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Chapter 9
Regulation of Metabolism
Metabolism:
Metabolism in the living organism contains many
pathways. Catabolism: to generate energy
Anabolism: to use eneragy
Cells maintain a dynamic homeostasis, the living
organism modulates various metabolisms in intensity,
direction and velocity, in order to adapt changes of
enviroment inside and outside the body.
Metabolism regulation is an important character of
life, being an adaptation formed in evolution over a
long –term.
There are three hierarchies in metabolism
regulations:
1) Metabolism regulation in the cell level
2) Hormone regulation of metabolism
3) Regulation of metabolism in level of the whole
Section 1
Metabolism Regulation in Cell Level
1. Basic manner of metabolism regulation in cells
1) Integration and orientation of metabolism enzymes
and pathways
ⅰ. Regional distribution of enzymes in cells
Eukaryote cells have many inner-membrane systems,
so enzyme distribution presents compartmentation,
which not only avoids interference among enzymes in
different metabolism pathways but also benefits
harmonious operation of enzymes.
Table 1 Compartment distribution of main metabolism
pathways (enzymes in eukarote cell)
Metabolism pathways distribution
Metabolism pathways
distribution
Cictric acid cycle
Mitochondrion
Urea synthesis
Mit, Cytosol
Protein synthesis
Cytosol
DNA synthesis
Nucleus
mRNA synthesis
Nucleus
Glycolysis
P.P.P
Cytosol
Glycogenolysis
Cytosol
Glycogenesis
Cytosol
Gluconeogenesis
Cytosol
FA β-oxidation
FA synthesis
Respiratory chain
Mitochondrion
Cytosol
Mitochondrion
Oxidative phosphrylation Mitochondrion
tRNA synthesis
rRNA synthesis
Ch synthesis
PL synthesis
Heme synthesis
Hydrolytic enzymes
ER, Cytosol
Nucleoplasma
Nucleus
ER, Cytosol
ER
Cytosol, Mit
Lysosome
ⅱ. Multienzyme System and Multifunctional Enzyme
Monomeric enzyme
Oligameric enzyme
Multienzyme System:
Pyruvate dehydrogenase complex
Multifunctional Enzyme:
FA synthase system
ⅲ. Isoenzyme
Isoenzymes (isozymes ) : are different forms of an
enzyme which catalyze the same reaction, but which
exhibit different physical or kinetic properties, such as
isoelectric point, pH optimum, substrate affinity or
effect of inhibitors.
Examples:
LDH (lactate dehydrogenase )
H4
H3M
H2M2
HM3
M4
Heart
Muscle
Different tissues express different isoenzyme forms
(by regulating tissues express different isoenzyme
forms) , as appropriate to their particular metabolic
needs.
2) Basic manner of metabolism regulation
Metabolism speed or direction often lies up on
activities of some key enzymes.
The enzyme that catalyzes the reaction at the slowest
speed, whose activities is modulated by substrates,
metabolites(products or effectors), is called regulatory
enzyme, key enzyme or rate-limiting enzyme.
Table 2 Rate-limiting enzymes of some important
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
ⅰ. Feedback Regulation
The substrates or products in metabolism pathways
often affect the initial enzymes in the pathway.
Feedback regulation is one of the finest acting
manners of regulatory enzymes.
Negative feedback:
Positive feedback:
Glucogenolysis : Gn
Glycogen
synthase
Glycogen
phosphorylase
UDPG
(—)
G1P
G6P
(+)
G
ⅱ. Substrate Cycle
In a metabolism pathway, the direction of reversible
reaction is controlled by different enzyme.
ATP
(+)
F-6-P
AMP
(-)
Pi
ADP
FPK-1
(-)
F-2,6-2P
F-1,6-2P
(+)
Fructose biposphatase-1
ⅲ. Cascade Reaction
In a chain reaction, when an enzyme is
activated, other enzymes are activated in turn
to bring primal signal amplifying.
