• We study two key metabolic pathways
- b-oxidation of fatty acids
– glycolysis
________
________
Glycolysis
• Glycolysis: from the Greek, glyko, sweet,
and lysis, splitting
– a series of 10 enzyme-catalyzed reactions by
which glucose is oxidized to two molecules of
pyruvate
O
C6 H12 O 6
Glucos e
glycolys is
ten enzymecatalyzed steps
2 CH3 CCO 2 - + 2 H+
Pyruvate
Glycolysis - Rexn 1
• reaction 1: phosphorylation of -D-glucose
HO
HO
CH2 OH
O
O O
+ -O-P-O-P-O-AMP
OH
-D-Glucose OH
HO
HO
-
O
-
O
ATP
CH 2 OPO3
O
hexokinase
2+
Mg
2-
OH
OH
-D-Glucose 6-phosphate
O
+ - O-P-O-AMP
O
ADP
Glycolysis - Rexn 2
• reaction 2: isomerization of glucose 6phosphate to fructose 6-phosphate
6
HO
HO
6
2-
CH2 OPO3
O
2
OH
phosphoglucoisomerase
1
OH
-D-Glucose 6-phosphate
CH2 OPO3 2 1
CH2 OH
O
H HO
2
H
OH
HO
H
-D-Fructose 6-phosphate
Glycolysis - Rexn 2
– this isomerization is most easily seen by
considering the open-chain forms of each
monosaccharide; it is one keto-enol
tautomerism followed by another
1
CHO
H 2 OH
HO
H
H
OH
H
OH
2CH2 OPO3
Glucose 6-phosphate
H C OH
C OH
HO
H
H
OH
H
OH
2CH2 OPO3
(An enediol)
1
CH2 OH
2
C O
HO
H
H
OH
H
OH
2CH2 OPO3
Fructose 6-phosphate
Glycolysis - Rexn 3
• reaction 3: phosphorylation of fructose 6phosphate
6
CH2 OPO3 2 1
CH2 OH
O
H HO
+ ATP
H
OH
HO
H
-D-Fructose 6-phosphate
phosphofructokinase
Mg 2+
6
CH2 OPO3 2 1
2CH2 OPO3
O
+ ADP
H HO
H
OH
HO
H
-D-Fructose 1,6-bisphosphate
Glycolysis - Rexn 4
• reaction 4: cleavage of fructose 1,6bisphosphate to two triose phosphates
CH2 OPO3
CH2 OPO3
2-
aldolase
H
OH
OH
CH2 OPO3
Dihydroxyacetone
phosphate
C=O
C=O
HO
H
H
2-
CH2 OH
CHO
2-
Fructose 1,6-bisphosphate
H C OH
CH2 OPO3
2-
D-Glyceraldehyde
3-phosphate
Glycolysis - Rexn 4b
• reaction 4b: isomerization of triose
phosphates
– catalyzed by phosphotriose isomerase
– reaction involves two successive keto-enol
tautomerizations
– only the D enantiomer of glyceraldehyde 3phosphate is formed
CH2 OH
C= O
CHOH
C-OH
CH2 OPO 3 2 -
CH2 OPO 3 2 -
CH2 OPO 3 2 -
Dihydroxyacetone
phosphate
An enediol
intermediate
D-Glyceraldehyde
3-phosphate
CHO
H C OH
Glycolysis - Rexn 5
• Reaction 5: oxidation of the -CHO group of
D-glyceraldehyde 3-phosphate
– the product contains a phosphate ester and a
high-energy mixed carboxylic-phosphoric
anhydride
CHO
H C OH
2CH2 OPO3
D-Glyceraldehyde
3-phosphate
+ NAD+ + Pi
glyceraldehyde
3-phosphate
dehydrogenase
O
C-OPO3 2 H C OH
+ NADH
2CH2 OPO3
1,3-Bisphosphoglycerate
Glycolysis - Rexn 6
• Reaction 6: transfer of a phosphate group
from 1,3-bisphosphoglycerate to ADP
O
2C-OPO3
+
H C OH
CH2 OPO3 2 1,3-Bisphosphoglycerate
O
O-P-O-AMP
O-
phosphoglycerate kinase
Mg2 +
ADP
COOO O
+ -O-P-O-P-O-AMP
H C OH
2O O
CH2 OPO3
3-Phosphoglycerate
ATP
Glycolysis - Rexn 7 & 8
• Reaction 7: isomerization of 3phosphoglycerate to 2-phosphoglycerate
-
COO
H C OH
phosphoglycerate
mutase
CH2 OPO3 23-Phosphoglycerate
-
COO
H C OPO3 2 CH2 OH
