glycolysis, gluconeogenesis, citricacid cycle

1. CARBOHYDRATE
METABOLISM
Wilhelmina Annie Mensah
Dept. Of Med. Bchem.
UGMS , Ghana

2. Key words
• Dextrins - mixture of short, branched
and unbranched oligosaccharides
• Fascilitated diffusion
• ATP
• NAD

3. Lecture Content
1. Digestion
 1.1. Absorption
 1.2.1 Glucose transporters
 1.2.Disorders of
carbohydrate digestion
2. Glycolysis
 Energy investment stage
 Energy generation stage
 Key notes of various steps
 Fate of pyruvate
 Regenration of NAD+
 lactic acidosis
 Regulation of glycolysis
 Inhibition
3. Conversion of
pyruvate to acetyl co A

4. Digestion is a process by which large complex
organic molecules of food are disintegrated
into small absorbable forms
Enzymes break the α (1-4)glycosidic bonds in
polysaccharides
Humans lack the enzymes that break β (1-4
) and α (1-6) glycosidic bonds present in
cellulose and branched amylopectin and
glycogen .
1.1. Digestion

5. Products or
substrate
Substrates
or product
Substrates
Enzyme
Enzyme
products
Enzyme
2- Duodenum
1- Mouth
3- Ileum
1.1.Digestion

6. 1.2.1. Glucose Transporters (carriers) : responsible for the
absorption of most of the products of digestion
A. Na+ -independent facilitated diffusion transport
system
Down a concentration gradient ( No energy required)
 Family of 14 glucose transporters (GLUT -1 to GLUT 14 )
Characteristics
 1. Tissue specificity, examples
– GLUT -1 –- Erythrocytes and blood-brain barrier
– GLUT – 2 –liver, kidney and B-cells of pancreas
– GLUT -3 ---Neurons
– GLUT- 4 -- Adipose tissues
1.2. Absorption

7. 1.2.1. Glucose transporters
A. Na+ -independent facilitated diffusion transport
system
Characteristics
2. Specialized function isoforms
–GLUT -1 , GLUT -3, GLUT- 4 are involved uptake
of glucose from blood
–GLUT – 2 --- transport glucose into or out of
cells
–GLUT – 5 – uptake of fructose in small
intestines and testes
1.2.Carbohydarate Absorption

8. 1.2.1. Glucose transporters
 B. Sodium –monosaccharide co-transport system
Against a concentration gradient (Requires
energy)
 Sodium dependent glucose transporter (SGLT)
The glucose or galactose is coupled to the conc
gradient of Na+ and transported into the cell at
the same time.
Location
Occurs in epithelial of intestines, renal tubules
and chorioid plexus.
1.2.Carbohydarate Absorption

9. 1.2.1. Transporters
Glu and Gal–SGLT -1
Fructose –GLUT-5
Into circulation -
GLUT - 2
1.2.Carbohydarate
Absorption

10. 1. Lactose Intolerance
– Genetic deficiency of lactase activity causes non-
utilization of lactose
– Lactose accumulates in the large intestines and draws
water by osmosis causing
– Symptoms like
• Osmotic diarrhoea
• Bacteria fermentation producing CO2 and H2 which
gives abdominal cramps and flatulence
– Treatment : Withdrawal or reduce milk consumption
1.3. Disorders of carbohydrate digestion

11. 2. Congenital Sucrase-Isomaltase deficiency
– Genetic deficiency of of surcrase and isomaltase
activity causes non- utilization of lactose
3. Disacchariduria
– It is due to deficiency of disaccharidases
– It is characterized by excretion of large amounts of
disaccharide in urine.
1.3. Disorders of carbohydrate digestion

12. 2. Glycolysis( Embden-Meyerhof pathway)

13. 2. Glycolysis( Embden-Meyerhof pathway)
Site - cytoplasm of the
cell
Source of glucose: from
the digestion of dietary
carbohydrates enter
liver
Purpose :
converts glucose to 2
pyruvate molecules
Produces ATP in tissues
that lack mitochondria
Two stages (10 steps)
– Energy ( 2ATP) investment
phase
– Energy generation phase
(4ATP, 2NADH+)
Conditions
Aerobic : Pyruvate – CO2
Anaerobic(fermentation) :
pyruvate – lactate

14. 2.1. Glycolysis( Embden-Meyerhof pathway)
1. Energy ( 2ATP) investment phase
1
2
3
4
5

15. 2.2. Glycolysis( Embden-Meyerhof pathway)
– 2. Energy generation phase (4ATP, 2NADH)
5
6
7
10
9
8

16. 2.Glycolysis( Embden-Meyerhof pathway)

17. Key Notes
Step 1
Can also be catalysed by liver glucokinase under
conditions of high glucose conc such as after a high
meal
Phosphorylation of glucose is important to
prevent glucose from being transported out of the
cell
Step 3
Rate limiting and Commited step of glycolysis
PFK is a important regulatory enzyme
PFK is regulated by high ATP and F6P
2.3 Glycolysis( Embden-Meyerhof pathway)

