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Chapter 15
Metabolism: Basic Concepts
and Design
The tiny ruby-throated hummingbird can
store enough fuel and converts fuels into
the cellular energy currency, ATP.
Fly 500 miles across Gulf of Mexico
without resting!
1
Outline
15.1 Metabolism is composed of many coupled,
interconnecting reactions
15.2 ATP Is the universal currency of free energy in
biological systems
15.3 The oxidation of carbon fuels is an important
source of cellular energy
15.4 Metabolic pathways contain many recurring
motifs
2
All Cells Transform Energy :
• Cells extract energy from their environment and
use this energy to convert simple molecules into
cellular components
Metabolism :
• A highly integrated network of chemical pathways
that enables a cell to extract energy from the
environment and use this energy for biosynthetic
purposes.
3
Metabolism answers the
questions
• How does a cell extract energy and reducing power
from its environment?
• How does a cell synthesize the building blocks of its
macromolecules and then the macromolecules
themselves ?
4
General Principles and motifs of
metabolism
1. Metabolic pathways : step by step
2. Common energy currency: ATP, links energyreleasing pathways with energy-requiring
pathways
3. Oxidation of carbon fuels powers the formation
of ATP
4. Unifying themes : a limited number of types of
reactions and particular intermediates are
common to many pathways
5. Metabolic pathways are highly regulated
5
15.1 Metabolism Is Composed of Many
Coupled, Interconnecting Reactions
• Why Do Organisms Need Energy?
– Performance of mechanical work in muscle contraction
and cellular movements
– Active transport of molecules and ions
– Synthesis of macromolecules and other biomolecules
from simple precusors
6
15.1 Metabolism Is Composed of Many
Coupled, Interconnecting Reactions
•Phototrophs or photosynthetic organisms (光合有機體):
– Organisms that obtain energy by trapping sunlight and
convert light energy into chemical energy
• Chemotrophs (化能生物)
– Organisms that obtain energy by the oxidation of
foodstuffs generated by phototrophs
7
Metabolism consists of energy-yielding
and energy-requiring reactions
•Metabolism
–a linked series of chemical reactions that begins with
a particular molecule and converts it into some
other molecule or molecules in a carefully defined
fashion
–Many such defined pathways in the cell
•These pathways are interdependent
•Their activity is coordinated by allosteric enzyme
8
無氧
有氧
CO2
Fig 15.1 Glucose metabolism
9
Fig 15.1 Metabolic pathways
10
• Metabolic pathways:
– Catabolism (分解代謝) or catabolic reaction
• Convert energy from fuels into cellular energy
– Anabolism (合成代謝) or anabolic reactions
• Require energy to synthesizes molecules from simpler
precursors
catabolism
Fuel (carbohydrate, fats)
CO2 + H2O + useful energy
Useful energy + simple precursor
anabolism
Complex molecules
 Amphibolic pathways: some pathways can be either anabolic
or catabolic, depending on the energy conditions in the cells
 General principle of metabolism is that biosynthetic and
degradative pathways are almost always distinct
11
Catabolism and Anabolism
氧化、放熱
還原、吸熱
NADH
Fig. 17.6 in Garrett and Grisham, Biochemistry, 4th edition (2009)
12
A Thermodynamically Unfavorable Reaction Can
Be Driven by a Favorable Reaction
• A pathway must satisfy minimally two criteria:
(1) Individual reactions must be specific
• Yield only one particular product or sets of products
(2) Entire set of reactions that constitute the pathway must
be thermodynamically favored
• Free energy (ΔG) is negative
13
Free-energy change: Spontaneity
(自發性) but not the rate of reaction
 ΔG tells us if the reaction can occur spontaneously:
1. If G is negative, reaction spontaneous, exergonic
2. If G is zero, no net change, system at equilibrium
3. If G is positive, free energy input required, endergonic
 G of a reaction depends only on free-energy of
