Transcript Document

29
Organic
Chemistry
William H. Brown &
Christopher S. Foote
29-1
29
The Organic
Chemistry of
Metabolism
Chapter 29
29-2
29 Introduction
 We
have now studied the typical reactions of the
major classes of organic compounds, and the
structure and reactions of carbohydrates and
lipids
 Now let us apply this background to the study of
the organic chemistry of metabolism
 We study two key metabolic pathways
• -oxidation of fatty acids
• glycolysis
29-3
29 Five Key Participants
 Five
of the compounds participating in these and
a great many other metabolic pathways:
• ATP, ADP, and AMP are universal carriers of phosphate
groups
• NAD+/NADH and FAD/FADH2 are coenzymes involved
in the oxidation/reduction of metabolic intermediates

Coenzyme: a low-molecular weight, nonprotein
molecule or ion that binds reversibly to an
enzyme, functions as a second substrate, and is
regenerated by further reaction
29-4
29 Adenosine Triphosphate
 ATP
is the most important compound involved in
the transfer of phosphate groups
phosphoric
ester group
N H2
N
adenine
N
O
O
O
N
O-P- O-P- O-P- O-CH2
N
O
O- O- OH
H
 - N-glycosidic bond
H
phosphoric
H
anhydride
HO
OH -D-ribofuranose
groups
29-5
29 Adenosine Triphosphate
• hydrolysis of the terminal phosphate of ATP gives ADP
and phosphate
• in glycolysis, the phosphate acceptors are -OH groups
of glucose and fructose
O
-
O
O
O- P- O- P-O- A MP + H2 O
O
O-
ATP
Water
(a phosphate
acceptor)
-
O- P-O- A MP + H2 PO 4 O
ADP
29-6
29 Nicotinamide adenine dinucleotide
• a biological oxidizing agent
O
CNH2
+
O
-
O-P-O-CH2
N
O
H
H
H
H
HO
O
OH
a -N-glycosidic bond
NH2
N
O=P-O-CH2
-
O
N
O
H
nicotinamide,
N
adenine
N
H
H
H
HO
OH
29-7
29 NAD+/NADH
 NAD+
is a two-electron oxidizing agent, and is
reduced to NADH
O
H O
CNH 2
H
CNH 2
+ H+ + 2 e :
N+
Ad
NAD+
(oxidized form)
N
Ad
NADH
(reduced form)
29-8
29 NAD+/NADH
• NAD+ is involved in a variety of enzyme-catalyzed
oxidation/reduction reactions; we deal with two of
these in this chapter
OH
O
C
C
H
A secondary
alcohol
A ketone
O
C
+
2 H+
+
2 e-
O
H
+
An aldehyde
H2 O
C
OH
+
2 H+
+
2 e-
A carboxylic
acid
29-9
29 NAD+/NADH
-:
H
B
e nzy m e
B
H
O
O
C
C
O
C-N H2
H
+
H
H
C-N H2
reduction
oxidation
O
:
N
:
H
e nzy m e
N
Ad
Ad
NAD+
NADH
29-10
29 FAD/FADH2
O
H3 C
N
H3 C
N
H
H
H
H
H
N
N
H
OH
OH
OH
flavin
O
ribitol
NH2
CH2
O
N
O
O=P-O-P-O-CH2
O-
O-
N
O
H
N
adenine
N
H
H
H
HO
OH
adenosine
29-11
29 FAD/FADH2
 FAD
participates in several types of enzymecatalyzed oxidation/reduction reactions
• our concern is its participation in the oxidation of a CC bond in a fatty acid hydrocarbon chain to a C=C
bond as shown in these balanced half-reactions
Oxidation of the hydrocarbon chain:
- CH 2 - CH2 - CH = CH - + 2 H+ + 2 e Reduction of FAD:
FAD + 2 H+ + 2 e -
FAD H2
29-12
29 FAD oxidation of C-C
Enz
Enz
B: H
H
C
R2
C
R1
H
H
H3 C
N
H3 C
N
B
H
H
R2
O
C
R1
C
H
H
N
N
H
O
Ad
H
B Enz
O
H3 C
N
H3 C
N
N
Ad
H
N
H
O
-:
B Enz
29-13
29 Fatty Acids and Energy
 Fatty
acids in triglycerides are the principle
