Download 20 - Wiley

20
Introduction to
Organic
Chemistry
2 ed
William H. Brown
20-1
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20
The Organic
Chemistry of
Metabolism
Chapter 20
20-2
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20 Introduction
• We have now studied the typical reactions of the
major classes of organic compounds and the
structure and reactions of carbohydrates and
lipids
• We now apply this background to the study of
the organic chemistry of metabolism
• b-oxidation of fatty acids
• glycolysis
20-3
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20 Five Key Participants
• Five compounds participating in these and a
great many other metabolic pathways are:
• ATP, ADP, and AMP are universal carriers of phosphate
groups
• NAD+/NADH and FAD/FADH2 are coenzymes involved
in oxidation/reduction of metabolic intermediates
• Coenzyme: a low-MW, nonprotein molecule or
ion that binds reversibly to an enzyme, functions
as a second substrate, and is regenerated by
further reaction
20-4
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20 Adenoside Triphosphate
• ATP is the most important of the compounds
involved in the transfer of phosphate groups
phosphoric
ester group
O
-
O
O
-
N
N
O
O- P - O- P - O- P - O- CH2
-
O
O
-
N
O
H
H
H
phosphoric
anhydride
groups
HO
Adenine
NH2
OH
N
a b-N-glycoside
bond
H b-D-ribofuranose
Adenosine
20-5
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20 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- P - O- P - O- A MP
OOAdenosine triphosphate
(ATP)
+ H2 O
Phosphate
acceptor O
- O- P - O- A MP
+ H2 P O4 -
OAdenosine diphosphate
(ADP)
20-6
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20 NAD+/NADH
• Nicotinamide adenine dinucleotide (NAD+) is a
biological oxidizing agent
The plus sign on NAD+
represents the positive
charge on this nitrogen
O
CNH2
O
-
O- P - O- CH2
A MP
N+
O
H
O
H
H
H
HO
Nicotinamide,
derived
from niacin;
a b-N-glycoside
bond
OH
20-7
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20 NAD+/NADH
• NAD+ is a two-electron oxidizing agent, and is reduced
to NADH
O
CNH2
N+
Ad
+ H+ + 2 e -
NAD +
(oxidized form)
H O
CNH2
H
N
Ad
NADH
(reduced form)
20-8
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20 NAD+/NADH
• we will discuss two types of oxidations involving NAD+
OH
O
C
C
H
A secondary
alcohol
+ 2e-
A ketone
O
C H +
An aldehyde
+ 2 H+
O
H2 O
C OH +
A carboxylic
acid
2 H+
+
2e-
20-9
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20 Oxidation by NAD+
-
B
E
H
H
O
O
C
C
H
H
O
CNH2
N+
reduction
oxidation
Ad
NAD +
B
E
H H O
CNH2
N
An electron
pair is added
to nitrogen
Ad
NADH
20-10
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20 FAD/FADH2
• Flavin adenine
dinucleotide
(FAD) is also a
biological
oxidizing agent
Riboflavin
20-11
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20 FAD/FADH2
• we discuss one type of oxidation involving FAD,
namely oxidation of the hydrocarbon chain of a fatty
acid
Oxidation of the hydrocarbon chain:
- CH2 - CH2 - CH= CH- + 2 H+ + 2 e Reduction of FAD:
FA D + 2 H+ + 2 e -
FA DH2
20-12
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20 Oxidation by FAD
the hydrocarbon
chain of the fatty acid
E
B
-
H
C
C
H
R2
R1
H
H
O
H3 C
N
H3 C
N
FAD
Ad
N
N
H
O
H
B
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
E
20-13
20 Oxidation by FAD
A trans carbon-carbon
double bond
E
B
H
H
R2
C
H3 C
H3 C
R1
C
H
H O
N
N
N
Ad
H
FADH 2
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
N
H
O
-
B
E
20-14
20 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
Energy
(kcal/g)
C6 H12 O6 + 6 O2
Glucose
CH3 (CH 2 ) 14 CO 2 H + 23 O2