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
2. Regulation of Enzymatic Activity in Cells
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
Table 3
Metabolism
pathway
Glycolysis
Cictric acid cycle
Gluconeogenesis
Glycogenolysis
Glycogenesis
FA synthesis
Some allosteric enzymes and effectors in enzyme
systems of metabolism pathways
Allosteric
enzymes
Activator
Inhibitor
HK
PFK-1
PK
Citrate synthase
AMP, ADP, FBP, Pi
G6P
FBP
Citrate
FBP
ATP, Acetyl CoA
AMP
ATP,
long-chain fatty acyl CoA
Isocitrate DHase
AMP, ADP
ATP
Pyr carboxylase
Acetyl CoA, ATP
FBPase
Citrate
AMP
Glycogen phosphorylase AMP, G1P, Pi
ATP, G6P
Glycogen synthase
G6P
Acetyl CoA carboxylase Citrate, Isocitrate long-chain
fatty acyl CoA
Cholesterol synthesis HMG CoA reductase
Cholesterol
Amino acid metabolism GLDH
ADP, Leu, Met ATP, GTP, NADH
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
Allosteric effect of fructose-1,6-biphosphatase
FDP
FDP
FDP
FDP
FDP
AMP
(allosteric inhibitor)
AMP
FDP
FDP
Glyceraldehydes-3-phosphate
Fatty acid –carrier protein
Citrate
AMP
(allosteric activator)
T state
(high activity)
AMP
AMP
FDP
R state
(low activity)
2). Covalent Modification(rapid regulation）
It means the reversible covalent
attachment of a chemical.
 Types of Covalent Modification:
phosphorylation / dephosphorylation
adenylylation/deadenylylation
methylation/demethylation
acetylation/deacetylation
－SH / －S－S , etc
Covalent Modification
Pi
Protein
phosphatase
H2 O
Protein-OH
O-
ATP
Protein kinase
Protein-O-P=O
O-
The reversible phosphorylation and
dephosphorylation of an enzyme
ADP
Table 4
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
Phosphorylation/dephosphorylation Activity/inactivity
HMG CoA reductase
Phosphorylation/dephosphorylation Inactivity/activity
Acetyl CoA carboxylase Phosphorylation/dephosphorylation Inactivity/activity
Key points:
 Change of a covalent bond
 The most common is the phosphorylation or
dephosphorylation. Enzymes----protein kinases or
phosphatases
 The activity of an enzyme after the modification can
increase or decrease.
 The modification is a rapid, reversible and effective
process.
Covalent modification of phosphorylase
2ATP
2Pi
Phosphorylase b
(dimer)
Inactivity
Phosphorylase
b kinase
phosphatase
2ADP
P
P
Phosphorylase a
(dimer)
High activity
P
P
P
P
Phosphorylase a
(tetramer)
Activity
3. Regulation of Enzyme Content 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.
1) Induction and repression of Enzyme Protein Synthesis
Induction: the activation of enzyme synthesis.
Repression: the shutdown of enzyme synthesis.
Genetic control of enzyme activity 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.
ⅰ. Repressor
Repressors are regulatory proteins that block
transcription of mRNA, by binding to the operator that
lies downstream of promoter.
This biding will prevent RNA polymerase from
passing the operator the and transcribing the coding
sequence for the enzymes.------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. They work by two ways:
Some repressor readily bind to the operator and
block transcription: lac operon
When no lactose:
Promotor Operator gene
Structural gene
I
Z
repressor gene
A
RNA
polymerase
mRNA
mRNA
repressor
protein
Y
NH2
When there is lactose:
I
repressor gene
P
O
Structural gene
A
Y
Z
RNA
polymerase
mRNA
mRNA
NH2
NH2
NH2
repressor
protein
lactose
Z
Y
A
Some repressor can not bind to the operator
directly : Trp operon
When Trp
trpR
P
Structural gene
O
RNA
polymerase
mRNA
repressor
protein
When Trp
Structural gene
trpR
P
O
RNA
polymerase
mRNA
repressor
protein
Trp
( corepressor )
ⅱ. Activators
Activators promote the transcription of mRNA.
Activator is an allosteric protein which can not
bind to the activator-binding site normally.
When no inducer:
activator-binding site
P
Structural gene
O
mRNA
activator
RNA
polymerase
When inducer:
activator-binding site
P
Structural gene
O
mRNA
RNA
polymerase
activator
inducer
ⅲ.
