1. Carbohydrates Metabolism
Chapter 4
Carbohydrates Metabolism

2. § 1 Overview
Carbohydrates in general are polyhydroxy aldehydes or ketones or compounds which yield these on hydrolysis.

3. Biosignificance of Carbohydrates
The major source of carbon atoms and energy for living organisms.
Supplying a huge array of metabolic intermediates for biosynthetic reactions.
The structural elements in cell coat or connective tissues.

4. Glucose transporters (GLUT)
GLUT1: RBC
GLUT4: adipose tissue, muscle

5. The metabolism of glucose
glycolysis
aerobic oxidation
pentose phosphate pathway
glycogen synthesis and catabolism
gluconeogenesis

6. glycogen
Glycogenesis
Glycogenolysis
starch
lactate
Glycolysis
Digestion absorption
glucose
aerobic oxidation
Lactate, amino acids, glycerol
Gluconeo-genesis
H2O+CO2
Pentose phosphate pathway
Ribose, NADPH

7. §2 Glycolysis

8. Glycolysis
The anaerobic catabolic pathway by which a molecule of glucose is broken down into two molecules of lactate.
glucose →2lactic acid (lack of O2)
All of the enzymes of glycolysis locate in cytosol.

9. 1. The procedure of glycolysis
glycolytic pathway
pyruvate
lactic acid

10. 1) Glycolytic pathway :
G → pyruvate
including 10 reactions.

11. (1) G phosphorylated into glucose 6-phosphate
Phosphorylated G cannot get out of cell
Hexokinase , HK (4 isoenzymes) ,
glucokinase, GK in liver ;
Irreversible .

12. Comparison of hexokinase and glucokinase
hexokinase glucokinase
occurrence in all tissues only in liver
Km value mmol/L mmol/L
Substrate G, fructose, glucose
mannose
Regulation G-6-P Insulin
Comparison of hexokinase and glucokinase

13. (2) G-6-P → fructose 6-phosphate

14. (3) F-6-P → fructose 1,6-bisphosphate
The second phosphorylation
phosphofructokinase-1, PFK-1

15. (4) F-1,6-BP → 2 Triose phosphates
Reversible

16. (5) Triose phosphate isomerization
G→2 molecule glyceraldehyde-3-phosphate, consume 2 ATP .

17. (6) Glyceraldehyde 3-phosphate → glycerate 1,3-bisphosphate

18. (7) 1,3-BPG → glycerate 3-phosphate
Substrate level phosphorylation

19. (8) Glycerate 3-phosphate → glycerate 2-phosphate

20. (9) Glycerate 2-phosphate → phosphoenol pyruvate

21. (10) PEP →pyruvate
Second substrate level phosphorylation
irreversible

22. 2) Pyruvate → lactate

23. Summary of Glycolysis

24. Total reaction: Formation of ATP:
C6H12O6 + 2ADP + 2Pi CH3CHOHCOOH + 2ATP + 2H2O
Formation of ATP:
The net yield is 2 ~P or 2 molecules of ATP per glucose.

25. 2. Regulation of Glycolysis
Three key enzymes catalyze irreversible reactions : Hexokinase, Phosphofructokinase & Pyruvate Kinase.

26. 1) PFK-1
The reaction catalyzed by PFK-1 is usually the rate-limiting step of the Glycolysis pathway.
This enzyme is regulated by covalent modification, allosteric regulation.

27. bifunctional enzyme

28. 2) Pyruvate kinase Allosteric regulation:
F-1,6-BP acts as allosteric activator；
ATP and Ala in liver act as allosteric inhibitors;

29. Covalent modification:
phosphorylated by Glucagon through cAMP and PKA and inhibited.

30. 3) Hexokinase and glucokinase
This enzyme is regulated by covalent modification, allosteric regulation and isoenzyme regulation.
Inhibited by its product G-6-P.
Insulin induces synthesis of glucokinase.

31. 3. Significance of glycolysis
1) Glycolysis is the emergency energy-yielding pathway.
2) Glycolysis is the main way to produce ATP in some tissues, even though the oxygen supply is sufficient, such as red blood cells, retina, testis, skin, medulla of kidney.
In glycolysis, 1mol G produces 2mol lactic acid and 2mol ATP.

32. § 3 Aerobic Oxidation of Glucose

33. The process of complete oxidation of glucose to CO2 and water with liberation of energy as the form of ATP is named aerobic oxidation.
The main pathway of G oxidation.

34. 1. Process of aerobic oxidation

35. 1) Oxidative decarboxylation of Pyruvate to Acetyl CoA
irreversible;
in mitochodria.