Palmitic acid
6 CO 2 + 6 H2 O
-3.8
16 CO2 + 16 H2 O -9.3
20-15
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20 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: a series of four enzyme-catalyzed
reactions that cleaves carbon atoms two at a
time, from the carboxyl end of a fatty acids
20-16
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20 Activation of Fatty Acids
• Begins in the cytoplasm with formation of a
thioester
• formation of the thioester is coupled with the
hydrolysis of ATP to AMP and pyrophosphate
O
A TP
R- C- O- + HS - Co A
Fatty acid
Coenzyme A
(as anion)
A MP + P 2 O7 4 O
R- C- S - Co A + OH An acyl-CoA
derivative
20-17
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20 Activation of Fatty Acids
• activation involves reaction with ATP
O
O
O
R- C- O- + A d - O- P - O- P - O- P - O
OO- OFatty acid
ATP
(as anion)
O
intermediate
with one
phosphorus
bonded to
five groups
O
O
O
O
R- C- O- P - O- Ad + - O- P - O- P - OOOOAn acyl-AMP
Pyrophosphate
(a mixed anhydride)
20-18
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20 Activation of Fatty Acids
• and then reaction with coenzyme A
O
O
Co A - S H + R- C- O- P- O- Ad
Coenzyme A
OAn acyl-AMP
O
tetrahedral
carbonyl
addition
intermediate
O
R- C- S - Co A + O- P - O- Ad
OAn acyl-CoA
AMP
20-19
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20 b-Oxidation
• Reaction 1: oxidation of a carbon-carbon single bond
to a carbon-carbon double bond
O
b

R- CH2 -CH2 - C-S Co A + FAD
An acyl-CoA
H
O
C- S Co A
C C
+
FA DH2
R
H
A trans- enoyl-CoA
20-20
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20 b-Oxidation
• Reaction 2: hydration of the carbon-carbon double
bond; only the R-enantiomer is formed
O
H
OH
C- S Co A
C C
+
H2 O
R
H
A trans- enoyl-CoA
C
O
CH2 - C- S Co A
H
R
(R)-b-Hydroxyacyl-CoA
20-21
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20 b-Oxidation
• Reaction 3: oxidation of the b-hydroxyl group to a
carbonyl group
OH
C
H
O
CH2 - C- S Co A +
R
(R)-b-Hydroxyacyl-CoA
NAD+
O
O
R- C- CH2 - C- S Co A + NA DH
b-Ketoacyl-CoA
20-22
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20 b-Oxidation
• Reaction 4: cleavage of the carbon chain by a reverse
Claisen condensation
O
O
R- C- CH2 - C- S Co A
+ Co A - S H
b-Keto acyl-CoA
Coenzyme A
O
O
R- C- S Co A + CH3 C- S Co A
An acyl-CoA
Acetyl-CoA
20-23
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20 b-Oxidation
• mechanism of the reverse Claisen condensation
O
O
R- C- CH2 - C- S Co A
-
S - Enz
O-
O
R- C- CH2 - C- S Co A
S - En z
Tetrahedral carbonyl
addition intermediate
O
O-
R- C- S - Enz + CH2 = C- S Co A
An enzyme
Enolate anion of
thioester
acetyl-CoA
20-24
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20 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
O
CH3 ( CH2 ) 1 4 COH +
Hexadecanoic acid
(Palmitic acid)
8 Co A - S H A T P
7 NA D +
7 FA D
A MP + P 2 O7 4 -
O
8 CH3 CS Co A +
Acetyl coenzyme A
7 NA DH
7 FA DH2
20-25
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20 Glycolysis
• Glycolysis: a series of 10 enzyme-catalyzed
reactions by which glucose is oxidized to two
molecules of pyruvate
C6 H1 2 O6
Glucose
glycolysis
ten enzymecatalyzed steps
O
2 CH3 CCO2 - + 2 H+
Pyruvate
• glycolysis is a 4-electron oxidation; it occurs in two
separate 2-electron oxidation steps
O
glycolysis
C 6 H1 2 O6
2 CH3 CCO2 - + 6 H+ + 4 e Glucose
Pyruvate
20-26
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20 Glycolysis - Rexn 1
• Reaction 1: phosphorylation of -D-glucose
HO
HO
CH2 OH
O
O
+
O
- O- P - O- P- O- A MP
OH
OH
-D-Glucose
HO
HO
O-
hexokinase
Mg 2 +
OATP
CH2 OPO3 2 O
O
+
OH
OH
-D-Glucose 6-phosphate
