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FERMENTATION: Anaerobic
Glycolysis
CATABOLIC FATES OF PYRUVATE
HOMOLACTIC FERMENTATION
• Utilized by muscles when the demand for ATP is
high and oxygen availability is low.
• ATP is rapidly regenerated compared to oxidative
phosphorylation.
• The reaction is freely reversible
HOMOLACTIC FERMENTATION
• Net reaction:
Glucose + 2ADP+ 2Pi
2 lactate + 2ATP+ 2H2O+ 2H+
• Lactate formed can either exported from the
cell or converted back to pyruvate
• The lactate formed in muscles is carried by the
blood to the liver, where it is converted to
glucose
CORI CYCLE
ALCOHOLIC FERMENTATION
• The NAD+ regenerated in this reaction will be
utilized by GAPDH
• TPP is an important cofactor of Pyruvate
decarboxylase
ALCOHOLIC FERMENTATION
GLYCOLYSIS AND CANCER
• Utilization of glucose and glycolysis proceed
faster in cancer cells
• Because of hypoxia, cancer cells depend on
anaerobic glycolysis for ATP production
• Tumor cells also have smaller amount of
mitochondria
• Some tumor cells overproduce several glycolytic
enzymes due to the presence of HIF-1
• HIF-1 acts at the level of mRNA synthesis to
stimulate the production of at least 8 glycolytic
enzymes
Entry of other sugars: Lactose
Entry of other sugars: Fructose
SYNTHESIS OF ACETYL Co-A
Pyruvate dehydrogenase complex is
composed of 3 enzymes and requires 5
coenzymes
PYRUVATE DEHYDROGENASE
COMPLEX
• E1: pyruvate
dehydrogenase (30
heterodimers)
• E2: dihydrolipoamide
transacetylase (20 trimers)
• E3: dihydrolipoamide
dehydrogenase (12 dimers)
• ~10,000 kD
PYRUVATE DEHYDROGENASE
COMPLEX
Thiamine = Vitamin B1
(ribo)Flavin = Vitamin B2
Niacin = Vitamin B3
Pantothenic Acid = Vitamin B5
REGULATION OF THE COMPLEX
• The eukaryotic complex contains two regulatory
enzymes: a kinase that phosphorylates three serine
residues in E1 and the phosphatase that removes those
phosphates
• The activity of the complex is controlled by allosteric
inhibition and covalent modification that is in turn
controlled by the energy state of the cell.
• ATP is an allosteric inhibitor of the complex; AMP is an
activator
• E2 is inhibited by acetyl-CoA and activated by CoA-SH
• E3 is inhibited by NADH and activated by NAD+
REGULATION OF THE COMPLEX
• Regulation also occurs by covalent
modification of E1 (de/phosphorylation)
• NADH and acetyl-CoA activate the pyruvate
dehydrogenase kinase which phosphorylates
the 3 specific serine residues in E1 rendering it
inactive
• Pyruvate dehydrogenase phosphatase
removes the phosphate groups. This enzyme
is activated by Ca2+ and Mg2+
KREBS CYCLE
• Aka tricarboxylic acid cycle and citric acid cycle
• central oxidative pathway
• Composed of 8 reactions that oxidizes acetyl
CoA to 2 molecules of CO2
• Occurs in the mitochondrial matrix
Citrate Synthase Reaction (First)
O
O
O
SCoA
acetyl CoA
O
H2O
O
CoASH
O
O
HO
O
+
citrate synthase
O
O
O
O
oxaloacetate
citrate
• Claisen condensation
• OAA must bind first before Acetyl-CoA (sequential
mechanism)
• -32.2kJ
Aconitase Reaction
O
O
O
O
HO
O
O
HO
O
O
aconitase
O
O
citrate
O
O
isocitrate
• Forms isocitrate
• Goes through alkene intermediate (cis-aconitate)
– elimination then addition
• 13.3kJ
Isocitrate Dehydrogenase
O
O
O
HO
O
NAD
NADH
CO2
O
O
O
O
O
isocitrate
•
•
•
•
isocitrate dehydrogenase
O
O
alpha ketoglutarate
All dehydrogenase reactions make NADH or FADH2
Oxidative decarboxylation
-20.9kJ
Energy from increased entropy in gas formation
α-ketoglutarate dehydrogenase
O
O
O
O
SCoA
CoASH
O
CO2
NAD
alpha ketoglutarate
NADH
O
O
O
dehydrogenase
alpha ketoglutarate
• Same as pyruvate dehydrogenase reaction
• Formation of thioester
– endergonic
– driven by loss of CO2
• increases entropy
• exergonic
• -33.5kJ
succinyl CoA
Succinyl CoA synthetase
SCoA
O
O
O
GDP
O
succinyl CoA
GTP
CoASH
succinyl CoA
synthetase
• Hydrolysis of thioester
– Releases CoASH
– Exergonic
• Coupled to synthesis of GTP
– Endergonic
– GTP very similar to ATP and interconverted later
• -2.9kJ
O
O
O
succinate
Succinate dehydrogenase
O
O
FAD
O
FADH2
O
H
H
succinyl CoA
O
O
dehydrogenase
succinate
• Dehydrogenation
• Uses FAD
– NAD used to oxidize oxygen-containing groups
• Aldehydes
• alcohols
– FAD used to oxidize C-C bonds
– 0kJ
O
O
fumarate
Fumarase
H2O
O
O
O
O
OH
H
H
fumarate
O
fumarase
O
• Addition of water to a double bond
• -3.8kJ
O
O
malate
Malate Dehydrogenase
O
O
NADH
NAD
O
O
O
O
OH
malate
O
O
malate
•
•
•
•
dehydrogenase
O
oxaloacetate
Oxidation of secondary alcohol to ketone
Makes NADH
Regenerates oxaloacetate for another round
29.7 kJ
REGULATION OF KREBS CYCLE
• 3 rate determining enzymes: citrate synthase, isocitrate
dehydrogenase and alpha-ketoglutarate
dehydrogenase
• 3 mechanisms used by the enzymes:
– Substrate availability (acetyl CoA and oxaloacetate)
– Product inhibition (NADH)
– Competitive feedback inhibition by intermediates (citrate
and succinyl CoA
• ADP is an effector of isocitrate dehydrogenase
• Ca2+ activates pyruvate dehydrogenase phosphatase,
isocitrate dehydrogenase and alpha-ketoglutarate
dehydrogenase
Counting ATP’s: one molecule of glucose
ATP
Glycolysis 2
PDC (X2) 0
TCA (X2) 2
NADH
2
2
6
FADH2
0
0
2
TOTAL
After
OxPhos
10
30 ATPs
2
4 ATPs
4
4