storage form of energy for most organisms
• carbon chains are in a highly reduced form
• the energy yield per gram of fatty acid oxidized is
greater than that per gram of carbohydrate
Yield of Energy
(kJ/mol) (kJ/g)
C6 H1 2 O6 + 6 O 2
6 CO 2 + 6 H 2 O
-2870
-15.9
Glucose
CH3 ( CH2 ) 1 4 COOH + 2 3 O 2
1 6 CO 2 + 1 6 H 2 O
-9791
-38.9
Palmitic acid
29-14
29 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
• -oxidation
 -Oxidation:
a series of four enzyme-catalyzed
reactions that cleaves carbon atoms two at a
time, from the carboxyl end of a fatty acids
29-15
29 Activation of Fatty Acids
• activation begins in the cytoplasm with formation of a
thioester between the carboxyl group of the fatty acid
and the sulfhydryl group of coenzyme A
• formation of the acyl-CoA derivative is coupled with
the hydrolysis of ATP to AMP and pyrophosphate
O
R- C-O -
A TP
+
HS-CoA
Fatty acid Coenzyme A
(as anion)
A MP + P2 O 7 4 -
O
R- C-S- CoA
+
OH-
An acyl-CoA
derivative
29-16
29 -Oxidation
• Reaction 1: stereospecific oxidation of a carboncarbon single bond a, to the carbonyl group
O
O

a
R- CH2 -CH2 - C-SCo A + FAD
H
C-SCo A
C
+ FAD H2
C
R
H
A trans-enoyl-CoA
An acyl-CoA
• Reaction 2: regiospecific and stereospecific hydration
of the carbon-carbon double bond; only the Renantiomer is formed
O
H
OH
C-SCo A
C
C
R
H
A trans-enoyl-CoA
+ H2 O
C
O
H
CH2 -C- SCoA
R
(R)--Hydroxyacyl-CoA
29-17
29 -Oxidation
• Reaction 3: oxidation of the -hydroxyl group
OH
C
H
O
CH2 -C- SCoA
R
(R)--Hydroxyacyl-CoA
+
N AD +
O
O
R- C-CH2 - C- SCoA
+
N AD H
-Keto acyl-CoA
• Reaction 4: cleavage of the carbon chain by a reverse
Claisen condensation
O
O
R- C-CH2 - C- SCoA
-Ketoacyl-CoA
+ CoA -SH
Coenzyme A
O
R- C-SCo A
An acyl-CoA
+
O
CH3 C-SCo A
Acetyl-CoA
29-18
29 -Oxidation
• mechanism of the reverse Claisen condensation
O
O
R- C-CH2 - C- SCoA
-
S-Enz
O-
O
R- C-CH2 - C- SCoA
S- Enz
Tetrahedral carbonyl
addition intermediate
O-
O
R- C-S- Enz
An enzymethioester
+
CH2 = C-SCo A
Enolate anion of
acetyl-CoA
29-19
29 -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
O
CH3 ( CH2 ) 1 4 COH
Hexadecanoic acid
(Palmitic acid)
8 CoA -SH
+
A TP
A MP + P2 O 7 4 -
7 N AD +
7 FAD
O
8 CH3 CSCoA
+
Acetyl coenzyme A
7 N AD H
7 FAD H2
29-20
29 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 H1 2 O6
Glucose
glycolysis
2 CH3 CCOO ten enzymecatalyzed steps Pyruvate
+
6 H+
29-21
29 Glycolysis - Rexn 1
• phosphorylation of a-D-glucose
HO
HO
CH2 OH
O
O
+
OH
a-D-Glucose
-
O
hexokinase
O- P-O- P-O- AMP
Mg 2 +
O- O-
OH
ATP
CH2 OPO 3 2 O
HO
HO
O
+ - O- P-O- AMP
OH
a-D-Glucose 6-phosphate
OOH
ADP
29-22
29 Glycolysis - Rexn 2
• isomerization of glucose 6-phosphate to fructose 6phosphate
6
HO
HO
CH2 OPO 3 2 O
2
OH
phosphoglucoisomerase
1
OH
a-D-Glucose 6-phosphate
6
CH2 OPO 3 2 1
CH2 OH
O
H
HO
2
H
OH ( a)
H
HO
a-D-Fructose 6-phosphate
29-23
29 Glycolysis - Rexn 2
• this isomerization is most easily seen by considering
the open-chain forms of each monosaccharide
CHO
H C
OH
CH2 OH
C
C
HO
O
H
H
HO
OH
H
HO
OH
H
H
OH
H