products minus free-energy of reactants
1. G of a reaction is independent of path (or molecular
mechanism) of the transformation
2. G provides no information about the rate of a reaction
----The rate of a reaction depends on the free energy
of activation (G‡ 活化能), which is largely
unrelated to the G of the reaction----
14
15
Standard Free-energy change of a reaction
is related to the equilibrium
(化學反應標準自由能變化與平衡常數有關, Go’ of a reaction is related to K’eq)
•To determine G of an enzyme catalyzed reaction , must consider
nature of both reactants and products as well as their
concentrations
Consider this reaction
A+BC+D
G is given by
G = Go + RTln([C][D]/[A][B])
(1)
G change in free energy
Go is standard free-energy change (Go’ : Go at pH7)
R is the gas constant,
T is the absolute temperature
[C][D]/[A][B] are the molar concentrations of the reactants
G of a reaction depends on the nature of the reactants
16
(Go ) and on their concentrations
For biochemical reactions:
• At equilibrium, G = 0
and so
0 = Go’ + RTln([C][D]/[A][B])
Go’ = - RTln([C][D]/[A][B])
(2)
(3)
•Equilibrium constant under standard conditions, K’eq, is defined as
K’eq = [C][D]/[A][B]
(4)
Substituting equation 4 into equation 3 gives
Go’ = - RTlnK’eq
K’eq =10 -Go’/RT
(5)
(6)
R = 1.987x10-3 kcal mol-1 deg-1 and T = 298K (=250C)
K’eq =10 -Go’/2.47
(7)
17
A Thermodynamically Unfavorable Reaction
Can Be Driven by a Favorable Reaction
• Free energy changes are additive: Overall ΔG for a
series of reactions is equal to the sum of ΔG of each
individual reaction
A  B + C ΔG o’ = +21kJ/mol
BD
ΔG o’ = -34kJ/mol
A  C + D ΔG o’ = -13kJ/mol
A thermodynamically unfavorable reaction can be
driven by a thermodynamically favorable reaction to
which it is coupled
18
15.2 ATP Is the universal currency of free
energy in biological system
ATP (adenosine triphosphate)
– Acts as the free-energy donor in most energy-requiring
process
• Motion
• Active transport
• Biosynthesis
• Most of catabolism consists of reactions that extract
from fuels and convert it into ATP
19
ATP structure
•ATP is a nucleotide consisting of an adenine, a ribose, and
a triphosphate unit
•The active form of ATP is usually a complex of ATP with
Mg2+ or Mn2+
•ATP is an energy-rich molecule
–its triphosphate unit contains two phosphoanhydride bonds
–ATP hydrolysis is exergonic (ΔG < 0)
•ATP + H2O  ADP + Pi ΔGo’ = -30.5kJ/mol
•ATP + H2O  AMP + Pii ΔGo’ = -45.6kJ/mol
triphosphate unit
Anhydride bond
adenine
The precise ΔGo’ of ATP hydrolysis
depends on:
• Ionic strength of the medium
• The concentration of Mg2+ or
other metal ions
[ MgADP  ]  [ Pi ]
20
G  G '  RT ln
2
[ MgATP ]
0
Actual ΔG of ATP hydrolysis in human
erythrocyte
•pH 7, temperature 37 oC
**ATP is formed from ADP and Pi ATP-ADP cycle
The fundamental mode of energy exchange in biological systems
21
Active form of ATP
• A complex of ATP with Mg2+ or Mn2 +
– True substrate for the enzyme
22
The role of ATP in energy metabolism is
paramount
• Some biosynthetic reactions are drive from GTP, UTP
or CTP
– Nucleoside monophosphate kinase catalyzed the
phosphorylation
– All the nucleotide triphosphates are energetically
equivalent
• NAD+ and FAD (electron carriers) are derivatives of
ATP
NMP
Nucleoside
monophosphate
NDP
Nucleoside
diphosphate
Nucleoside monophosphate
kinase
+ ATP
NDP
Nucleoside diphosphate
kinase
+ ATP
NTP
+ ADP
+ ADP
23
ATP Hydrolysis Drives Metabolism by Shifting the
Equilibrium of Coupled Reactions
•A thermodynamically unfavorable reaction can be
driven by a thermodynamically favorable reaction to
which it is coupled reaction
24
ATP Hydrolysis Drives Metabolism by Shifting the
Equilibrium of Coupled Reactions
– Coupling the hydrolysis of ATP with the conversion of A into B under standard
conditions has changed equilibrium ratio of B to A by a factor of about 105.
– If used the energy of ATP hydrolysis under cellular conditions, ratio of [B] to [A]
changes by about 108 (if using -50.2 kJ mol-1).
