Biochemistry
Dept. of Biochemistry
and Molecular Biology
Professor Wu Yaosheng
2009-10
還沒有來得及準備好接受這一地的金黃，
秋天就這樣悄無聲息的來到了我們的身邊。
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Chapter 9
Regulation of Metabolism
Main Contents
1. Metabolic Regulation at Cell Level
2. Metabolic Regulation at Hormone
Level
3. Regulation of Metabolism at Integral
Level
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Key Points and difficulties
◆ Some important metabolism molecules
◆ Key enzymes and their distribution
◆ Mutual relationship of carbohydrate, TG, Pr
◆ Regulation levels and fashion of substance
metabolism
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Introduction
Characteristics of Substance Metabolism
1.Mutual interknit among various metabolism
pathways
Digestion Absorption
Lipid
Sugar
H2O
Salt
Protein
Vitamin
Middle metabolism
Waste excretion
各种物质代谢之间互有联系，相互依存。
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2. Metabolism processes regulated constant
finely
Inside and outside
of circumstances
To fit in with the change
of circumstances
To influence organism
metabolism
Subtle regulation
mechanisms to regulate
metabolism intensity,
direction, velocity
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3. Various tissues and organs have themselves
metabolism characters
Different structures
Different
organs
Different metabolism
pathways
Different enzymes
and contents
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4. Each common metabolism pool

For example：
gluconeogensis
Various tissues
glycogen
degradation
Blood sugar
Sugar digested and
absorbed
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5. ATP is the common form for energy store and
utilization
To
release
energy
ADP+Pi
Directly
supply
energy
Nutriment
decomposition
ATP
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6. NADPH can supply the reduction equation for
anabolism

For example：
Pentose phosphate pathway
NADPH + H+
Acetyl CoA
Fatty acids,
cholesterol
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Questions
1. How to relate carbohydrate metabolism
with lipid or protein metabolism by
some important interim molecules?
What are metabolic interrelationships?
2. What are the important significances of
ATP during substance metabolism?
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Section One
Metabolic Regulation at
Cell Level
1.1 Distribution of Enzymes in Cells
•代谢途径有关酶类常常组成多酶体系，分布于
细胞的某一区域 。
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Distribution of enzymes in main metabolic pathways
Metabolic pathways
Distribution
Metabolic pathways
Distribution
Glycolysis
Cytosol
Oxidation
phosphorylation
Mitochondrion
Citric acid cycle
Mitochondrion
Protein synthesis
ER
Pentose phosphate
pathway
Cytosol
Urea synthesis
Mitochondrion,
cytosol
Gluconeogenesis
Cytosol
DNA synthesis
Nucleus
Glycogenesis and
glycogenolysis
Cytosol
mRNA synthesis
Nucleus
Fatty acid β-oxidation
Mitochondrion
tRNA synthesis
Nucleoplasm
Fatty acid synthesis
Cytosol
rRNA synthesis
Nucleus
Respiratory chain
Mitochondrion
Heme synthesis
Cytosol, Mitochon.
Phospholipid synthesis
Endoplasmic
reticulum
Hydrolytic enzymes
Lysosome
Cholesterol synthesis
ER, Cytosol
Bilirubin synthesis
ER, cytosol
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Distribution of enzymes in main metabolic pathways
Compartmentalization of enzymes in cells
Significances
◆To
avoid interference among enzymes in
different metabolic pathways
◆ To
be benefit to harmonious operation of
enzymes
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1.2 Multienzyme system, Multifunctional Enzymes,
and Isoenzymes
1.2.1 Multienzyme System and Multifunctional
Enzymes
Multienzyme system is an enzyme complex
assembled by several different functional enzymes.