OH
H
OH
H
OH
H
OH
H
OH
CH2 OPO 3 2 Glucose 6-phosphate
(an aldohexose phosphate)
CH2 OPO 3 2 An enediol
CH2 OPO 3 2 Fructose 6-phosphate
(a 2-ketohexose phosphate)
29-24
29 Glycolysis - Rexn 3
• phosphorylation of fructose 6-phosphate
HO
H
H
CH2 OH
CH2 OPO 3 2 -
C O
C O
H
OH
OH
+ A TP
CH2 OPO 3 2 Fructose 6-phosphate
phosphofructokinase
Mg
2+
HO
H
H
H
OH
OH
+ A DP
CH2 OPO 3 2 Fructose 1,6-bisphosphate
29-25
29 Glycolysis - Rexn 4
• cleavage of fructose 6-phosphate to two triose
phosphates
HO
H
H
CH2 OPO 3 2 -
CH2 OPO 3 2 -
C O
C= O
H
OH
OH
aldolase
CH2 OPO 3 2 Fructose 1,6-bisphosphate
CH2 OH
Dihydroxyacetone
phosphate
+
H C= O
H C OH
Glyceraldehyde
3-phosphate
CH2 OPO 3 2 -
29-26
29 Glycolysis - Rexn 4
• reaction 4 is a reverse aldol reaction
• a key intermediate is an imine formed by the C=O
group of fructose 1,6-bisphosphate and an -NH2 group
of the enzyme catalyzing this reaction
CH2 OPO 3 2 C O
HO
H
H
OH
H
+
+ H 3 N Enz
OH
CH2 OPO 3 2 -
Fructose 1,6-bisphosphate
: B-
( - H2 O)
CH2 OPO 3 2 +
C= NH Enz
HO
H
:BH
O H
H
OH
CH2 OPO 3 2 -
Protonated imine
29-27
29 Glycolysis - Rexn 4
• reverse aldol reaction gives two three-carbon
fragments, one as an imine
2-
H
OH
CH 2 OPO 3 2 -
Protonated imine
:
CH 2 OPO 3 2 +
C= NH Enz (enzyme-catalyzed
reverse aldol
HO
H
reaction)
:B H
O H
CH 2 OPO 3
C-N H
Enz
CHOH
B
+
H
H C= O
H C OH
CH2 OPO 3 2 Glyceraldehyde
3-phosphate
29-28
29 Glycolysis - Rexn 4
• hydrolysis of the imine gives dihydroxyacetone
phosphate and regenerates the -NH2 group of the
enzyme
CH2 OPO 3 2 -
:
C-N H
Enz
CHOH
B
H
CH2 OPO 3 2 -
+
C= NH
Enz
CH2 OH
:B-
Protonated imine
+ H2 O
CH2 OPO 3 2 C= O
CH2 OH
+
+ H 3 N Enz
:B-
Dihydroxyacetone
phosphate
29-29
29 Glycolysis - Rexn 5
• isomerization of triose phosphates
CH2 OH
C=O
CH2 OPO3 2 -
CHOH
C-OH
CH2 OPO3 2 -
Dihydroxyacetone
phosphate
An enediol
intermediate
CHO
H C OH
CH2 OPO3 2 Glyceraldehyde
3-phosphate
29-30
29 Glycolysis - Rexn 6
• oxidation of the -CHO group of glyceraldehyde 3phosphate
• the -CHO group is oxidized to a carboxyl group
• which is in turn converted to a carboxylic-phosphoric
mixed anhydride
• the oxidizing agent is NAD+, which is reduced to NADH
A two-electron oxidation
O
G- C- H + H2 O
A two-electron reduction
N AD + + H+ + 2 e -
O
G- C- OH + 2 H+ + 2 e -
N AD H
29-31
29 Glycolysis - Rexn 6
We divide this reaction into three stages
• stage 1: formation of a thiohemiacetal
O
OH
G- C- H + HS-Enz
G- C- S- Enz
H
A thiohemiacetal
• stage 2: oxidation of the thiohemiacetal by NAD+
H
O
G- C- S- Enz
O
G- C- S- Enz
O
H
H
H
CNH 2
an enzyme-bound
thioester
O
CNH 2
:
N+
N
Ad
Ad
29-32
29 Glycolysis - Rexn 6
• stage 3: conversion of the thioester to a mixed
anhydride
G- C- S- Enz
+
-:
O- P-OH
O-
:
O
O
O- O
G- C- O-P- OH
Enz- S
OO
O
G- C- O-P- OH
+
Enz -S: -
O1,3-Bisphosphoglycerate
(a mixed anhydride)
29-33
29 Glycolysis - Rexn 7
• transfer of a phosphate group from 1,3bisphosphoglycerate to ADP
O
C-OPO3 2 -
phosphoO
glycerate kinase
+ O-P-O-AMP
H C OH
Mg2 +
2O
CH2 OPO3
ADP
1,3-Bisphosphoglycerate
