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Phosphorylation of Glucose
Glucose + ATP Glucose–6–phosphate + ADPGlucokinase
Glucose + ATP Glucose–6–phosphate + ADPHexokinase
• There are TWO enzymes
Trapping of Glucose by phosphorylation
Glucokinase enzyme
Glucose + ATP Glucose-6-phosphate + ADPGlucokinase
Comparison between Hexokinase & Glucokinas
GlucokinaseHexokinaseFactorNo
Glucose onlyAll HexosesSubstrate1
Liver onlyAll tissuesDistribution2
Not inhibited by
Glucose-6-phosphate
Inhibited by
Glucose-6-phosphate
Product
inhibition3
High Km
(Low affinity)
Low Km
(High affinity)
Km for
glucose4
ActivatedNot affectedEffect of
Insulin5
ActivatedNot affectedEffect of
Carbohydrate6
InhibitedNot affectedEffect of
Starvation7
(Sigmoidal Curve)
(Hyberbolic Curve)
(Substrate concentration)
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
(Lactate Fermentation)(Ethanol + CO2)
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Stages of Glycolysis
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Stages of Glycolysis
Stages of Glycolysis
Stages of
Glycolysis
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Substrate-Level
Phosphorylation
Rx 7 and 10
Substrate-Level Phosphorylation
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
1
2
3
5 4
6
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Metabolic Significance of Glycolysis
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1
2
3
• The three irreversible enzymes (Glucokinase,
Phosphofructokinase, Pyruvate kinase) are
under the control of Insulin
• Insulin induces the synthesis of:
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)
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
Fate of Pyruvate
Pyruvate Decarboxylase
& Alcohol DHLactate DHPyruvate DH
In CytoplasmIn Mitochondria
2 Pyruvate
< TD>
< TD>
We
< TD>
Lactate Dehydrogenase
For regeneration of NAD
• Lactate DH in Heart & Muscles:
Heart (H4) Muscle (M4)
H HHH
M MMM
H MMM
H HMH
H HMM
LDH5LDH1
LDH2 LDH3 LDH4
Different forms of Lactate Dehydrogenase
• Lactate DH in different tissues:
H3M H2M2 HM3
LDH
CPK
CatabolismAnabolism
LDH5 & LDH4LDH1 & LDH2
< TD>
TPP
12
Cyto
so
l
Acetaldehyde
Lactate & Ethanol Fermentation
Decarboxylase
Alcohol
Dehydrogenase
Rapoport-Luebering Cycle in RBCs
(R-L Cycle)
1,3- Diphosphoglycerate 2,3- Diphosphoglycerate
(2,3-DPG)
3- Phosphoglycerate
Mutase
Phoshatase
3-Phoshoglycerate
kinaseADP
ATP
Pi
• To meet this deficiency in ATP
synthesis, glycolysis rate in
RBCs increases.
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Complete glycolysis
1
Rapoport-Luebering Cycle in RBCs
(R-L Cycle) 1
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.
3
1.Malate shuttle (Dicarboxylic Acid Shuttle).
2.Glycerol phosphate shuttle.
Two Shuttle Pathways for
Oxidation of Cytoplasmic NADH
(Dicarboxylic
Acid Shuttle)
Cytoplasm
(3 ATP)
(From glycolysis)1.Malate Shuttle
electron respiratory chain
2- -Glycerol Phosphate Shuttle
Cytosol
2 ATP(2 ATP)
2- -Glycerol Phosphate Shuttle
2 ATP
< TD>
Fate of Pyruvate
2 pyruvate + 2 NAD+ + 2 CoA ----> 2 acetyl CoA + 2 NADH + 2 carbon dioxide
H
Oxaloacetate
Alanine
Alanine
Cyto
pla
sm
Mit
och
on
dri
a
12
5
3
46
Acetaldehyde
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
• Pyruvate Dehydrogenase:
• A multienzyme complex has 3 Functions:
Pyruvate Acetyl CoA
FAD CoA-SH
NADH + CO2
NAD+ TPP
Lipoic acid
Pyruvate DH
1 2
3
5
4
NADH & ATP
Pyruvate Acetyl CoA1- Product inhibition
2- Covalent modification
(Protein kinase)
(directly)
3- Insulin
Pyruvate DH
Regulation of Pyruvate DH
-
--
+
Pyruvate Acetyl CoA
Insulin Allosterically
Pyruvate DH
Carboxylation of Pyruvate
-
+
Pyruvate OxaloacetateBiotin
ATP + CO2 ADP + Pi
Pyruvate
Carboxylase
-+
OH
(( ) )CH3
-Alanine Pantoic acid
Sources & Fate of
Acetyl CoA
12
3
5
Ketone
Bodies
4
Cholesterol
Tricarboxylic Acid Cycle (TCA)
Citric Acid Cycle, Kreb’s Cycle
Third Stage of Metabolism
2 Carbons4 Carbons
6 Carbons
5 Carbons4 Carbons
4 Carbons 4 Carbons
4 Carbons
3 Carbons
3 ATP
3 ATP
1 ATP
2 ATP
3 ATP
• 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.
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.
Comments & Biological
Significance of TCA 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.
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.
Regulation of Kreb’s Cycle
Effect of Fluroacetate on TCA
• Flurocitrate inhibits aconitase
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4
Metabolic Significance of Kreb’s Cycle
1
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.
aspartate -ketoglutarate oxaloacetate glutamate
Aminotransferase (Transaminase)
COO
CH2
CH2
C
COO
O
COO
CH2
HC
COO
NH3+
COO
CH2
CH2
HC
COO
NH3+
COO
CH2
C
COO
O + +
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.
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.
Fate of CO2