For example, pyruvate dehydrogenase complex
Multifunctional enzyme is an enzyme with different
enzymatic functions in a single polypeptide. For
example, fatty acid synthase system
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The fatty acid synthase complex has 7 active sites:
Acetyl CoA-ACP transacetylase (AT)
b-ketoacyl-ACP synthase (KS)
Malonyl CoA-ACP transferase (MT)
b-ketoacyl-ACP reductase (KR)
b-hydroxyacyl-ACP dehydratase (HD)
Enoyl-ACP reductase (ER)
Acyl carrier protein (ACP)
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1.2.2 Isoenzymes
Enzymes catalyzing the same reaction with different
components and different physicochemical properties
are named as isoenzymes. For example, LDH
H H
H H
H H
H M
M M
H H
H M
M M
M M
M M
LDH1
(H4)
LDH2
(H3M)
LDH3
(H2M2)
LDH4
(HM3)
LDH5
(M4)
lactate dehydrogenase, LDH isoenyzmes
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
Example Two
B B
M B
CK1(BB)
CK2(MB)
brain
M M
CK3(MM)
cardiac muscle skeleton muscle
肌酸激酶 (creatine kinase, CK) 同工酶
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1.3 Basic Manners of Metabolic Regulation at
Cell Level
1.3.1 Rate-Limiting Enzyme and Rated-Limiting
Step
Definition for rate-limiting enzyme:
An enzyme with relatively low activity catalyzing
the relatively low reaction speed for control the rate
of the whole pathway is named rate-limiting enzyme.
A
E1
B
E2
C
E3
D
E4
E
E5
F
E6
G
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Rate-limiting enzymes of some metabolism pathways
Metabolism pathway
Rate-limiting enzymes
Glycolysis
HK , PFK-1, PK
P.P.P
G6PD
Gluconeogenesis
Pyr carboxylase, PEP carboxykinse, FBPase,
G6Pase
Cictric acid cycle
Citrate synthase, Isocitrate DHase, α-KG DHase
Glycogenesis
Glycogen synthase
Glycogenolysis
Glycogen phosphorylase
Triacylglycerol hydrolysis
Triacylglycerol lipase
FA synthesis
Acetyl CoA carboxylase
Ketogenesis
HMG CoA synthase
Cholesterol synthesis
HMG CoA reductase
Urea synthesis
Argininosuccinate synthase
Heme synthesis
ALA synthase
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1.3.2 Feedback Regulation
The end-products in metabolism pathways often affect
the activities of the initial enzymes.
Feedback regulation is one of the finest acting
manners of regulatory enzymes.
Negative feedback: most key enzymes
Positive feedback: F-1,6-BP to 6-FPK-1
Glucogenolysis : Gn
Glycogen
synthase
Glycogen
phosphorylase
UDPG
(—)
G1P
G6P
G
(+)
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1.3.3 Substrate Cycle
Substrate cycle is the reversible interconversion between
two substrates catalyzed by distinct enzymes for unilateral
reactions.
ATP
(+)
F-6-P
AMP
(–)
Pi
FPK-1
ADP
(+)
F-2,6-2P
F-1,6-2P
(–)
Fructose biposphatase-1
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1.3.4 Cascade Reactions
In a chain reaction, when an enzyme is
activated, other enzymes are activated in
turn to bring primal signal amplifying.
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hormones（glucagon 、epinephrine）+ receptor
Adenyly cyclase
（inactive）
Adenyly cyclase
（active）
ATP
cAMP
PKA
(inactive)
Phosphorylase b
kinase
Phosphorylase b
inactive
PKA
(active)
Phosphorylase b
kinase-P
Phosphorylase a-P
active
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激素（胰高血糖素、肾上腺素等）+ 受体
腺苷环化酶
（无活性）
腺苷环化酶（有活性）
ATP
cAMP
Pi
磷酸化酶b激酶
PKA
PKA
(无活性)
(有活性)
磷蛋白磷酸酶-1
磷酸化酶b激酶-P
糖原合酶
Pi
糖原合酶-P
磷酸化酶b
磷蛋白磷酸酶-1
–
Pi
–
磷酸化酶a-P
磷蛋白磷酸酶-1
–
磷蛋白磷酸酶抑制剂-P
PKA（有活性）
磷蛋白磷酸酶抑制剂
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1.4 Regulation of Enzymatic Activity in Cells
1.4.1 Allosteric Regulation ( rapid regulation）
when some metabolites combine reversibly
to an regulating site of an enzyme and change
the conformation of the enzyme, resulting in the
change of enzyme activity.
