2. Phosphorylation of Glucose
• There are TWO enzymes
Glucokinase
Glucose + ATP Glucose–6–phosphate + ADP
Hexokinase
Glucose + ATP Glucose–6–phosphate + ADP
6. Comparison between Hexokinase & Glucokinas
No Factor Hexokinase Glucokinase
1 Substrate All Hexoses Glucose only
2 Distribution All tissues Liver only
Product Inhibited by Not inhibited by
3 inhibition Glucose-6-phosphate Glucose-6-phosphate
Km for Low Km High Km
4
glucose (High affinity) (Low affinity)
Effect of Not affected Activated
5
Insulin
Effect of Not affected Activated
6 Carbohydrate
Effect of Not affected Inhibited
7 Starvation
8. Comparison between Glucokinase & Hexokinase
Tissue-specific distribution of the two
enzymes ensures that:
At low blood glucose concentrations, liver
is prevented from utilizing glucose until the
nutrient requirements of other tissues are
satisfied
22. PFK is the regulatory (key) enzyme in glycolysis!
• The second irreversible reaction of glycolysis
• Large negative ∆G, means PFK is highly regulated
• PFK is regulated by:
– Citrate is an allosteric inhibitor
– ATP also inhibits PFK, while AMP activates PFK
– Fructose-2,6-bisphosphate is allosteric activator
– PFK increases activity when energy status is low
– PFK decreases activity when energy status is high
25. • The three irreversible enzymes (Glucokinase,
Phosphofructokinase, Pyruvate kinase) are
under the control of Insulin
• Insulin induces the synthesis of:
1
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2
3
26. Regulation of Glycolysis
1- After carbohydrate meal:
Blood glucose level Stimulates insulin
secretion Increases synthesis of
glucokinase, phosphofructokinase & pyruvate kinase
Enhances glycolysis
2- During fasting:
Blood glucose level Inhibits insulin
secretion & stimulates glucocorticoid secretion
Increases the synthesis of the four enzymes that
reverse glycolysis (Stimulate gluconeogenesis)
27. 3- Pasteur effect:
Increased oxygen inhibits glycolysis, since
increased citrate and ATP or increased ATP/ADP
ratio which inhibit phosphofructokinase (the rate
limiting enzyme of glycolysis)
Decreased ATP/ADP ratio or increased ADP, AMP
& Pi activates phosphofructokinase
4. Glycolysis inhibited by iodoacetate, fluroacetate &
arsenite, since they inhibit Kreb’s cycle
28.
29. Fate of Pyruvate
2 Pyruvate
Pyruvate Decarboxylase
Pyruvate DH Lactate DH
& Alcohol DH
In Mitochondria In Cytoplasm
34. Different forms of Lactate Dehydrogenase
• Lactate DH in Heart & Muscles:
Heart (H4) Muscle (M4)
LDH1 LDH5
H H M M
H H M M
• Lactate DH in different tissues:
H3M H2M2 HM3
LDH2 LDH3 LDH4
H H H H H M
H M M M M M
39. 1 Rapoport-Luebering Cycle in RBCs
(R-L Cycle)
Mutase
1,3- Diphosphoglycerate 2,3- Diphosphoglycerate
(2,3-DPG)
3-Phoshoglycerate
ADP kinase Phoshatase
7
Pi
ATP
3- Phosphoglycerate
Complete glycolysis • To meet this deficiency in ATP
synthesis, glycolysis rate in
RBCs increases.
41. 3 Pyruvate Kinase Deficiency in RBCs
1. The genetic deficiency of pyruvate kinase in
RBCs leads to hemolytic anemia.
2. This is due to inhibited (reduced rate of)
glycolysis and lowered level of ATP synthesis.
3. So the rate of synthesis of ATP is inadequate to
meet the energy needs of the cell to maintain
the structural integrity of erythrocytes.
42. Two Shuttle Pathways for
Oxidation of Cytoplasmic NADH
1. Malate shuttle (Dicarboxylic Acid Shuttle).
2. Glycerol phosphate shuttle.
47. Fate of Pyruvate Alanine
Cytoplasm
3
Acetaldehyde
2 1
5
Mitochondria
6
4
Alanine
H
Oxaloacetate
2 pyruvate + 2 NAD+ + 2 CoA ----> 2 acetyl CoA + 2 NADH + 2 carbon dioxide
48. • Pyruvate Dehydrogenase:
• A multienzyme complex has 3 Functions:
1. Decarboxylation: Removal of CO2 (Decarboxylase,
TPP as coenzyme).
