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Glycolysis and Krebs Cycle
Assoc. Prof. Dr. Suzana Makpol
Dept. of Biochemistry
Faculty of Medicine, UKM
Session 2011/2012
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Objectives (Glycolysis)
describe the glycolysis pathway
describe the regulation of glycolysis differentiate between aerobic and
anaerobic glycolysis
explain the role of 2,3-bisphosphoglycerate in red blood cells
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CARBOHYDRATE METABOLISM AFTER MEAL
Acetyl CoA
Glu
CO2 + H2O + ATP (Na+ / ATPase at neuron
cells membrane)
Rice
Small intestine
Insulin
Glucagon
Portal vein
Adipose cell
TG
TG
LactatePyruvate
Glucose
Glycogen
Acetyl CoA
CO2 + H2O
+ ATP
Glycogen
LIVER
CO2 + H2O + ATP
Lactate
Pyruvate
GluBRAIN RBC
Carbohydrate
Glucose VLDL
Glucose
Gly 3-P
-Amylase
-amylase pancreasedisaccharidases (lactase,maltase, sucrase)
MUSCLEGlu
Acetyl
CoA
Fatty acids
+ Glycerol
LDL
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Session 2007/2008 4
Release of Chemical Energy Cellular
respiration
occurs in three
sets of reactions:
glycolysis, the
citric acid cycle,
and the electron
transport chain
(oxidative
phosphorylation)
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Glucose pyruvate/lactate It occurs in the cytosol
Glycolysis aerobic & anaerobic (does not requireoxygen to proceed)
1st stage of glycolysis - glucose is phosphorylatedat two places, requiring ATP
2nd stage - the 6-carbon glucose is split into two 3-carbon pyruvate molecules
ATP is synthesized at the second stage of glycolysisat two places
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Role of Glycolysis
- provide ATP
- precursor for ribose sugar
- glycerol 3-phosphate triacylglycerol
- pyruvate FA synthesis
amino acid synthesis
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Glucose
Glucose 6-phosphate
ATP*
ADP
Hexokinase/Glucokinase (Hexokinase IV)
Fructose 6-phosphate
Fructose 1,6-bisphosphate/diphosphate
ATP*
ADP
Dihydroxyacetone P Glyceraldehyde 3-P
Glycerol
Glycerol 3-P
Phosphoglucose isomerase
Aldolase A
Triose phosphate isomerase
PHASE I
*irreversible reaction: endergonic
Glycerol-3P dehydrogenase
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1,3-Bisphosphoglycerate
3-Phosphoglycerate
Phosphoenolpyruvate (PEP)
Pyruvate
NAD+
NADH
ADP
ATP
ATP
Pyruvate kinase
Glyceraldehyde 3-P
Lactate
ADP
NADH NAD+
Glyceraldehyde-3P dehydrogenase
Phosphoglycerate kinase
2-Phosphoglycerate
2,3-Bisphosphoglycerate
1,3-Bisphosphoglycerate mutase
2,3-Bisphosphoglycerate
phosphatase
Phosphoglycerate mutase
Enolase
Lactate dehydrogenase
Pi
PHASE II
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Role of 2,3-BPG in RBC
Bisphosphoglycerate shunt
2,3-BPG as allosteric inhibitor of
oxygen binding to heme. 2,3-BPG reenters the glycolytic
pathway via dephosphorylation to
3-phosphoglycerate
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Session 2007/2008 10
Regulation of Glycolysis
Glu 6-P
ATP, Citrate
ATP,asetil KoA
AlanineGlucagon
_
_
_
F2,6-BP, AMP+
+ F1,6-BP
Phosphofructokinase 2
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Session 2007/2008 11
When oxygenis available,
pyruvatemoves from
cytosol intomitochondria.
Aerobic Respiration
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In the absence of O2
, lactate formed (eg. RBC,muscle of a sprinter muscle cramps)
Pyruvate ---> Lactate (NADH -->NAD+; LDH)
Source of NADH (G3-P --> 1,3-BPG)
Formation of NAD+ is important for glycolysis to goon
Formation of ATP 100x faster than aerobicrespiration (citric acid cycle and oxidativephosphorylation)
Lactate will return to the liver to be converted topyruvate and to undergo gluconeogenesis - CoriCycle
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Cori Cycle
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Overall reaction for Glycolysis
Glucose + 2ATP + 2 ADP + 2PO4
- + 2NAD+
2 Pyruvate + 2 NADH + 2H2O + 4 ATP
Net Energy : 2ATP
Anaerobic : no NADH
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Session 2011/2012 15
Objectives (Krebs Cycle)
explain how pyruvate enters the mitochondria
explain the conversion of pyruvate to acetyl CoA
describe the Krebs Cycle and its regulation explain how reduced coenzymes fuel the
production of ATP
describe the entry and exit of metabolites in
Krebs Cycle
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Krebs Cycle (overview)
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Function of Krebs cycle:
Release of chemical energy (NADH,FADH2, GTP/ATP)
Its intermediates are precursors ofmany important compounds(Succinyl CoA = heme synthesis,
OAA aspartate, an amino acid)
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Krebs cycle
Acetyl CoA combines with oxaloacetate to formcitrate.