-
O- P - O- A MP
OADP
20-27
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20 Glycolysis - Rexn 2
• Reaction 2: isomerization of glucose 6phosphate to fructose 6-phosphate
6
HO
HO
CH2 OP O3 2 O
2
OH
phosphoglucoisomerase
1
OH
-D-Glucose 6-phosphate
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
6
CH2 OP O3 2 - 1
CH2 OH
O
H HO
2
H
OH ( )
HO
H
-D-Fructose 6-phosphate
20-28
20 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
CH2 OP O3 2 Glucose 6phosphate
1 CH OH
2
2C O
H C OH
C OH
HO
H
H
OH
H
OH
CH2 OP O3 2 (An enediol)
HO
H
H
H
OH
OH
CH2 OP O3 2 Fructose 6phosphate
20-29
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20 Glycolysis - Rexn 3
• Reaction 3: phosphorylation of fructose 6phosphate
CH2 OH
CH2 OP O3 2 -
C O
C O
HO
H
H
OH
H
OH
+
ATP
CH2 OP O3 2 Fructose 6phosphate
phosphofructokinase
Mg 2 +
HO
H
H
OH
H
OH
CH2 OP O3 2 -
Fructose 1,6bisphosphate
20-30
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20 Glycolysis - Rexn 4
• Reaction 4: cleavage of fructose 1,6bisphosphate to two triose phosphates
a carbonyl
group
CH2 OPO 3 2 -
Dihydroxyacetone
phosphate
CH2 OPO 3 2 C= O
C= O
CH2 OH
HO
H
aldolase
+
H
OH
a b-hydroxyl H
H C= O
OH
group
H C OH
CH2 OPO 3 2 CH2 OPO 3 2 Fructose 1,6Glyceraldehyde
bisphosphate
3-phosphate 20-31
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20 Glycolysis - Rexn 4
• reaction 4 is a reverse aldol reaction
• an 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= NH En z
CH2 OPO 3 2 C= O
HO
H
H
+
+ H3 N
H
O- H
OH
CH2 OPO 3 2 Fructose 1,6bisphosphate
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
En z
B
-
( - H2 O)
HO
H
H
H
BO H
OH
CH2 OPO 3 2 Protonated imine
20-32
20 Glycolysis - Rexn 4
• reverse aldol reaction gives two three-carbon
fragments, one as an imine
CH2 OPO 3 2 +
En z
C= NH
BHO
H
H
O H
H
OH
CH2 OPO 3 2 (enzyme-catalyzed
reverse aldol
reaction)
CH2 OPO 3 2 Protonated imine
C- NH
En z
CHOH B
H C= O H
H C OH
CH2 OPO 3 2 Glyceraldehyde
3-phosphate
20-33
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20 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
CH2 OPO 3 2 +
C= N H Enz
En z
CHOH B
H
H2 O
CH2 OH B
Protonated imine
CH2 OPO 3 2 C= O
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
+
+ H3 N
En z
CH2 OH
B-
Dihydroxyacetone
phosphate
20-34
20 Glycolysis - Rexn 5
• Reaction 5: isomerization of triose phosphates
CH2 OH
CHOH
CHO
C= O
C- OH
H C OH
CH2 OPO 3 2 -
CH2 OPO 3 2 -
Dihydroxyacetone
phosphate
An enediol
intermediate
CH2 OPO 3 2 Glyceraldehyde
3-phosphate
20-35
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20 Glycolysis - Rexn 6
• Reaction 6: oxidation of the -CHO group of
glyceraldehyde 3-phosphate
• the -CHO group is oxidized to a carboxyl group
• which is in turn converted to a carboxylic-phosphoric
mixed anhydride
• the oxidizing agent, NAD+, is reduced to NADH
A two-electron oxidation
O
G- C- H + H2 O
A two-electron reduction
NA D + + H+ + 2 e -
O
G- C- OH + 2 H+ + 2 e NADH
20-36
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20 Glycolysis - Rexn 6
• We divide this reaction into three steps
• step 1: formation of a thiohemiacetal
O
G- C- H +
Glyceraldehyde
3-phosphate
OH
HS - Enz
G- C- S - En z
H
A thiohemiacetal
20-37
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20 Glycolysis - Rexn 6
• step 2: oxidation of the thiohemiacetal by NAD+
H
O
O
G- C- S -En z
G- C- S- Enz
O
H
an enzyme-bound
thioester
CNH 2
N+
Ad
H H O
CN H2
N
Ad
20-38
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20 Glycolysis - Rexn 6
• step 3: conversion of the thioester to a mixed
anhydride
O
O
G- C- S - En z +
-
O- P-OH
O-
O- O
G- C- O- P- OH
En z - S