◆allosteric
enzyme
◆ allosteric
site
Allosteric activator
◆ allosteric effectors
Allosteric inhibitor
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Some allosteric enzymes and their effectors in metabolism pathways
Metabolism
Allosteric enzymes
Activator
HK
Glycolysis
Inhibitor
G-6-P
6-FPK-1
AMP, ADP, F-1,6-BP,
F-2,6-BP
Citrate, ATP
Pyruvate kinase
F-1,6-BP
ATP, alanine
Citrate synthase
ADP
ATP, citrate, NADH
Isocitrate dehydrogenase
ADP
ATP, Ca2+
Pyruvate carboxylase
Acetyl CoA
ADP
F-1,6-bisphosphatase
Citrate
AMP, F-2,6-BP
Glycogenolysis
Glycogen phophorylase b
AMP, G-1-P, Pi
ATP, G-6-P
Glycogenesis
Glycogen sythase
G-6-P
FA biosynthesis
Acetyl CoA carboxylase
Citrate, isocitrate
Cholesterol
biosynthesis
HMG-CoA carboxylase
AA metabolism
L-glutamate
dehydrogenase
ADP, leucine,
methionine
ATP, GTP, NADH
Purine synthesis
PRPP amidotransferase
PRPP
AMP, ADP, GMP, GDP,
Pyrimidine synthesis
Aspartate
transcarbomoylase
CTP
ALA synthase
Heme
Citric acid cycle
Gluconeogenesis
Heme synthesis
Long-chain fatty acyl-CoA
Cholesterol
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General Properties of Allosteric Enzymes
Key points:
 An allosteric enzyme is regulated by its effectors
(activator or inhibitor).
 Allosteric effectors bind noncovalently to the enzyme.
 Allosteric enzymes are often multi-subunit proteins.
 A plot of V0 against [S] for an allosteric enzyme gives
a sigmoidal-shaped curve.
 The binding of allosteric enzyme with an effector will
induce a conformational change
Does not consume energy
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Allosteric effect of fructose-1,6-biphosphatase
FDP
FDP
FDP
FDP
FDP
AMP
(allosteric inhibitor)
AMP
FDP
FDP
Glyceraldehydes-3-phosphate
FA –carrier protein
(allosteric activator)
AMP
T state
(high activity)
AMP
AMP
FDP
R state
(low activity)
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1.4.2 Covalent Modification (rapid regulation）
It means the reversible covalent attachment
of a chemical group.