A series of reactions regenerate oxaloacetate
and produce ATP, NADH + H+
, FADH2, andcarbon dioxide.
This cycle can be repeated as long as oxygenand pyruvate are available
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Session 2007/2008 19
Glucose
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PEP
ATP
Pyruvate
= Pyruvate kinase
Lactate
Pyruvate
Acetyl CoA
Citrate
Oxaloacetate
IsocitrateNADHAKGSuccinyl CoA
Fumarate
MalateNADH
FADH2
NADH
O2H2O
Malate/ Aspartate
Malate/ Aspartate
OAA
P
E
N
G
A
N
GK
U
T
EL
E
KT
R
O
N
GLUCONEOGENESIS
CITRIC ACIDCYCLE
Carboxykinase
ATP
P
E
N
G
A
N
GK
U
T
EL
E
KT
R
O
N
P
E
N
G
A
N
GK
U
T
T
R
A
NS
P
O
R
T
E
L
E
C
T
R
O
N
Glucose
ADP
Pyruvate dehydrogenase
Citrate synthase
Aconitase
Isocitrate dehydrogenaseAKG dehydrogenase
Succinate dehydrogenase
Fumarase
Malate dehydrogenase
NADH
GTP
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Pyruvate dehydrogenase
Pyruvate
Acetyl CoAMulti enzyme complex
Pyruvate dehydrogenase/pyruvatedecarboxylase (2 forms:nonphosphorylated is the active form whilethe phosphorylated is inactive form )
Dihydrolipoyl transacetylase
Dihydrolipoyl dehydrogenase
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E1complex
E3 complex
E2 complex
lipoamide
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Pyruvate dehydrogenase (PDH)
OOCCOCH3
+ CoA + NAD+ CoASCOCH3
+ CO2
+ NADH + H+
Pyruvate Acetyl CoA
Co-factors: TPP, coenzyme A, NAD+, lipoic acid (vitamins)
Acetyl CoA dan NADH, are negative effectors while ADP a positiveeffector
PDH deficiency (genetic disorder) lactic acid accumulates damageof neuron cells mental retardation (rare)
Treatment: supplement with thiamine if E1 is abnormal; lipoic acid ifE2 is abnormal and give diet low in carbohydrate
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Session 2007/2008 24
Regulation of
PDH complex
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Regulation of TCA Cycle
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Citrate synthase
Acetyl CoA + OAA citrate
An allosteric enzyme
Positive effector: ADPNegative effectors: ATP, NADH,
succinyl CoA, acyl CoA derivative ,
fatty acids
Isocitrate dehydrogenase:
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Isocitrate dehydrogenase:rate limiting TCA
CO2NADH (need Mg2+)
ADP , positive effectorATP and NADH negative effector
K t l t t d h d
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-Ketoglutarate dehydrogenase
Multi enzyme complex:
a-ketoglutarate dehydrogenase/decarboxylase,transsuccinylase,lipoamide dehydrogenase
a-Ketoglutarate Succinyl CoARelease of CO2Formation of NADHCoenzymes: NAD+, TPP, lipoic acid and CoA
Negative effectors: ATP , GTP, NADH andSuccinyl CoASimilar structure to PDH
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Succinate dehydrogenase
Succinate Fumarate
Formation of FADH2
Malonate is an analogue ofsuccinate, a competitive inhibitor ofthe enzyme
OVERALL REACTION OF CITRIC ACID
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O C O O C C CCYCLE
Acetyl CoA + 3NAD+ + FAD + GDP + Pi + 2H2O
2CO2 + 3NADH + FADH2 + GTP + 3H+ + CoA
Regulatory enzymes (mainly allosteric): citrate
synthase, isocitrate dehydrogenase, a-ketoglutarate dehydrogenase
Regulatory factors:
ADP/ATP and NAD+
/NADH = regulate respirationfor energy formation
Efflux of intermediates from TCA
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Session 2007/2008 31
Efflux of intermediates from TCA
cycle
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Thank You