–ATP’s action as an energy-coupling
agent
25
A + ATP + H2O  B + ADP + Pi
•The hydrolysis of n ATP molecules changes the equilibrium
ratio of a couple reaction by a factor of 108n
•A thermodynamically unfavorable reaction sequence can
be converted into a favorable one by coupling it to the
hydrolysis of a sufficient number of ATP molecules in a new
reaction
•A and B may represent activated and unactivated
conformations of a protein
• Activated by phosphorylation with ATP
• Such as: Muscle contraction
•A and B may refer to the concentrations of an ion or
molecule on the outside and inside of a cell
• Active transport of Na+ and K+ across membrane
26
ATP has a higher phosphoryl transfer potential
(phosphoryl-group transfer potential)
ATP + H2O  ADP + Pi + H+
∆Go’=-30kJ mol-1 (-7.3kcalmol-1)
Glycerol 3-phosphate + H2O  glycerol + Pi
∆Go’=-9.2kJ mol-1 (-2.2kcalmol-1)
•ATP has a stronger tendency to transfer its terminal
phosphoryl group to water than does glycerol 3phosphate
•ATP has a higher phosphoryl transfer potential
(phosphoryl-group transfer potential) than does
glycerol 3-phosphate.
27
Higher phosphoryl transfer potential of ATP
due to ATP structure
1. Resonance Stabilization
– ADP and Pi, have greater resonance stabilization than
does ATP
2.Electrostatic repulsion :
– At physiological pH 7 , charge on ATP is -4
– Strong electrostatic repulsion between them
– Repulsion is reduced when ATP is hydrolyzed
3.Stabilization due to hydration:
– More water can surround ADP and Pi than ATP,
increasing stabilization by hydration
Fig 15.5 Improbable resonance structure :
28
Fig 15.4 Resonance structures of orthophosphate terminal part of ATP- γ phosphate group
ATP is not the only compound with a
high phosphoryl-transfer potential
29
Creatine Phosphate
• Creatine phosphate in vertebrate muscle serves as
a reservoir of high-potential phosphoryl groups
that can be readily transferred to ATP
Creatine kinase
Creatine phosphate + ADP  ATP + creatine
ΔG°’ = -12.6 kJ/mol
Keq = 162
30
Sources of ATP during exercise
Fig 15.7 Source of ATP during exercise
•ATP stored in muscle can sustain activity for < 1 sec
•Creatine phosphate also stored in muscle
– major source of ATP regeneration for next several seconds
• Initially exercise is powered by existing ATP and creatine phosphate
•Subsequently the ATP must be generated by metabolic pathways
31
15.3 The oxidation of carbon fuels is an
important source of cellular energy
•ATP is an immediate donor of free
energy, not a long-term storage form
of free energy (ATP提供能量但非儲存能量分子)
–ATP is consumed within ~1 min of its
formation (Total ATP ~100 g
–turnover is very high
•In 24 hours, a resting human turns over
40 kg ATP
•During exercise, turnover up to 0.5 kg
ATP/min
•Regenerating ATP
–Oxidation of fuel molecule
–Electrons are used to regenerate from
ADP and Pi.
Fig 15.18 ATP-ADP cycle
32
General Principles and motifs of
metabolism
1. Metabolic pathways : step by step
2. Common energy currency: ATP, links energy-releasing
pathways with energy-requiring pathways
3. Oxidation of carbon fuels powers the formation of
ATP
4. Unifying themes : a limited number of types of
reactions and particular intermediates are
common to many pathways
5. Metabolic pathways are highly regulated
33
Free Energy of Oxidation of Single-Carbon
Compounds
•In aerobic organisms, the ultimate electron acceptor in the
oxidation of carbon is O2 and the oxidation product is CO2
•The more reduced a carbon is to begin with, the more free
energy is release by its oxidation
O 陰電性 > C陰電性
C 陰電性 > H陰電性
most oxidized
most reduced
e-
8
6
4
2
0
•Oxidation: loss of electrons Fig 15.9 free energy of oxidation of
34
Reduction: gain of electrons single-carbon compounds
•Fats are a more efficient fuel source than
carbohydrates such as glucose because the carbon in
fats is more reduced
–A fuel molecules: oxidation takes place one carbon at a time
–Fatty acid is more reduced than glucose and can produce
more energy
•Carbon oxidation energy is used to create
–Compounds with high phosphoryl-transfer potential
–Ion gradient
•End point is formation of ATP
Fig 15.10 Prominent fuels
35
High Phosphoryl transfer potential compounds can couple
carbon oxidation to ATP synthesis
•Energy released in the oxidation of a carbon compounds is
converted into ATP
•Example GAP (G-3-P) a metabolite of glucose
aldehyde
acid
+ energy
(GAP)
-however the oxidation does not take place directly
36
high phosphoryl transfer potential
•The carbon oxidation generates an acyl phosphate, 1,3bisphosphoglycerate
•The electron released are captured by NAD+.