 Types of Covalent Modification:
phosphorylation / dephosphorylation
adenylylation/deadenylylation
methylation/demethylation
acetylation/deacetylation
－SH / －S－S , etc
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Covalent Modification
Pi
Protein
phosphatase
H2 O
Protein-OH
O-
ATP
Protein kinase
Protein-O-P=O
O-
ADP
The reversible phosphorylation and
dephosphorylation of an enzyme
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Regulation of covalent modification in enzyme activities
Enzyme
Reactive type
Effect
PFK-1
Phosphorylation/dephosphorylation
Inactivity/activity
Pyr DHase
Phosphorylation/dephosphorylation
Inactivity/activity
Pyr decarboxylase
Phosphorylation/dephosphorylation
Inactivity/activity
Glycogen phosphorylase Phosphorylation/dephosphorylation
Activity/inactivity
Phosphorylase b kinase
Phosphorylation/dephosphorylation
Activity/inactivity
Protein phosphatase
Phosphorylation/dephosphorylation
Inactivity/activity
Glycogen synthase
Phosphorylation/dephosphorylation
Inactivity/activity
Triacylglycerol lipase
HMG CoA reductase
Phosphorylation/dephosphorylation
Phosphorylation/dephosphorylation
Acetyl CoA carboxylase Phosphorylation/dephosphorylation
Activity/inactivity
Inactivity/activity
Inactivity/activity
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Key points:
 The activity state of an enzyme modulated can
interconvert reversely
 Change of a covalent bond catalyzed by E, and
can be modulated by hormones
 The modification is a rapid, reversible and
effective and amplified by cascade reaction
 The most common is the phosphorylation or
dephosphorylation. Enzymes----protein kinases
or phosphatases
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Covalent modification of phosphorylase
2ATP
Phosphorylase
b kinase
2Pi
phosphatase
Phosphorylase b
(dimer)
Inactivity
2ADP
P
P
Phosphorylase a
(dimer)
High activity
P
P
P
P
Phosphorylase a
(tetramer)
Activity
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1.5 Regulation of Enzyme Level in Cells
(Genetic Control)
The amount of enzyme present is a balance
between the rates of its synthesis and degradation.
The level of induction or repression of the gene
encoding the enzyme, and the rate of degradation of its
mRNA, will alter the rate of synthesis of the enzyme
protein.
Once the enzyme protein has been synthesized, the
rate of its breakdown (half-life ) can also be altered as a
means of regulating enzyme activity.
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1.5.1 Induction and repression of E Pr Synthesis
Induction: the activation of enzyme synthesis.
Repression: the shutdown of enzyme synthesis.
Genetic control of enzyme leverl means to
controlling the transcription of mRNA needed for an
enzyme’s synthesis.
In prokaryotic cells, it also involves regulatory
proteins that induce or repress enzyme’s synthesis.
Regulatory proteins bind to DNA, and then block or
enhance the function of RNA polymerase. So, regulatory
proteins may function as repressors or activators.
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Repressor
Repressors are regulatory proteins that block
transcription of mRNA, by binding to the operator that
lies downstream of promoter.
This binding will prevent RNA polymerase from
passing the operator and transcribing the coding
sequence for the enzyme.------Negative control.
Regulatory proteins are allosteric proteins. Some
special molecules can bind to regulatory proteins and
alter their conformation, and then affect their ability to
bind to DNA.
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For example: lac operon
When no lactose:
Promotor Operator gene
Structural gene
I
Z
repressor gene
A
RNA
polymerase
mRNA
mRNA
repressor
protein
Y
NH2
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When lactose presents:
I
repressor gene
P
O
Structural gene
A
Y
Z
RNA
polymerase
mRNA
mRNA
NH2
NH2
NH2
repressor
protein
Z
Y
A
lactose
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Inducers
Inducers promote the transcription of mRNA.
Activator is an allosteric protein which is unable to bind to
promoter to transcribe relative genes directly in eukaryotes.
When no inducer:
activator-binding site
P
Structural gene
O
mRNA
Activator
RNA
polymerase
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When inducer:
activator-binding site
P
Structural gene
O
mRNA
RNA
polymerase
activator
inducer
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Bacteria also Use Translational Control of Enzyme
Synthesis
The bacteria produces antisense RNA that
is complementary to the mRNA coding for the
enzyme.
When the antisense RNA binds to the
mRNA by complementary base paring, the
mRNA cannot be translated into protein.
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1.5.2 Degradation of Enzyme Proteins
Cellular enzyme proteins are in a dynamic state with
change of enzyme synthesis and degradation so that
ultimately determine enzyme level at any point in time.
In many instances, transcriptional regulation
determines the concentrations of specific enzyme, with
enzyme proteins degradation playing a minor role.
In other instances, protein synthesis is constitutive,
and the amounts of key enzymes and regulatory proteins
are controlled via selective protein degradation.
In addition, it also involves the abnormal enzyme
proteins ( biosynthetic errors or post-synthetic damage).