18. Key Notes
Step 6
Phosphorylation of G3P is by
inorganic phosphate (Pi ) not ATP
Step 7 and 10
sbstrate level phosphorylation : enzyme transfer a
high energy Phosphate from a substrate to ADP to
form ATP
2.3. Glycolysis( Embden-Meyerhof pathway)

19. Glycolysis energetics
– Net ATP Produced
• Net reaction for glycolysis
1 NADH = 3 ATPS
2NADH = 6 ATPS + 2 ATP
Total number of ATPs = 8
2.4. Glycolysis( Embden-Meyerhof pathway)

20. 2.5. Glycolysis( Embden-Meyerhof pathway)
 5 Fates of Pyruvate
Lactate
 Acetyle- CoA
Ethanol
Oxaloacetate
Alanine

21. Regeneration of NAD+
Essential because accumulation of NADH can stop
glycolysis
Ways of regeneration
Aerobic conditions - Electron transport system
Anaerobic conditions: formation of alcohol
Anaerobic conditions - Formation of lactate
under regenerates NAD+
2.6. Glycolysis( Embden-Meyerhof pathway)

22. LACTIC ACIDOSIS :
Accumulation of lactic acid in the
muscles
Causes: Lack of O2 in the tissues
• Vigorous exercise
• Collapsed circulation
• Myocardial infarction
• Uncontrolled hemorrage
• Shock
Causing Aenarobic glycolysis;
Pyruvate to Lactate
 to regenerate NAD+
1.4. Glycolysis( Embden-Meyerhof pathway)

23. Glycolysis in Cancerous cells
High rate of glycolysis than other cells
Experience hypoxia (limited oxygen supply),
initial lack an extensive capillary network to supply
the tumor with oxygen.
Depend on anaerobic glycolysis for much of their
ATP production until capillaries are formed
Convert glucose to pyruvate and then to lactate as
they recycle NADH
1.4. Glycolysis( Embden-Meyerhof pathway)

24. 1.4. Glycolysis( Embden-Meyerhof pathway)
 Regulation of glycolysis
Allosteric regulation
Up regulation
F16B on pyruvate kinase
AMP on PFK1
F26B on PFK1
Down regulation
G6P on hexokinase
Citrate on PFK1
ATP on PFK1

25. 1.4. Glycolysis( Embden-Meyerhof pathway)
 Regulation of glycolysis
Hormonal control
• Insulin increases rate of glycolysis by
increasing concentration of
glucokinase,
phosphofructokinase-1
pyruvate kinase

26. 1.4. Glycolysis( Embden-Meyerhof pathway)
Inhibitors of Glycolysis
1.Glyceraldehyde -3-phosphate
dehydrogenase inhibitors (step 2)
They combine with-SH of active site and makes
enzyme inactive
Iodoacetate,
arsenate
 heavy metals like Hg2+,
Ag+.
2.Enolase ( step 9): inibited by fluoride.

27. Quick reminder
1. Digestion
 1.1. Absorption
 1.2.1 Glucose transporters
 1.2.Disorders of
carbohydrate digestion
2. Glycolysis
 Energy investment stage
 Energy generation stage
 Key notes of various steps
 Fate of pyruvate
 Regenration of NAD+
 lactic acidosis
 Regulation of glycolysis
 Inhibition
3. Conversion of
pyruvate to acetyl co A

28. 2.Glycolysis( Embden-Meyerhof pathway- Quick Reminder)

29. Quick Reminder

30. 3. Conversion of pyruvate to acetyl Co A
Players
– Coenzyme A
– NAD+
– TPP
– Lipoamide/lipoate
– FAD
– Pyrvate dehydrogenase

31. 3. Conversion of pyruvate to acetyl Co A
Site : Mitochondria Matrix
Energetics : 2 NADH = 6 ATP

32. 3. Conversion of pyruvate to acetyl Co A
Regulation

33.  Role of B Enzymes
3. Conversion of pyruvate to acetyl Co A

34. 4 . Citric Acid Cycle (TCA or Kreb Cycle)

35. Site : Mitochondrial matrix
Purpose
– Conversion of 2 acetyl-CoA to CO22
– Generates reducing equivalents (NADH, FADH2)
and GTP to be oxidized in the respiratory chain to
generate ATP
Stages : 8 steps
4. Citric Acid Cycle (TCA or Kreb Cycle)

36. Regulation
4. Citric Acid Cycle (TCA or Kreb Cycle)
citrate
Acetyl Co A
Citrate synthase
ATP
Isocitrate
Isocitrate dehydrogenase
ATP, NADH
ADP
Succinyl CO A
α- ketoglutarate
α- ketoglutarate
dehydrogenase
ATP, NADH

37. 4. Citric Acid Cycle (TCA or Kreb Cycle)
Energetics : 2 Acetyl CoA from 2 Pyruvate
 1 NADH =3 ATP
 1FADH= 2 ATP
 1 GTP = 1 ATP
× 2 = 24