37
Compounds with high phosphoryl-transfer potential can
couple carbon oxidation to ATP synthesis
• The cleavage of 1,3-BPG can be coupled to synthesis of ATP
• The energy of oxidation is initially trapped as a high-energy
phosphate compound (1,3-bisphosphoglycerate) and then used to
form ATP
38
Ion gradients across membranes provide an
important form of cellular energy that can be
coupled to ATP synthesis
•Electrochemical potential of
ion gradients across
membranes, produced by the
oxidation of fuel molecule or
by photo-synthesis, is the most
common way to form ATP
•In animals, proton gradients
account for > 90% ATP
generation, called oxidative
phosphorylation
Fig 15.11 Proton gradient
39
Energy from foodstuffs is
extracted in three stage
•First stage (preparation)
– Large molecules in food are
broken down into smaller units
digestion
– Absorbed by intestine cells and
throughout the body
– No useful energy is captured
•Second stage
– The small molecules are
degraded to simple units that
play a central role in metabolism
– Convert into acetyl CoA
– Some ATP is generated
•Third stage
– ATP is produced from the
complete oxidation of acetyl CoA
– Citric acid cycle
– Oxidative phosphorylation
Fig 15.12 Stages of catabolism gradient40
• In stage III
– Acetyl CoA brings acetyl units into the citric acid cycle
– then oxidized to CO2
• 3 e- pairs transferred to NAD+, 1 e- pair transferred to FAD
for each acetyl group
– proton gradient is generated as electron flow from
NADH and FADH2 to O2, gradient used to make ATP
(oxidative phosphorylation)
Generating high-energy
electrons
Converting the energy of these
electrons into ATP
41
General Principles and motifs of
metabolism
1. Metabolic pathways : step by step
2. Common energy currency: ATP, links energy-releasing
pathways with energy-requiring pathways
3. Oxidation of carbon fuels powers the formation of
ATP
4. Unifying themes : a limited number of types of
reactions and particular intermediates are
common to many pathways
5. Metabolic pathways are highly regulated
42
15.4 Metabolic pathways contain
many recurring motifs
Activated Carriers
•Activated carrier of phosphoryl groups
–ATP (because phosphoryl transfer from ATP is an
exergonic process)
•Activated carriers of electrons for fuel oxidation
•Nicotinamide adenine dinucleotide (NAD+)
•Flavin adenine dinucleotide (FAD)
•Activated carriers for reductive biosynthesis : NADPH
•Activated carriers of two-carbon fragments: Coenzyme A
43
Many Enzyme require Cofactors
•A complete, catalytically active enzyme together with its
bound coenzyme and/or metal ions is called a holoenzyme
Apoenzyme + cofactor = holoenzyme
•An enzyme without its cofactor
is referred to as an apoenzyme.
•Cofactors can be subdivided into
two groups: (1) small organic
molecules called coenzyme (2)
metals
•Tightly bound coenzyme are
called prosthetic groups; loosely
associated coenzymes are
cosubstrates.
•Enzymes that use the same
coenzyme usually perform
catalysis by similar mechanisms
:vitamins
44
Activated carriers of electrons for fuel
oxidation
•In aerobic organisms, the ultimate electron acceptor in
the oxidation of fuel molecules is O2
–fuel molecules transfer electrons to special carriers,
which are either NADH or FADH2 (intermediate carriers)
–And then transfer electron to O2
45
Activated carriers of electrons for fuel
oxidation
•NADH (nicotinamide adenine
dinucleotide)
–Reactive part: nicotinamide ring
•pyridine derivative synthesis from the
vitamin niacin (維他命B3, 菸鹼酸)
•Accepts a hydrogen ion and two electrons
R = H for NAD+
adenine
R = PO32- for NADP+
Fig 15.13 Structures of the oxidized
forms of nicotinamide-derived
electron carriers
oxidized form
Dehydrogenation
Dehydrogenase
Reduced form
46
NAD+ and NADP+
(a) Nicotinamide adenine dinucleotide, NAD+, and its phosphorylated analog NADP+
undergo reduction to NADH and NADPH, accepting a hydride ion (two electrons and one
proton) from an oxidizable substrate. The hydride ion is added to either the front (the A
side) or the back (the B side) of the planar nicotinamide ring. (b) The UV absorption
spectra of NAD+ and NADH. Reduction of the nicotinamide ring produces a new, broad
absorption band with a maximum at 340 nm. The production of NADH during an enzymecatalyzed reaction can be conveniently followed by observing the appearance of the47
absorbance at 340 nm.