2. Oxidation of the remaining two-carbon compound
and reduction of NAD+ (Dehydrogenase, CoASH,
FAD & NAD+).
3. Trans-acetylating function: Attachment of CoA with
a high energy thio-ester bond to form Acetyl CoA
(Transacetylase, Lipoic acid).
Pyruvate + NAD+ + CoASH Acetyl CoA + NADH + CO2
49. Pyruvate DH
Pyruvate Acetyl CoA
1 FAD 2 CoA-SH
NADH + CO2
3 NAD+ 4 TPP
5 Lipoic acid
50. Regulation of Pyruvate DH
- NADH & ATP
Pyruvate DH
Pyruvate Acetyl CoA
- - 1- Product inhibition
2- Covalent modification
(Protein kinase) +
(directly)
3- Insulin
51. Carboxylation of Pyruvate
-
Pyruvate DH
Pyruvate Acetyl CoA
+
Insulin + Allosterically
-
Pyruvate
Carboxylase
Pyruvate Oxaloacetate
Biotin
ATP + CO2 ADP + Pi
60. Comments & Biological
Significance of TCA Cycle
• It is the final pathway for oxidation (3rd stage) of
all foodstuffs to CO2 + H2O + Energy.
• It is important for the interconversion of
carbohydrates, fats & proteins.
• All reactions are reversible except: Citrate
synthase, Isocitrate DH & -Ketoglutarate DH.
• The rate limiting enzyme is Citrate synthase.
61. Comments & Biological
Significance of TCA Cycle
• Mitochondrial isocitrate DH is NAD+
linked, while cytoplasmic isocitrate DH is
NADP+ linked.
• TCA is the major source of succinyl Co A
which used for:
– Heme synthesis.
– Ketolysis.
– Detoxication reactions.
62. Regulation of Kreb’s Cycle
1. Insulin activates pyruvate DH & inhibits pyruvate
carboxylase, thus directing pyruvate towards complete
oxidation through kreb’s cycle.
2. Acetyl Co A inhibits pyruvate DH & activates pyruvate
carboxylase, thus directing pyruvate & glucose towards
formation of oxaloacetate to combine with excess Acetyl
Co A for the optimal activity of kreb’s cycle.
• During starvation (glucose supply is low & fat oxidation
provides excess Acetyl Co A), so oxaloacetate is
required.
63. Regulation of Kreb’s Cycle
3. So, Kreb’s cycle is inhibited by:
a) Starvation (No carbohydrates).
b) Diabetes mellitus (No insulin).
c) Anaerobic conditions (No oxygen).
4. It is inhibited in vitro by fluroacetate & iodoacetate
which form flurocitrate & iodocitrate that inhibits
aconitase.
5. Malonic acid is a competitive inhibitor of succinate
dehydrogenase.
6. Arsenite inhibits Kreb’s cycle.
66. Sources of Oxaloacetate
1. Pyruvate, in mitochondria (Pyruvate
carboxylase).
2. Malate, in mitochondria, by malate DH.
3. Citric acid, in cytoplasm (ATP-Citrate lyase).
4. Aspartic acid, by transamination in both
cytoplasm & mitochondria.
67. COO COO
COO CH2 COO CH2
CH2 CH2 CH2 CH2
HC NH3+ + C O C O + HC NH3+
COO COO COO COO
aspartate -ketoglutarate oxaloacetate glutamate
Aminotransferase (Transaminase)
68. Fate of Oxaloacetate
1. Aspartic acid, by transamination.
2. Citric acid, by citrate synthase.
3. Malate, in mitochondria, by malate DH.
4. Phosphoenolpyruvate by reversal glycolysis
(Gluconeogenesis), in cytoplasm, by PEP
Carboxykinase.
69. Fate of CO2
1. Excretion through lungs (main fate).
2. Combined with ammonia to form urea.
3. Combined with ammonia to form pyrimidine.
4. Enters in the formation of C6 of purines.
5. Fixation into organic acids (Carboxylation):
1. Pyruvic acid + CO2 Oxaloacetic acid.
2. Acetyl Co A + CO2 Malonyl Co A.
3. Propionyl Co A + CO2 Methylmalonyl Co A.