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There are two pathways to degrade
enzyme protein in cells:
1. Lysosomal pathway
ATP independent
2. Proteasome pathway
ATP, Ubiquitin dependent
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Questions
1. Which one of the following metabolism
pathways is not present in cytoplasm?
A. Glycolysis
B. Phosphate pentose pathway
C．Glycogenesis and glycogenolysis
D．Fatty acid β-oxidation
E．Fatty acid synthesis
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Questions
2. All gluconeogenesis, ketone body
biosynthesis and urea synthesis exist in
A. Heart
B．Kidney
C．Brain
D．Liver
E．Muscle
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Can you fill in these blanks?
Substrate cycle is the reversible interconversion between
two substrates catalyzed by distinct enzymes for unilateral
reactions.
ATP
(+)
F-6-P
AMP
(–)
Pi
FPK-1
ADP
(+)
F-2,6-2P
F-1,6-2P
(–)
Fructose biposphatase-1
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Questions
1. Why some persons who are easely drunk can
turn to endure alcohol after they have
experience to drink wine?
2. Why some persons who need hypnotics (安眠
药）would become more and more dependent to
drugs?
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Section Two
Metabolic Regulation at
Hormone Level
Hormones are generally secreted by
endocrine glands, travelled by blood stream to
specific target cells.
By these mechanisms, hormones regulate
the metabolic processes in various organs and
tissues; facilitate and control growth,
differentiation, reproductive activities, learning
and memory; and help organisms coping with
changing conditions and stresses to around
environment.
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Hormonal regulation depends upon the transduction
of the hormonal signal across the plasma membrane to
specific intracellular sites, particularly the nucleus.
Many steps in these signal across the signalling
pathway involve phosphorylation of Ser, Thr, and Tyr
residues on target proteins.
According to receptor’s location in a cell, hormones
are divided into two classes:
Hormones act on cell membrane receptors
Hormones act on intracellular receptors
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Hormones act on cell membrane receptors
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Hormones act on intracellular receptors
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2.1 Regulation of Hormones to Receptors on
Cell Membrane
Hormones act on membrane receptors, as the first
messenger, to activate various signal transduction
pathways that mobilize various second messengers----cAMP, cGMP, Ca2+, IP3 , DG that activate or inhibit enzymes
or cascade of enzymes in specific ways.
The first messengers:
Peptide or protein hormones: GH, Insulin, etc
Amino acid derivatives: epinephrine, norepinephrine
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H
Adenylate cyclase
cAMP
Ｒ
R
β
β
γ
α
γ
AA
CC
GDP
GTP
ATP
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Hormone
receptor
Ｇ protein
Enzyme
The second messenger
Protein kinase
Enzyme or other protein
Biological effects
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2.2 Regulation of Hormones to Receptors in
Cells
Hormones to act on intracellular receptors:
Steroid hormones: Glucocorticoids
Mineralocorticoids
Vit D
Sex hormones
Amino acid derivatives: T3, T4
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Hormone
Can you give
some examples?
receptor
Ｇ protein
Enzyme
The second messenger
Protein kinase
Enzyme or other protein
Biological effects
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Section Three
Regulation of Metabolism
at Integral Level
Living in a constantly changing
environment, human must have the ability
to adapting to the environment.
Why and how?
The metabolism of body has to be
regulated through neurohumoral pathways
to satisfy energy needs and to maintain
homeostasis of the internal environment.
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3.1 Metabolism Regulation in Starvation
3.1.1 Starvation in Short-term (1-3 days)
Glycogen reserve
Blood Glucose
Insulin
glucagon
corticosteroid
a series of
metabolic changes
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(1) Protein Metabolism
Protein degradation ↑,
Amino acid
Protein
Glucose
degradation
gluconeogenesis
Amino acid
Pyruvate
deamination
transamination
Pyruvate
transamination
Alanine
Muscle
Glucose
Alanine
Liver
Blood
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(2) Carbohydrate Metabolism
Gluconeogenesis
Lactic acid 30%
Glycerol 10%
Amino acids 40%
Liver : 80%
Renocortical : 20%
Tissue utilize glucose
In brain , glucose is still the main fuel
source.