• Flavin Adenine Dinucleotide (FAD)
– consists of a flavin mononucleotide (FMN) unit and an
AMP unit
– Reactive part: isoalloxazine ring
• A derivative of vitamin riboflavin (維生素B2,核黃素)
• Accepts two electrons and two protons
oxidized
form
Reduced
form
adenine
Fig 15.14 Structures of the oxidized forms
of flavin adenine dinucleotide (FAD)
Fig 15.15 Structures of the reactive parts
of FAD and FADH2
48
Activated carriers for reductive biosynthesis: NADPH
•High-potential electrons are required in most
biosyntheses because the precursors are more oxidized
than the products
•Electron donor in most reductive biosyntheses is
NADPH (nicotinamide adenine dinucleotide phosphate)
–2’-hydroxyl group of adenosine moiety is esterified with
phosphate
–is used almost exclusively for reductive biosyntheses
(NADH is used primarily for the generation of ATP)
•NADPH provides electrons
49
•NADPH operates chiefly with enzymes that catalyze anabolic
reactions, supplying the high-energy electrons needed to
synthesize energy-rich biological molecules
•NADH, as an intermediate in the catabolic system of reactions
that generate ATP through the oxidation of food molecules
•The genesis of NADH from NAD+ and that of NADPH from
NADP+ occurs by different pathways that are independently
regulated, so that the cell can adjust the supply of electrons for
these two contrasting purposes
•Inside the cell, the ratio of NAD+ to NADH is kept high, whereas
the ratio of NADP+ to NADPH is kept low. This arrangement
provides plenty of NAD+ to act as an oxidizing agent and plenty
of NADPH to act as a reducing agent—as required for their
special roles in catabolism and anabolism
50
Essential Cell Biology by Alberts, Bray, Hopkin, Johnson, Lewis, Raff, Roberts, and Walter, 4th edition published by Garland Science
Activated carriers of two-carbon fragments:
Coenzyme A
•Coenzyme A (CoA-SH)
–a carrier of acyl groups derived from the vitamin
pantothenate (維生素B5,泛酸)
•The terminal sulfhydryl group is the reactive site
•Acyl groups are important constituents both in catabolism and
in anabolism
– Acyl groups are linked to CoA by thioester bonds
– An acyl group often linked to CoA is the acetyl unit acetyl CoA
– Acetyl CoA carries an activated acetyl group, just as ATP carries
an activated phosphoryl group.
thioester bonds
51
52
Acetyl CoA has a high acetyl grouptransfer potential
•The ΔGo’ for the hydrolysis of acetyl CoA has a large
negative value
•The hydrolysis of a thioester is thermodynamically more
favorable than that of an oxygen ester
Acetyl CoA + H2O  acetate + CoA + H+
ΔGo’ = -7.5kcal mol-1(-31.4kJ mol-1)
oxygen ester
thioester
•Acetyl CoA carries an
activated acetyl group
•Transfer of the acetyl group
is exergonic
•Acetyl CoA has a high acetyl
53
group-transfer potential
•Use of activated carriers illustrates two key aspects of
metabolism:
–NADH, NADPH, and FADH2 react slowly with O2 in the
absence of a catalyst
kinetic stability
it enables enzymes to control the flow of free energy and
reducing power
–Most interchanges of activated groups in metabolism are
accomplished by a rather small set of carriers
•The existence of a recurring set of activated carriers in all
organisms is one of the unifying motifs of biochemistry
54
Important!!!!
