1. Storage Mechanisms and Control of Carbohydrate Metabolism

2. Learning Objectives 1. How Is Glycogen Produced and Degraded?
2. How Does Gluconeogenesis Produce Glucose from Pyruvate?
3. How Is Carbohydrate Metabolism Controlled?
4. Why Is Glucose Sometimes Diverted through the Pentose Phosphate Pathway?

4. Why do animals store any energy as glycogen
Why do animals store any energy as glycogen? Why not convert all excess fuel into fatty acids?
Why not store energy as free glucose?

5. Adult Human 70 kg Triacylglyceride: 100.000 kcal
Protein (muscle): kcal
Glycogen: kcal
Glucose: kcal
Triacylglyceride: approx 11 kg
of body weight
glycogen storage instead of fat:
increase in weight: kg!!

6. Glycogen Breakdown ”glycogenolysis”
Glycogen is cleaved by glycogen phosphorylase by adding phosphate to give a-D-glucose-1-phosphate (phosphorolysis)
No ATP is involved in this phosphorolysis
Occur in the liver maintains blood glucose

7. Enzyme-catalyzed isomerization converts the
1-phosphate to the 6-phosphate
Note: more ATP is produced from glucose of glycogen
glycolysis

9. Glycogen transferase enzyme transfers three glucose residues from (limit branch) to another branch, where they are removed by glycogen phosphorylase
Glycogen debranching enzyme then hydrolyzes the a(1,6) glycosidic bond of the last glucose residue remaining at the point of branching.

11. Glycogenesis glycogenin
Glucose 1-phosphate reacts with uridine triphosphate to give UDPG and pyrophosphate
Glucose-
1-phosphate +
UTP
UDP G PP i
UDP-glucose
pyrophosphorylase O - P H C 2 N
Uridine diphosphate glucose
(UDPG )

12. Coupling of UDPG formation with hydrolysis of pyrophosphate drives formation of UDPG to completion

13. Exchange of phosphate from ATP regenerates UTP
Uridine diphosphate glucose (UDPG) then adds its glucose unit to the growing glycogen chain
Exchange of phosphate from ATP regenerates UTP
HO(Glucose) n OH +
Glycogen
UDP G
glycogen
synthase
HO-Glucose-
O(Glucose)
new glucose
unit added
α (1-4 bond)

14. Glycogenesis
Branching enzyme transfers about seven glucose residue-long segment from growing branch to a new branch point via α(1-6) glycosidic bond

15. Control of Glycogen Metab
Glycogen phosphorylase - a major control point
(Dephosphorylated form(
(Phosphorylated form)

16. Coordinate Control of Glycogen Metabolism
Inactive forms are shown in red, and active ones in green.

17. Control of Glycogen Metab
The activity of glycogen synthase is subject to the same type of covalent modification as glycogen phosphorylase
the response, however, is opposite
hormonal signals (glucagon or epinephrine) stimulate its phosphorylation
once phosphorylated, glycogen synthase becomes inactive at the same time the hormonal signal is activating glycogen phosphorylase
glycogen synthase can be phosphorylated by several other enzymes including glycogen synthase kinase
dephosphorylation is by phosphoprotein phosphatase

18. Glycogen Loading ??

19. Glycogen storage diseases.
Type I Von Gierke’s disease
Deficiency of glucose-6-phosphatase
Liver cells and renal tubule cells loaded
with glycogen. Hypoglycemia, lactic acidemia,
ketosis, hyperlipemia.

20. Summary
Glycogen is the storage form of glucose in animals, including humans. Glycogen releases glucose when energy demands are high
Glucose polymerizes to form glycogen when the organism has no immediate need for the energy derived from glucose breakdown
Glycogen metabolism is subject to several different control mechanisms, including covalent modification and allosteric effects

21. Gluconeogenesis

22. Gluconeogenesis
The synthesis of glucose from none carbohydrate sources like lactate, glycerol and amino acids.
gluconeogenesis is not the exact reversal of glycolysis; that is, pyruvate to glucose does not occur by reversing the steps of glucose to pyruvate
It is impossible to reverse any kinase reaction under physiological conditions.
gluconeogenesis occur in the cytosol & mitochondria
gluconeogenesis takes place in the liver 90%
and in kidneys 10%

23. Gluconeogenesis there are three irreversible steps in glycolysis
--- phosphoenolpyruvate to pyruvate + ATP
--- fructose-6-phosphate to fructose-1,6-
bisphosphate
--- glucose to glucose-6-phosphate
the net result of gluconeogenesis is reversal of these three steps, but by different reactions and using different enzymes (bypassing)

24. + 2 ATP
- 6 ATP

25. Gluconeogenesis Step 1: carboxylation of pyruvate (1st bypass)
requires biotin
pyruvate carboxylase is subject to allosteric control; it is activated by acetyl-CoA

27. Biotin
Biotin is a carrier of CO2 (carboxylation)

28. Gluconeogenesis
decarboxylation of oxaloacetate is coupled with phosphorylation by GTP to give PEP
the net reaction of carboxylation/decarboxylation is
net reaction is close to equilibrium: DG0’ = 2.1 kJ•mol-1

29. Gluconeogenesis
Second different reaction (2nd bypass) in gluconeogenesis
G° = -16.7•kJ mol-1
fructose-1,6-bisphosphatase is an allosteric enzyme, inhibited by AMP and F2,6P and activated by ATP