2-Phosphoglycerate
• Reaction 8: dehydration of 2phosphoglycerate
COO2-
enolase
COO-
2+ H2 O
H C OPO3
C
OPO
2+
3
Mg
CH2 OH
CH2
2-Phosphoglycerate
Phosphoenolpyruvate
Glycolysis - Rexn 9
• Reaction 9: phosphate transfer to ADP
-
COO
2C OPO3
+
CH2
Phosphoenolpyruvate
O
O-P-O-AMP
O
pyruvate
kinase
Mg2 +
ADP
O O
COO
C=O + O-P-O-P-O-AMP
O O
CH 3
ATP
Pyruvate
Glycolysis - Rexn 9
• Reaction 9: phosphate transfer to ADP
– stage 1: transfer of the phosphate group
CO 2 C OPO 3 2- +
CH 2
Phosphoenolpyruvate
O
-
O-P-O-AMP
OADP
pyruvate
kinase
Mg 2+
CO 2 C-OH
CH 2
Enol of
pyruvate
O
+
-
O
O-P-O-P-O-AMP
O-
O-
ATP
Glycolysis - Rexn 9
– stage 2: enolization to pyruvate
CO 2 -
CO 2 -
C-OH
C=O
CH2
CH3
Enol of
pyruvate
Pyruvate
Glycolysis
• Summing these 10 reactions gives the net
equation for glycolysis
C6 H1 2 O6 + 2 N A D+ + 2 HPO 4 2 - + 2 A DP
Glucose
O
2 CH3 CCOO - + 2 NADH +
Pyruvate
glycolysis
2 ATP + 2 H 2 O + 2 H +
Reactions of Pyruvate
• Pyruvate is most commonly metabolized in
one of three ways, depending on the type of
organism and the presence or absence of O2
12
aerobic conditions
plants and animals
Acetyl CoA
13
Citric acid cycle
OH
O
- 11 anaerobic conditions
CH3 CHCOO
CH3 CCOO
contracting muscle
Lactate
Pyruvate
10 anaerobic conditions
CH3 CH2 OH + CO2
fermentation in yeast
Ethanol
Pyruvate to Lactate
– in vertebrates under anaerobic conditions, the
most important pathway for the regeneration of
NAD+ is reduction of pyruvate to lactate
O
CH3 CCOO - + NA DH + H +
Pyruvate
lactate
dehydrogenase
OH
CH3 CHCOO- + NA D+
Lactate
– lactate dehydrogenase (LDH) is a tetrameric
isoenzyme consisting of H and M subunits; H4
predominates in heart muscle, and M4 in
skeletal muscle
Pyruvate to Lactate
– while reduction to lactate allows glycolysis to
continue, it increases the concentration of
lactate and also of H+ in muscle tissue
C6 H1 2 O6
Glucose
lactate
fermentation
OH
2 CH3 CHCOO- + 2 H+
Lactate
– when blood lactate reaches about 0.4 mg/100
mL, muscle tissue becomes almost completely
exhausted
Pyruvate to Acetyl-CoA
– under aerobic conditions, pyruvate undergoes
oxidative decarboxylation
– the carboxylate group is converted to CO2
– the remaining two carbons are converted to the
acetyl group of acetyl CoA
oxidative
O
decarboxylation
CH 3 CCOO - + NA D + + CoA SH
Pyruvate
O
CH 3 CSCoA + CO2 + N A DH
Acetyl-CoA
Pyruvate to Acetyl-CoA
• Oxidative decarboxylation of pyruvate to acetylCoA is considerably more complex than the
previous equation suggests
• In addition to NAD+ (from the vitamin niacin) and
coenzyme A (from the vitamin pantothenic acid),
it also requires
– FAD (from the vitamin riboflavin)
– pyridoxal phosphate (from the vitamin pyridoxine)
– lipoic acid
Catabolism of Glycerol
• Glycerol enters glycolysis via
dihydroxyacetone phosphate
CH2 OH
CHOH
CH2 OH
Glycerol
ATP
ADP
NAD+
CH2 OH
CHOH
2-
CH2 OPO3
Glycerol
1-phosphate
NADH
CH2 OH
C=O
2-