36. Pyruvate dehydrogenase complex:
E1 pyruvate dehydrogenase
Es E2 dihydrolipoyl transacetylase
E3 dihydrolipoyl dehydrogenase
thiamine pyrophosphate, TPP (VB1)
HSCoA (pantothenic acid)
cofactors lipoic Acid
NAD+ (Vpp)
FAD (VB2)

37. Pyruvate dehydrogenase complex:
HSCoA
NAD+

38. pyruvate dehydrogenase complex
The structure of
pyruvate dehydrogenase complex

40. HSCoA

41. CO2
NADH
+H+
NAD+
CoASH

42. 2) Tricarboxylic acid cycle, TCAC
The cycle comprises the combination of a molecule of acetyl-CoA with oxaloacetate, resulting in the formation of a six-carbon tricarboxylic acid, citrate. There follows a series of reactions in the course of which two molecules of CO2 are released and oxaloacetate is regenerated.
Also called citrate cycle or Krebs cycle.

43. (1) Process of reactions

47. Citrate cycle

48. Summary of Krebs Cycle ①
Reducing equivalents

49. ② The net reaction of the TCAC:
acetylCoA+3NAD++FAD+GDP+Pi+2H2O
→ 2CO2+3NADH+3H++FADH2+GTP+ HSCoA
③ Irreversible and aerobic reaction
④ The enzymes are located in the mitochondrial matrix.

50. ⑤ Anaplerotic reaction of oxaloacetate

51. (2) Bio-significance of TCAC
① Acts as the final common pathway for the oxidation of carbohydrates, lipids, and proteins.
② Serves as the crossroad for the interconversion among carbohydrates, lipids, and non-essential amino acids, and as a source of biosynthetic intermediates.

52. Krebs Cycle is at the hinge of metabolism.

53. 2. ATP produced in the aerobic oxidation
acetyl CoA → TCAC : 3 (NADH+H+) + FADH2 + 1GTP → 12 ATP.
pyruvate →acetyl CoA: NADH+H+ → 3 ATP
1 G → 2 pyruvate : 2(NADH+H+) → 6 or 8ATP
1mol G： 36 or 38mol ATP
（12＋3 ）×2 ＋ 6（ 8 ）＝36（ 38 ）

54. 3. The regulation of aerobic oxidation
The Key Enzymes of aerobic oxidation
The Key Enzymes of glycolysis
Pyruvate Dehydrogenase Complex
Citrate synthase
Isocitrate dehydrogenase (rate-limiting )
-Ketoglutarate dehydrogenase

55. (1) Pyruvate dehydrogenase complex

56. (3) Isocitrate dehydrogenase
(2) Citrate synthase
Allosteric activator: ADP
Allosteric inhibitor: NADH, succinyl CoA, citrate, ATP
(3) Isocitrate dehydrogenase
Allosteric activator: ADP, Ca2+
Allosteric inhibitor: ATP
(4) -Ketoglutarate dehydrogenase
Similar with Pyruvate dehydrogenase complex

58. Oxidative phosphorylation→TCAC↑
ATP/ADP↑ inhibit TCAC,
Oxidative phosphorylation ↓
ATP/ADP↓，promote TCAC，
Oxidative phosphorylation ↑

59. 4. Pasteur Effect The key point is NADH ： NADH mitochondria
Under aerobic conditions, glycolysis is inhibited and this inhibitory effect of oxygen on glycolysis is known as Pasteur effect.
The key point is NADH ：
NADH mitochondria
Pyr TCAC CO2＋H2O
Pyr can’t produce to lactate.

60. §4 Pentose Phosphate Pathway

61. 1. The procedure of pentose phosphate pathway/shunt
In cytosol

62. 1) Oxidative Phase

63. 2) Non-Oxidative Phase Transketolase: requires TPP Transaldolase
Ribose 5-p
Fructose 6-p
Glycolysis
Fructose 6-p
Xylulose 5-p
Xylulose 5-p
Glyceraldehyde 3-p
Transketolase: requires TPP
Transaldolase

64. 2. Regulation of pentose phosphate pathway
The net reation:
3G-6-P + 6NADP+ →
2F-6-P + GAP + 6NADPH + H+ + 3CO2
2. Regulation of pentose phosphate pathway
Glucose-6-phosphate Dehydrogenase is the rate-limiting enzyme.
NADPH/NADP+↑, inhibit;
NADPH/NADP+↓, activate.