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(3) Triacylglycerol Metabolism
Fat mobilization
Fatty acid
Ketone bodies
Heart
Skeletal muscle
Renal cortex
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3.1.2 Change of Metabolism in Long-term
Starvation ( >7 days)
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Starvation in Long-term
(1) Protein Metabolism
Muscle protein degradation
Amino acid , but Glu deamination
In urine
Urea
NH3
Acidism(酸中毒）
( by ketosis 酮症)
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(2) Carbohydrate Metabolism
In kidney :
Gluconeogenesis
( almost equal to that in liver )
The main materials of gluconeogenesis in
liver:
Lactic acid
Pyruvate
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(3) Triacylglycerol Metabolism
Fat mobilization
Fatty acid
Ketone bodies
Skeletal muscle: FA as an energy source to
ensure that adequate amounts of ketone bodies
are available in brain.
Brain: gradually adapts to using ketone
bodies as fuel.
This may reduce utilization of glucose and
gluconeogenesis of amino acid, so decrease
the breakdown of protein.
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After starvation in Long-term, if the
person is given a big meal with a lot of
meat and wine in short time, what
case would occur?
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3.2 Metabolism Regulation in Stress
Stress is a tense state of an organism in
response to unusual stimulus.
Effect:
Stimulus
injury
Excitation of sympathetic nerves
pain
Adrenal medullary/cortical hormones
frostbite
Epinephrine, glucagons, growth hormone
oxygen deficiency
Insulin
toxicosis
Metabolism of
carbohydrates
infection
lipids
out-of-control rage
proteoins
Catabolism
change
Anabolism
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(1) Change of Carbohydrate Metabolism
Hyperglycemia
catecholamine
glucagon
growth hormone
corticosteroid
Glycogenolysis
Gluconeogenesis
Stress hyperglycemia
Stress glucosuria
Insulin
Blood glucose
If exceeds renal
threshold of glucose
(8.96 mmol/L)
Glucosuria
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(2) Change of Triacylglycerol Metabolism
Adrenaline
Noradrenaline
Glucagon
Fat mobilization
Fatty acid
Ketone bodies
Tissue utilize FA as energy
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(3) Change of Protein Metabolism
Protein hydrolysis
Amino acid: as material for Gluconeogenesis
Urea synthesis
Equilibrium of
negative nitrogen
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Liver
Glycogenolysis
Glycerophosphate
Ketogenesis
Stress
Sympathetic
excitation
Adrenal cortex/
medulla hormone
FA
LA
glucose
Gluconeogenesis
Pyruvate
Ureogenesis
Alanine
NH3
FA LA Alanine Urea Glucose
Glycerophosphate
Kidney
Blood vessel
Glucosuria
TG hydrolysis
Lipocyte
Muscle glycogenolysis
Muscle Protein degradation
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Questions
1. Which one of substance change in blood is
incorrect under stress ?
A. Glucose increase
B. Free fatty acid increase
C．Amino acid increase
D．Ketone body increase
E．VLDL increase
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Questions
2. When hungry, the false statement about
substance metabolism alternation is
A．Gluconeogenesis enhancement
B. Triglyceride mobilization enhancement
C．Ketone body synthesis enhancement
D．Insulin secretion increase
E. Glucagon secretion increase
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Questions
1.How does Ala turn to be glucose in vivo? When
does this case occur？
2. How does carbohydrate metabolism and amino
acid metabolism be modulated in liver cells to
adapt with those in skeleton muscles and in
cardiac muscle?
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Questions
3. How to compare allosteric regulation with
chemical modification?
4. Use several examples to explain some
diseases involved with abnormal metabolism.
5. What changes of metabolism in body would
occur in long-term starvation?
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