OA tetrahedral
carbonyl addition
intermediate
O
O
G- C- O- P- O -
+ En z - S -
O1,3-Bisphosphoglycerate
(a mixed anhydride)
20-39
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20 Glycolysis - Rexn 7
• Reaction 7: transfer of a phosphate group from
1,3-bisphosphoglycerate to ADP
O
C- OPO 3 2 H C OH
phosphoglycerate kinase
O
+
CH2 OPO 3 2 1,3-Bisphosphoglycerate
-
O- P-O- AMP
O-
Mg 2+
ADP
CO 2 H C OH
O
+
2-
CH2 OPO 3
3-Phosphoglycerate
-
O
O- P-O- P- O- AMP
O-
OA TP
20-40
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20 Glycolysis - Rexn 8
• Reaction 8: isomerization of 3-phosphoglycerate
to 2-phosphoglycerate
CO 2 H C OH
phosphoglycerate
mutase
CH 2 OPO 3 23-Phosphoglycerate
CO 2 2H C OPO 3
CH 2 OH
2-Phosphoglycerate
20-41
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20 Glycolysis - Rexn 9
• Reaction 9: dehydration of 2-phosphoglycerate
CO 2 H C OPO 3
2-
enolase
Mg 2+
CH2 OH
2-Phosphoglycerate
CO 2 C OPO 3 2-
+
H2 O
CH2
Phosphoenolpyruvate
20-42
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20 Glycolysis - Rexn 10
• Reaction 10: phosphate transfer to ADP
• stage 1: transfer of the phosphate group
CO 2 C OPO 3 2 - +
O
-
O- P-O- AMP
CH2
Phosphoenolpyruvate
OADP
CO 2 C- OH +
CH2
Enol of
pyruvate
pyruvate
kinase
Mg 2+
O
-
O
O- P-O- P-O- AMP
O - OATP
20-43
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20 Glycolysis - Rexn 10
• stage 2: enolization to pyruvate
CO 2 -
CO 2 -
C-OH
C=O
CH2
CH3
Enol of
pyruvate
Pyruvate
20-44
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20 Glycolysis
• Summing these 10 reactions gives the net
equation for glycolysis
+
C6 H1 2 O6 + 2 NA D + 2 HPO 4
Glucose
2-
+ 2 A DP
glycolysis
O
2 CH3 CCO2 - + 2 NADH +
Pyruvate
2 ATP + 2 H 2 O + 2 H +
20-45
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20 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
20-46
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20 Lactate Fermentation
• In vertebrates under anaerobic conditions, the
most important pathway for the regeneration of
NAD+ is reduction of pyruvate to lactate
O
CH3 CCO2 - + NA D H + H +
Pyruvate
lactate
dehydrogenase
OH
CH3 CHCO 2 - + NA D +
Lactate
20-47
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20 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
C 6 H 1 2 O6
Glucose
lactate
fermentation
OH
2 CH3 CHCO 2 - +
Lactate
2 H+
• when blood lactate reaches about 0.4 mg/100 mL,
muscle tissue becomes almost completely exhausted
20-48
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20 Pyruvate to Ethanol
• Yeasts and several other organisms regenerate
NAD+ by this two-step pathway
• decarboxylation of pyruvate to acetaldehyde
O
CH3 CCO2 Pyruvate
+
H+
pyruvate
decarboxylase
O
CH3 CH +
Acetaldehyde
CO 2
• reduction of acetaldehyde to ethanol
alcohol
O
+ dehydrogenase
CH3 CH + NA DH + H
Acetaldehyde
CH3 CH2 OH + NA D +
Ethanol
20-49
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20 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 CCO2 + NAD + Co A S H
Pyruvate
O
CH3 CS Co A + CO2 + NA DH
Acetyl-CoA
20-50
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20 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)
• thiamine pyrophosphate (from thiamine, B1)
• lipoic acid
20-51
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20 Pyruvate to Acetyl-CoA
O
NH2
CH2 CH2 O- P- O- P- O OOS
H3 C
N
H3 C
O
H
Thiamine pyrophosphate
CH2 CH2 CH2 CH2 CO2 S
S
Lipoic acid
(shown as the
carboxylate anion)
H
20-52
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
20
The Organic
Chemistry of
Metabolism
End Chapter 20
20-53
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.