30. Gluconeogenesis
Third different reaction (3rd bypass) in gluconeogenesis
G°’ = kJ•mol-1

31. The Cori Cycle The Cori cycle
under vigorous anaerobic exercise, glycolysis in muscle tissue converts glucose to pyruvate; NAD+ is regenerated by reduction of pyruvate to lactate
lactate from muscle is transported to the liver where it is reoxidized to pyruvate and converted to glucose
thus, the liver shares the stress of vigorous exercise

32. The Cori Cycle

33. Control of carbohydrate metabolism
How ?

34. Control of carbohydrate metabolism
Allosteric: fructose-2,6-bisphosphate (F2,6P)
high concentration of F2,6P stimulates glycolysis; a low concentration stimulates gluconeogenesis
concentration of F2,6P in a cell depends on the balance between its synthesis (catalyzed by phosphofructokinase-2) and its breakdown (catalyzed by fructose bisphosphatase-2)
AMP inhibits FBPase and stimulates PFK
each enzyme is controlled by phosphorylation/ dephosphorylation

35. Fructose-2,6-bisphosphate
Fructose-2,6-bisphosphate is an allosteric activator of phosphofructokinase (a glycolytic enzyme) and an allosteric inhibitor of fructose bisphosphate phosphatase (an enzyme in the pathway of gluconeogenesis).
p. 520

36. Reciprocal Regulation of Gluconeogenesis and Glycolysis in the Liver

37. Control of carbohydrate metabolism

38. Control of carbohydrate metabolism
Substrate cycling
opposing reactions can be catalyzed by different enzymes and each opposing enzyme or set of enzymes can be regulated independently

39. Major Control Points in Carbohydrate Metabolism
Three steps in glycolysis are major control points in glucose metabolism
Hexokinase
Inhibited by high levels of glucose 6-phosphate
Phosphofructokinase,
When glycolysis is inhibited through glucose 6-phosphate builds up, shutting down hexokinase
Pyruvate kinase (PK) is an allosteric enzyme
Inhibited by ATP and alanine
Activated by fructose-1,6-bisphosphate
PK has 3 different isoenzymes
M predominates in muscle, L in liver, and A in other tissues
Native PK is a tetramer
Liver isoenzymes are subject to covalent modification

40. Control of Pyruvate Kinase

41. Summary
A number of control mechanisms operate in carbohydrate metabolism. These include allosteric effectors, covalent modification, substrate cycles, and genetic control
In the mechanism of substrate cycling, the synthesis and the breakdown of a given compound are catalyzed by two different enzymes

42. Pentose Phosphate Pathway
As the name implies, five-carbon sugars, including ribose, are produced from glucose
The oxidizing agent is NADP+; it is reduced to NADPH, which is a reducing agent in biosyntheses e.g. lipid
PPP is composed from two reactions:
Oxidative reactions: begins with two oxidation steps (using NADP+) to give ribulose-5-phosphate
Non-oxidative reactions: a series of carbon-shuffling steps during which three-, four-, five-, six-, and seven-carbon monosaccharide phosphates are produced
ATP production is not an important concern

43. PPP oxidative reactionsH O 2 P 3 -
Glucose-6-phosphate
6-Phosphogluconate
NADP +
NADPH
Ribulose-5-phosphate

44. Non-oxidative reactionsH 2 O H
RNA C O C H O H O H O H H O H H O H H O H C H 2 O H O H H O H C H 2 O P 3 - C H 2 O P 3 - C O H O H
Ribose-5-phosphate
Sedoheptulose-
7-phosphate H O H C H 2 O P 3 - C H 2 O
Ribulose-5-
phosphate C O H O H C H O H O H H O H C H 2 O P 3 - C H 2 O P 3 -
Xylulose-5-phosphate
Glyceraldehyde-
3-phosphate

46. To glycolysis

47. Pentose Phosphate Pathway

48. Pentose Phosphate Pathway
the carbon-shuffling reactions are catalyzed by
---transketolase for the transfer of two-carbon units requires thiamine pyrophosphate as a coenzyme
---transaldolase for the transfer of three-carbon units
Control of the pentose phosphate pathway
glucose-6-phosphate (G6P) can be channeled into either glycolysis or the pentose phosphate pathway
if ATP needed, G6P is channeled into glycolysis
if NADPH or ribose-5-phosphate are needed, G6P is channeled into the pentose phosphate pathway

49. G-6-PD
More than 400 variants of G-6-PD have been characterized, which show less activity than normal.
G-6-PD is the most common human enzyme deficiency in the world. It affect an estimated 400 million people.
Hemolysis, abdominal pain, dizziness, headache, dyspnea, palpitation, neonatal jaundice

50. Precipitating Factors
Infection & other ac. Illness(diabetic ketoacidosis)
Drugs: Antimalarials, Antipyretics or Antibiotics
Fava beans “favism”
Neonatal jaundice : due to decrease hepatic catabolism or increase production of bilirubin.

51. Pentose Phosphate Pathway
Summary of oxidative reactions
Glucose-6-phosphate + 2 NADP
Ribulose-5-phosphate + CO2 + 2 NADPH
Summary of non-oxidative reactions
Reactant
Enzyme
Products C 5 +
Transketolase 7 3 6 4
Transaldolase 2
Net:

52. Relationship between PPP and Glycolysis

53. End
Chapter 18