reactions catalyzed by
gluconeogenic enzymes:
◦ conversion of pyruvate to PEP via
oxaloacetate, catalyzed by pyruvate
carboxylase and PEP carboxykinase;
◦ dephosphorylation of fructose 1,6-
bisphosphate by FBPase-1; and
◦ dephosphorylation of glucose 6-
phosphate by glucose 6-phosphatase.

16. Alternative paths from pyruvate to phosphoenolpyruvate
 Conversion of mitochondrial
pyruvate to cytosolic
phosphoenolpyruvate to
initiate gluconeogenesis.
 Oxaloacetate cannot pass
across the inner
mitochondrial membrane, so
it is reduced to malate, which
can do so.

17.  The initial irreversible step of glycolysis is bypassed by
glucose 6-phosphatase, which catalyzes the
dephosphorylation of glucose 6-phosphate to
form glucose
◦ This enzyme is mainly found in liver and kidney, the only two
organs capable of releasing free glucose into the blood.
◦ A special transporter (GLUT2) in the membranes of these
organs allows release of the glucose.
 Glycolysis and gluconeogenesis are reciprocally
regulated to prevent wasteful operation of both
pathways at the same time.

18. The Cori CycleThe Cori Cycle
 During vigorous exercise, pyruvate produced by glycolysis in muscle
is converted to lactate by lactate dehydrogenase.
 The lactate diffuses into the bloodstream and is carried to the liver.
 Here it is converted to glucose by gluconeogenesis. The glucose is
released into the bloodstream and becomes available for uptake by
muscle (as well as other tissues, including brain).

19. Pentose Phosphate PatwayPentose Phosphate Patway
 The pentose phosphate pathway (PPP), also called
the hexose monophosphate shunt, is an alternate
pathway of glucose metabolism that supplies the
NADPH required by many biosynthetic pathways.
◦ The main purpose of the PPP is to generate NADPH to be
used in pathways for synthesis of important molecules,
eg, amino acids, lipids, and nucleotides.
◦ NADPH derived from the PPP is also important for
detoxification of reactive oxygen species.
◦ The PPP also is responsible for synthesis of ribose 5-
phosphate for nucleotide biosynthesis
 The PPP operates in two phases: an oxidative phase
and a nonoxidative phase

21. Glycogen MetabolismGlycogen Metabolism
Glycogen is stored in muscle and liver as
large particles.
Contained within the particles are the
enzymes that metabolize glycogen, as well
as regulatory enzymes
Glycogen
granules in a
hepatocyte

22. glycogenolysisglycogenolysis
 the catabolic pathways from glycogen to glucose 6-phosphate
 Catalyzed by glycogen phosphorylase

24. Glycogen synthesisGlycogen synthesis
A glycogen chain is elongated by
glycogen synthase. The enzyme
transfers the glucose residue of
UDP-glucose to the nonreducing end
of a glycogen branch to make a new
(α-14) linkage

25. Branch synthesis in glycogen
The glycogen-branching enzyme (also called amylo (14) to (16)
transglycosylase or glycosyl-(46)-transferase) forms a new branch
point during glycogen synthesis

26. Glycogen degradation and glycogen synthesis are
reciprocally regulated by hormones.

27. Kinases and phosphatases control the activities of the
interconvertible enzymes glycogen phosphorylase and glycogen
synthase

28. THE CITRIC ACIDTHE CITRIC ACID
CYCLECYCLE
Hans Krebs (1900–1981). Krebs was
awarded the Nobel Prize in Physiology or
Medicine in 1953 for his discovery of the citric
acid cycle.

29. Catabolism of proteins, fats, andCatabolism of proteins, fats, and
carbohydrates in the three stagescarbohydrates in the three stages
of cellular respirationof cellular respiration

30. Production of Acetyl-CoA (Activated Acetate)Production of Acetyl-CoA (Activated Acetate)
 Pyruvate, the product of glycolysis, is converted to
acetyl-CoA, the starting material for the citric acid
cycle, by the pyruvate dehydrogenase complex.