CH2 OPO3
Dihydroxyacetone
phosphate
Fatty Acids and Energy
• Fatty acids in triglycerides are the principal
storage form of energy for most organisms
– hydrocarbon chains are a highly reduced form
of carbon
– the energy yield per gram of fatty acid oxidized
is greater than that per gram of carbohydrate
oxidized
Energy
Energy
-1
(kcal•mol ) (kcal•g-1)
C6 H1 2 O6 + 6 O2
Glucose
CH3 (CH2 ) 1 4 COOH + 2 3 O2
Palmitic acid
686
3.8
1 6 CO2 +1 6 H2 O 2,340
9.3
6 CO2 + 6 H2 O
Oxidation of Fatty Acids
• There are two major stages in the oxidation
of fatty acids
– activation of the free fatty acid in the cytoplasm
and its transport across the inner mitochondrial
membrane
b-oxidation
b-Oxidation
b-Oxidation: a series of five enzymecatalyzed reactions that cleaves carbon
atoms two at a time from the carboxyl end
of a fatty acid
– Reaction 1: the fatty acid is activated by
conversion to an acyl CoA; activation is
equivalent to the hydrolysis of two high-energy
phosphate anhydrides
O
R-CH2 -CH2 -C-OH + ATP + CoA-SH
A fatty acid
O
R-CH2 -CH2 -C-SCoA + AMP + 2 Pi
An acyl CoA
b-Oxidation
– Reaction 2: oxidation of the ,b carbon-carbon
single bond to a carbon-carbon double bond
acyl-CoA
O
b

dehydrogenase
R- CH 2 -CH2 - C-SCo A + FAD
O
An acyl-CoA
H
C-SCo A
+ FAD H2
C C
R
H
A trans enoyl-CoA
b-Oxidation
– Reaction 3: hydration of the double bond
O
OH
enoyl-CoA
H
C-SCo A
O
hydratase
C
+ H2 O
C C
R
CH2 -C- SCoA
H
R
H
A trans enoyl-CoA
An L-b-hydroxyacyl-CoA
– Reaction 4: oxidation of the 2alcohol to a
ketone
OH
C
H
O
CH2 -C-SCoA
R
b-Hydroxyacyl-CoA
+ NAD+
b-hydroxyacyl-CoA
dehydrogenase
O
O
R-C-CH2 -C-SCoA + NADH + H+
b-Ketoacyl-CoA
b-Oxidation
– Reaction 5: cleavage of the carbon chain by a
molecule of CoA-SH
O
O
R-C-CH2 -C-SCoA + CoA-SH
b-Ketoacyl-CoA
Coenzyme A
thiolase
O
O
R-C-SCoA + CH3 C-SCoA
An acyl-CoA
Acetyl-CoA
b-Oxidation
– this series of reactions is then repeated on the
shortened fatty acyl chain and continues until
the entire fatty acid chain is degraded to acetyl
CoA
eight cycles of
O
CH3 ( CH2 ) 1 6 C-SCoA +
Octadecanoyl-CoA
(Stearyl-CoA)
8 CoA-SH
+
8 NAD
8 FAD
b-oxidation
O
9 CH3 C-SCoA +
Acetyl-CoA
8 NADH
8 FADH2
b-oxidation of unsaturated fatty acids proceeds in
the same way, with an extra step that isomerizes
the cis double bond to a trans double bond
Ketone Bodies
• Ketone bodies: acetone, b-hydroxybutyrate,
and acetoacetate
– formed principally in liver mitochondria
– can be used as a fuel in most tissues and organs
• Formation occurs when the amount of
acetyl CoA produced is excessive compared
to the amount of oxaloacetate available to
react with it
– intake high in lipids and low in carbohydrates
– diabetes not suitably controlled
– starvation
Ketone Bodies
O
2 CH3 C-SCoA
Acetyl-CoA
HS-CoA
O
O
CH3 CCH2 C-SCoA
Acetoacetyl-CoA
O
NADH
-
CH3 -C-CH2 -COO
Acetoacetate
CO2
+
+
NAD + H
O
CH3 -C-CH3
Acetone
OH
CH3 -CH-CH2 -COO
b-Hydroxybutyrate