38.  ATP generation during oxidation of Glucose
• However, the amount depends on shuttle used for the
transfer of reducing equivalents from cytosol to
mitochondria.
4. Energetics

40. Quick Reminder

41. GLYCOGEN METABOLISM

42. Glycogen structure
it makes up 6% of
the body weight
Made of α D-
glucose molecules
Straight chains
made of α (1-4)
glycosidic bonds
Branches made of
α (1-6) glycosidic
bonds

43. 5.Glycogenolysis and Glycogenesis
Glycogenesis: is the synthesis of glycogen from glucose
due to sufficient ATP produced
Glycogenolysis : the breakdown of glycogen to produce
glucose (ATP) in a state of fasting or glucose depletion

44. 5.Glycogenesis
Glycogenesis: is the synthesis of glycogen from
glucose due to sufficient ATP produced
Site : Liver and skeletal muscles
Purpose : to serve as a ready source of glucose
for glycolysis because
Fat can not be oxidized under anaerobic condition.
 Acetyl-CoA of fat oxidation can not be converted to
glucose.
 Skeletal muscle is unable to mobilize fat rapidly.

45. 5.Glycogenesis
Hexokinase/glucokinase
Glucose
Glucose -6-P
UTP
PPi
Phosphogluco mutase
Glucose -1-P
ATP
ADP
UDP- Glu-
pyrophosphorylase
2Pi
UDP-Glucose
Glycogen primer Glcn
UDP
Glycogen synthase
Glycogen

46. • Glycogenin

47. 5.Glycogenolysis
Glycogenolysis: is the conversion of glycogen to
glucose
Site : Liver and skeletal muscles during fast and
excercise
Purpose : to serve as a ready source of glucose
for glycolysis because
Fat can not be oxidized under anaerobic condition.
 Acetyl-CoA of fat oxidation can not be converted to
glucose.
 Skeletal muscle is unable to mobilize fat rapidly.

48. Pi
5.Glycogenolysis
Phosphorylase
Glycogen( Glucose)n
Glucose -1-P
Phosphogluco mutase
Glucose -6-P
Glucose 6-phosphatase
Glucose
Glucose
Glycolysis
Transferase & Debranching enzyme
Pi
Muscle
Liver,
kidney,
intestines

49. P
P P P P
P
P
P
phosphorylase
Transferase activity of
Debranching enzyme
-1,6 glucosidase activity of
Debranching enzyme

50. Glycogen storage diseases
Causes : absence of major enzymes in glycogen
metabolism
Presentation : accumulation of abnormal amount
glycogen in the liver or muscles
Symptoms
Liver enlargement due to increased liver glycogen
Exercise intolenrant
Liver cirrhosis
Hypoglycemia
Cardiac and respiratory failure
Death

51. Disease Enzyme defect
Type I (von Gierke’s ) Glucose 6-phosphatase
Type II (Pome’s) Lysosomal glucosidase
Type III (Cori’s) Debranching enzyme
Type IV (Andersen’s) Branching enzyme
Type V (McArdle’s) Muscle phosphorylase
Type VI (Her’s) Liver phosphorylase
Type VII Musle phosphofructokinase
Type VIII Liver phosphofructokinase
Glycogen storage diseases

52. 5.Control of Glycogen Metabolism

53. Guconeogenesis
• The synthesis of glucose from non-carbohydrate
sources in the liver and kidney
• Begins in the mitochondria and ends in the
cytosol
• Notable precursors are
Pyruvate
Glycerol
Lactate
Amino acids

54. Pathway
Gluconeogenesis
- glycolysis going backwards
- 3 places differ- control points in glycolysis
- 4 new enzymes (eukaryotes)
- importance of near equilibrium reactions
- ATP energy, NADH reducing equivalents consumed
#3
#10
#1
**Gluconeogenesis Net Reaction:**
2 Pyruvate + 4 ATP + 2 GTP + 2 NADH + 2 H+ + 6 H2O
 Glucose + 4 ADP + 2 GDP+ 2 NAD+ + 6 Pi
Glycolysis Net Reaction:
Glucose + 2 ADP + 2 NAD+ + 2 Pi
 2 Pyruvate + 2 ATP + 2 NADH + 2 H+ + 2 H2O

55. Gluconeogenesis
6 ATP needed total
4 needed to overcome barrier
of production of 2 mol of PEP

56. Gluconeogenesis: The Irreversible Steps
Pyruvate  PEP; reversing the pyruvate kinase step of glycolysis.
4 subunits
Biotin
Allosteric
+ acetyl CoA
Transcriptional
regulation
+ glucagon (fasting)
- Insulin (fed state)
Indicates CAC
Backed-up
No allosteric reg
Hormonal induction

57. Gluconeogenesis
No ATP needed since
Fru-1,6-bisP not high energy
intermediate

58. Fru-1,6-biP  Fru-6-P; reversing the PFK-1 step of glycolysis.
Large – DG and irreversible
Allosteric modulation
- AMP
- 2,6-Fru bisP (opposing effect in glycolysis)

61. • Cori cycle: Synthesis from lactate
Guconeogenesis