31. Coenzyme ACoenzyme A

32. Oxidative decarboxylation of pyruvate to acetyl-CoA by
the PDH complex

33. Reactions of the citric acid cycleReactions of the citric acid cycle

34. Products of one turn of the citric acid cycleProducts of one turn of the citric acid cycle

35. Biosynthetic precursors produced by an incompleteBiosynthetic precursors produced by an incomplete
citric acid cycle in anaerobic bacteriacitric acid cycle in anaerobic bacteria

36. Role of the citric acid cycle in anabolismRole of the citric acid cycle in anabolism

37. Regulation of the Citric Acid CycleRegulation of the Citric Acid Cycle

38. THE GLYOXYLATETHE GLYOXYLATE
CYCLECYCLE

39.  The glyoxylate cycle is active in the germinating seeds
of some plants and in certain microorganisms that can
live on acetate as the sole carbon source.
 In plants, the pathway takes place in glyoxysomes in
seedlings.
 It involves several citric acid cycle enzymes and two
additional enzymes: isocitrate lyase and malate synthase.

40. Reaction of glyoxylate cycle

41. Relationship between the
glyoxylate and citric acid
cycles

42. Coordinated regulation of glyoxylate
and citric acid cycle

43. ELECTRON TRANSPORT
CHAIN (ETC)/ RESPIRATION

45.  Most energy from food obtained through stepwise
anaerobic oxidative processes to yield NADH or
FADH2 (reducing equivalent).
 Then
 NADH or FADH2 aerobically oxidized ( in ETC ).
 This energy is used to synthesize ATP
(phosphorylation).

46. But how the energy of ETC
(oxidation) is used to
synthesize ATP (phosphorylation)
The coupling of oxidation &
phosphorylation.

47. Peter Mitchell
Chemiosmotic Theory
A proton gradient is generated with energy from electron transport
by the vectorial transport of protons (proton
pumping) by Complexes I, III, IV from the matrix to
intermembrane space of the mitochondrion.

48. Mitochondrion or
the power house of cell
• Outer membrane
permeable to small molecules
• Inner membrane
Impermeable to small molecules.
• Cristae increase area
IT contains:
Electron transport system (ETC) and ATP synthase complex
embedded;
• Integrity required for coupling ETC to ATP synthesis
• Matrix contains Krebs cycle enzymes, β-oxidation enzymes;
also ATP, ADP, NAD, NADH2, Mg2+, etc
The size : (1-2μ)
The number: 1-1000s in each cell

49. Chemiosmotic TheoryChemiosmotic Theory

50. Ubiquinone and cytochrome c are mobile carriers. They ferry electrons from one
complex to the next
ETC= electron transport chain

51. • NADH
dehydrogenase
(NADH Q
reductase)
• Huge protein
– 25 pp
• FMN, Fe-S
• Electron  UQ
• NADH
dehydrogenase
(NADH Q
reductase)
• Huge protein
– 25 pp
• FMN, Fe-S
• Electron  UQ
 Iron-Sulfur Centers
 Transfer of electrons in variety of proteins such as NADH and
succinate dehydrogenase

52. 2H+
+ 2 e-
Coen
zyme
Q
Coen
zyme
Q
Coenzyme Q = Ubiquinone
a lipid in inner membrane
 carries electrons
 polyisoprene tail
 moves freely within membrane
Complex II: Succinate Q Recuctase (Succinate dehydrogenase)
Is the only membrane
bound enzyme in the
TCA cylce and
contains  FAD, Fe-S
II  electrons  UQ

53. Complex III= Cyt C reductase

54. Complex IV (Cytochrome C oxidase)
Heme A and Cu act together to
transfer electrons to oxygen
e- from cyt c to a
Cyto oxidase
Contains a, a3,
and CuA, CuB
The detail of this
electron transfer in
Complex IV is not
known
It also functions as a
proton pump
Cu(II)  Cu(I)