COOO O
+ O-P-O-P-O-AMP
H C OH
O- O2CH2 OPO3
ATP
3-Phosphoglycerate
29-34
29 Glycolysis - Rexn 8
• isomerization of 3-phosphoglycerate to 2phosphoglycerate
COOH C OH
CH2 OPO 3 2 3-Phosphoglycerate
phosphoglycerate COOmutase
H C OPO 3 2 CH2 OH
2-Phosphoglycerate
29-35
29 Glycolysis - Rexn 9
 dehydration
of 2-phosphoglycerate
COOH C OPO 3
CH2 OH
2-
enolase
Mg2+
2-Phosphoglycerate
COOC OPO 3 2 - + H2 O
CH2
Phosphoenolpyruvate
29-36
29 Glycolysis - Rexn 10
• phosphate transfer to ADP
-
pyruvate kinase
Mg2+
COO
C OPO 3 2 CH2
ADP
Phosphoenolpyruvate
ATP
COO-
COO-
C-OH
C= O
CH2
CH3
Enol of
pyruvate
Pyruvate
29-37
29 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
glycolysis
Glucose
O
2 CH3 CCOO - + 2 NADH + 2 ATP + 2 H 2 O
Pyruvate
29-38
29 Fates of Pyruvate
 Pyruvate
does not accumulate in cells, but rather
undergoes one of three enzyme-catalyzed
reactions, depending of the type of cell and its
state of oxygenation
• reduction to lactate
• reduction to ethanol
• oxidation and decarboxylation to acetyl-CoA
A
key to understanding the biochemical logic
behind two of these fates is to recognize that
glycolysis needs a continuing supply of NAD+
• if no oxygen is present to reoxidize NADH to NAD+,
then another way must be found to do it
29-39
29 Lactate Fermentation
• in vertebrates under anaerobic conditions, the most
important pathway for the regeneration of NAD+ is
reduction of pyruvate to lactate
O
CH3 CCOO- + NADH + H3 O+
Pyruvate
lactate
dehydrogenase
OH
CH3 CHCOO- + NAD+ + H2 O
Lactate
29-40
29 Pyruvate to Lactate
• while lactate fermentation does allow glycolysis to
continue, it increases the concentration of lactate and
also of H+ in muscle tissue, as seen in this balanced
half-reaction
C6 H1 2 O6 + 2 H2 O
Glucose
lactate
fermentation
OH
2 CH3 CHCOO- + 2 H3 O+
Lactate
• when blood lactate reaches about 0.4 mg/100 mL,
muscle tissue becomes almost completely exhausted
29-41
29 Pyruvate to Ethanol
 Yeasts
and several other organisms regenerate
NAD+ by this two step pathway
• decarboxylation of pyruvate to acetaldehyde
pyruvate
O
decarboxylase
CH3 CCO2 - + H+
Pyruvate
O
CH3 CH + CO 2
Acetaldehyde
• reduction of acetaldehyde to ethanol
alcohol
O
dehydrogenase
+
CH3 CH + N AD H + H
CH3 CH2 OH + NA D +
Acetaldehyde
Ethanol
29-42
29 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
CH3 CCOO- + NAD+ + CoASH
Pyruvate
O
CH3 CSCoA + CO2 + NADH
Acetyl-CoA
29-43
29 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
29-44
29 Pyruvate to Acetyl-CoA
NH2
N
N
S
O O
O-P-O-P-OO- O-
N
Thiamine pyrophosphate
H
COO-
S S
Lipoic acid
(as the carboxylate anion)
29-45
29 Prob 29.21
Describe the type of reaction involved in this biochemical
synthesis of -hydroxybutyrate.
O
2 CH3 CSCoA
Acetyl-CoA
CoA-SH
O
O
CH3 CSCoA CoA-SH
O
SCoA
Acetoacetyl-CoA
1
2
CO 2
O
OH 3 C OH O
-
SCoA
(S)-3-Hydroxy-3methylglutaryl-CoA
O
CH3 CSCoA
O
O
H+
4
-
3
O
Acetoacetate
NADH
5
NAD+
O
Acetone
OH O
O-Hydroxybutyrate
29-46
29
The Organic
Chemistry of
Metabolism
End Chapter 29
29-47