55
Many activated carriers derived from vitamins
•Almost all of the activated carriers that act as coenzymes are
derived from vitamins
– Vitamins are organic molecules that are needed in small amounts in
the diets of some higher animals
腳氣病
口角幹裂、皮膚炎
糙皮病
56
Structures of some of the B vitamins
:泛酸
CoA
FAD
:核黃素
NAD+
:菸鹼酸
57
• Humans beings require at least 12 vitamins in their diet
• Not all vitamins coenzymes, e.g., A, C, D, E, K
58
59
General Principles and motifs of
metabolism
1. Metabolic pathways : step by step
2. Common energy currency: ATP, links energy-releasing
pathways with energy-requiring pathways
3. Oxidation of carbon fuels powers the formation of
ATP
4. Unifying themes : a limited number of types of
reactions and particular intermediates are
common to many pathways
5. Metabolic pathways are highly regulated
60
Thousands of metabolic reactions can
be subdivided into just six types
氧化還原酶
轉移酶
水解酶
裂解酶
異構酶
連接酶
電子或質子轉移
官能基團的轉移
加水或脫水分子
共價鍵生成或裂解
同一分子內基團之轉移
消耗 ATP 生成分子
間新鍵
61
• Oxidation-reduction reactions
– Electron transfer
– Such as: oxidoreductase or dehydrogenase
– Useful energy is often derived from the oxidation of
carbon compounds
– FADH2 and NADH are electron carrier
Succinate
dehydrogenase
Malate
dehydrogenase
62
•Ligation reactions
–Require ATP cleavage
–Formation of covalent bonds (C-C bonds)
–Combine smaller molecules to form larger ones
pyruvate carboxylase
63
•Isomerization Reactions
–Rearrangement of atoms to form isomers
–Prepare the molecules for subsequent reactions
Aconitase
64
•Group-transfer reactions
–Transfer of a functional group from one molecule to
another
–Such as: phosphoryl group
hexokinase
65
•Hydrolytic Reactions
–Cleavage of bonds by the addition of waters
–Break down large molecules to facilitate further
metabolism or to reuse some of the components for
biosynthesis purposes
–Hydrolysis of a peptide to yield two smaller peptides
66
• Functional groups may be added to double bonds to
form singles bonds or removed from single bonds to
form double bonds
– Addition or removal of groups to form double bonds
– Such as: lyases
Aldolase
Enolase
67
General Principles and motifs of
metabolism
1. Metabolic pathways : step by step
2. Common energy currency: ATP, links energy-releasing
pathways with energy-requiring pathways
3. Oxidation of carbon fuels powers the formation of
ATP
4. Unifying themes : a limited number of types of
reactions and particular intermediates are
common to many pathways
5. Metabolic pathways are highly regulated
68
Metabolism is regulated in three principal ways
•Controlling the amounts of enzymes
–The amount of a particular enzyme depends on both its
rate of synthesis and degradation
–Adjusted by the transcription rate
•Presence of Lactose in E. coli induces synthesis of βgalactosidase
•Controlling catalytic activities
•Controlling accessibility of substrates
69
• Controlling catalytic activities
– Reversible allosteric control
• First reaction in biosynthetic pathways is allosterically
inhibited by end product of the pathway
• Feedback inhibition
– Inhibition of aspartate transcarbamoylase by cytidine
triphosphate
– Reversible covalent modification
• triggered by hormones and signal transduction.
– Glycogen phosphorylase is activated by serine phosphorylation
when glucose is scarce.
– Hormone coordinate metabolic relations between different
tissue
– Energy status of the cell
• Energy charge
– Range from 0 (all AMP) to 1 (all ATP)
– Cell range from 0.8 to 0.95
• Higher EC favors biosynthesis and lower EC favors catabolism.
• Alternative index: phoshorylation potential
Phosphorylation potential = [ATP]/ [ADP]+[Pi]
70
Fig 15.19 Energy charge regulates metabolism
•ATP-generating (catabolic) pathways are inhibited by a high energy
charge, whereas ATP-utilizing (anabolic) pathways are stimulated by a
high energy charge
•High concentrations of ATP inhibit the relative rates of a typical ATPgenerating (catabolic) pathway and stimulate the typical ATP-utilizing
(anabolic) pathway
71
• Controlling accessibility of substrates
– Metabolic regulation and flexibility are enhanced by
compartmentalization
• Compartmentalization segregates opposed reaction
– Fatty acid oxidation takes place in mitochondria
– Fatty acid synthesis takes place in cytoplasm
– The flux of the substrates
• Glucose breakdown can take place in many cells only if
insulin is present to promote glucose into the cell
• The transfer of substrates from one compartment of a
cell to another (from cytoplasm to mitochondria)
72