55. Membrane potential = 140 mV
pH gradient = 60 mV
Total proton motive force = 200 mV

56. ATP Synthase (F0 - F1 complex)
F0
FI
F0 = Oligomycin sensitive Fragment

57. ATP synthesis at FATP synthesis at F11 results fromresults from
repetitive comformational changesrepetitive comformational changes
asas γγ rotatesrotates

58. Uncoupling ProteinUncoupling Protein
The coupling of oxidation (to make Proton gradient)
and phosphorylation ( ADP+P) is needed for ATP
synthesis.
*Thermogenin is a proton carrier located at inner mitochondrial
membrane*
*This uncoupling protein
produced in
brown adipose tissue of newborn
mammals, and hibernating
mammals for cold adaptation.

59. Uncoupling ProteinUncoupling Protein
The uncoupling protein blocks development of a H+
electrochemical gradient, thereby stimulating
respiration. ∆G of respiration is dissipated as heat.
This "non-shivering thermogenesis" is costly in
terms of respiratory energy unavailable for ATP
synthesis, but provides valuable warming of the
organism.
The gene is activated by thyroid hormone
Different level of the hormone in different season
and areas

60. Poisons of OxidativePoisons of Oxidative
PhosphorylationPhosphorylation
1- OXIDATION (ETC) inhibitors.
2- PHOSPHORYLATION inhbitors.
3- Uncouplers.
4- ATP/ADP transporter
(tanslocators) inhibitors

61. Rotenone, amytal
Antimycin A
Dimercaprol
HCN, CO,
H2S
• Complex 1: Rotenone and Barbiturates such as amobarbital and amytal
inhibit NDAH- DH. They are fatal at sufficient dosage.
• 2- Complex 2: Malonate is competitive inhibitor of Suc- DH
• 2- Complex 3: Antimycin A and Dimercaprol inhibit cyt C reductase.
• 3- Complex 4: Classic poisons HCN, CO, H2S arrest respiration by inhibiting
cyt oxidase.
• Note: all the components of the respiratory chain before the block
become reduced, all the components
• downstream become oxidized.
ETC inhibitorsETC inhibitors

62. ATP Synthase and ATP/ADPATP Synthase and ATP/ADP
translocator inhibitorstranslocator inhibitors
 The antibiotic
Oligomycin completely
blocks F0 ( Oligomycin
sensitive Fragment) the
flow of H+ through the
F0 directly inhibiting ox-
phos.
 Atractyloside ATP/ADP
translocator.

63. Uncouplers are lipid-
soluble weak acids. E.g.,
H+
can dissociate from
the OH group of the
uncoupler
dinitrophenol.
Uncouplers dissolve in
the membrane and
function as carriers
for H+
.
OH
NO2
NO2
2,4-dinitrophenol
Uncoupling reagents (uncouplers)

64. It is very efficient processIt is very efficient process
•Recall living cells efficiency is ~ 42%, compared to about 3%
efficiency when burning oil or gasoline.
BUT HOW?
Separating carbohydrates, lipids, etc. from oxygen to
optimize recover of energy. In other words first they are
anaerobically oxidized to yield NADH and FADH2,
And then
 Stepwise aerobic oxidation of NADH and FADH2 through
ETC
And then
ATP synthesis by electrochemical energy.
How is the energy yield in livingHow is the energy yield in living
cellscells

65. SummarySummary
1. Oxidative Phosphorylation is carried out by
respiratory assemblies that are located in the
inner membrane...
2. Respiratory assemblies contain numerous
electron carriers, Such as cytochromes.
3. When electrons are transferred, H+
are
pumped out.
4. ATP is formed when H+
flow back to the
mitochondria.
5. Oxidation and phosphorylation are
COUPLED
6. The oxidation of NADH  3 ATP, and FADH2
 2 ATP