65. 3. Significance of pentose Phosphate pathway
1) To supply ribose 5-phosphate for bio-synthesis of nucleic acid;
2) To supply NADPH as H-donor in metabolism;
NADPH is very important “reducing power” for the synthesis of fatty acids and cholesterol, and amino acids, etc.

66. NADPH is the coenzyme of glutathione reductase to keep the normal level of reduced glutathione;
So, NADPH, glutathione and glutathione reductase together will preserved the integrity of RBC membrane.

67. Deficiency of glucose 6-phosphate dehydrogenase results in hemolytic anemia.
favism
NADPH serves as the coenzyme of mixed function oxidases (mono-oxygenases). In liver this enzyme participates in biotransformation.

68. §5 Glycogen synthesis and catabolism

69. Glycogen is a polymer of glucose residues linked by
 (1→4) glycosidic bonds, mainly
 (1→6) glycosidic bonds, at branch points.

71. 1. Glycogen synthesis (Glycogenesis)
The process of glycogenesis occurs in cytosol of liver and skeletal muscle mainly.

72. UDPG: G active pattern, G active donor.
In glycogen anabolism, 1 G consumes 2~P.
Glycogen synthase: key E.

73. UDPG

75. Branching enzyme

76. 2. Glycogen catabolism (glycogenolysis)
Phosphorylase: key E;
The end products: 85% of G-1-P and 15% of free G;
There is no the activity of glucose 6-phosphatase (G-6-Pase) in skeletal muscle.

77. Debranching enzyme: glucan transferase -1,6-glucosidase

78. (1→6) linkage
Nonreducing ends
Glycogen phosphorylase
Transferase activity of debranching enzyme
(1→6) glucosidase activity of debranching enzyme
Glucose

79. 3. Regulation of glycogenesis and glycogenolysis
1) Allosteric regulation
In liver:
G phosphorylase glycogenolysis
In muscle:

80. 2) Covalent modification
Glucagon
epinephrine
Adenylyl cyclase
receptor
G protein
cAMP
PKA
Phosphorylase
Glycogen synthase
glycogenolysis
Blood sugar
glycogenesis

82. §6 Gluconeogenesis

83. Concept:
The process of transformation of non-carbohydrates to glucose or glycogen is termed as gluconeogenesis.
Materials: lactate, glycerol, pyruvate and glucogenic amino acid.
Site: mainly liver, kidney.

84. 1. Gluconeogenic pathway
The main pathway for gluconeogenesis is essentially a reversal of glycolysis, but there are three energy barriers obstructing a simple reversal of glycolysis.

85. 1) The shunt of carboxylation of Pyr

86. 2) F-1, 6-BP →F-6-P

87. 3) G-6-P →G
2 lactic acid G consume ATP?

88. gluconeogenesis

89. 2. Regulation of gluconeogenesis
Substrate cycle:
The interconversion of two substrates catalyzed by different enzymes for singly direction reactions is called “substrate cycle”.
The substrate cycle produces net hydrolysis of ATP or GTP futile cycle

90. Key enzymes of gluconeogenesis
PEP carboxykinase
Pyr carboxylase
Fructose-bisphosphatase
Glucose-6-phosphatase

93. 3. Significance of gluconeogenesis
Replenishment of Glucose by Gluconeogenesis and Maintaining Normal Blood Sugar Level.
Replenishment of Liver Glycogen.
Regulation of Acid-base Balance.

94. First stages
(cytosol)
Second stages
(Mt.)
Third stages
(Mt.)

95. Lactic acid (Cori) cycle
Lactate, formed by the oxidation of glucose in skeletal muscle and by blood, is transported to the liver where it re-forms glucose, which again becomes available via the circulation for oxidation in the tissues. This process is known as the lactic acid cycle or Cori cycle.
prevent acidosis；reused lactate

96. Lactic acid cycle

97. §6 Blood Sugar and Its Regulation

98. 1. The source and fate of blood sugar

99. Blood sugar level must be maintained within a limited range to ensure the supply of glucose to brain.
The blood glucose concentration is 3.89～6.11mmol/L normally.

100. 2. Regulation of blood sugar level
1）insulin： for decreasing blood sugar levels.
2）glucagon：for increasing blood sugar levels.
3）glucocorticoid: for increasing blood sugar levels.
4）adrenaline：for increasing blood sugar levels.

101. 3. Abnormal Blood Sugar Level
Hyperglycemia: > 7.22～7.78 mmol/L
The renal threshold for glucose: 8.89～10.00mmol/L
Hypoglycemia: < 3.33～3.89mmol/L

102. Pyruvate as a junction point