Bacteria also Use Translational
Control of Enzyme Synthesis
The bacteric 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.
2) Regulation on Enzyme Protein Degradation
Cellular enzyme proteins are in a dynamic state of
turn over, with the relative rates of enzyme synthesis
and degradation ultimately determining the amount of
enzymes.
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).
There are two pathways to degrade enzyme
protein in cells:
ⅰ. Lysosomal pathway
ATP independent
ⅱ. Proteasome pathway
ATP, Ubiquitin dependent
Section 2
Hormone Regulation of Metabolism
Hormones are secreted by certain cells, usually
located in glands, either by simple diffusion or
circulation in the blood stream, to specific target cells.
By these mechanisms, hormones regulate the
metabolic processes of various organs and tissues;
facilitate and control growth, differentiation,
reproductive activities, learning and memory; and help
the organism cope with changing conditions and stress
in its environment.
Hormonal regularion 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 associating transmembrane receptors
Hormones associating intracellular receptors
1. Regulation Hormones Associating
Transmembrane Receptors
Hormones associating transmembrane receptors, as
the first messenger, which act by binding to membrane
receptors, 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 messenger:
Peptide or protein hormones: GH, Insulin, etc
Amino acid derivatives: epinephrine, norepinephrine
H
腺苷酸环化酶
cAMP
Ｒ
R
β
β
γ
α
γ
AA
CC
GDP
GTP
ATP
Hormone
receptor
Ｇ protein
Enzyme
The second messenger
Protein kinase
Enzyme or other protein
Biological effects
2. Regulation Hormones Associating
Intracellular Receptor
Hormones associating intracellular receptor:
Steroid hormones:
Glucocorticoids
Mineralococorticoids
Vit D
Sex hormones
Amino acid derivatives: T3, T4
Section 3 Regulation of Metabolism in
Relation to the Whole
Living in an ever-changing environment,
human must have the ability to adapting to the
environment, the body regulates metabolism
through neurohumoral pathways to satisfy
energy needs and maintain homeostasis of the
internal environment.
1. Metabolism Regulation in Stress
Stress is a tense state of an organism in response to
unusual stimulus.
Effect:
Stimulus
injury
pain
frostbite
oxygen deficiency
toxicosis
Excitation of sympathetic nerves
Adrenal medullary/cortical hormones
Epinephrine, glucagons, wrowth hormone
Insulin
Metabolism of
carbohydrates
lipids
infection
change
proteoins
out-of-control rage
Catabolism
Anabolism
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
In the beginning phase of stress:
Liver
Gluconeogenesis
Muscle
Glycogens
Most tissue utilizes glucose
In brain, it utilizes glucose normally.
Glycogens
2). Change of Triacylglycerol Metabolism
Adrenaline
Noradrenaline
Glucagon
Fat mobilization
Fatty acid
Ketone bodies
Tissue utilize FA as energy
3). Change of Protein Metabolism
Protein hydrolysis
Amino acid: as material for Gluconeogenesis
Urea synthesis
Equilibrium of
negative nitrogen
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
Muscle glycogenolysis
TAG hydrolysis
Lipocyte
Muscle
Protein degradation
2. Change of Metabolism in Starvation
1) Starvation in Short-term (1-3 days)
Glycogen reserve
Blood Glucose
Insulin
glucagon
corticosteroid
a series of
metabolic changes
ⅰ. Protein Metabolism
Protein degradation
,
Protein
Amino acid
Glucose
Glucose
degradation
gluconeogenesis
Amino acid
Pyruvate
deamination
transamination
Pyruvate
transamination
Alanine
Alanine
Muscle
Blood
Liver
ⅱ. 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.
ⅲ. Triacylglycerol Metabolism
Fat mobilization
Fatty acidG
Ketone bodies
Heart
Skeletal muscle
Renal cortex
2) Starvation in Long-term
ⅰ. Protein Metabolism
Muscle protein degradation
Amino acid , but Glu deamination
In urine
Urea
NH3
Acidism
( by ketosis )
ⅱ. Carbohydrate Metabolism
In kidney :
Gluconeogenesis
( almost equal to that in liver )
The main materials of gluconeogenesis
in liver:
Lactic acid
Pyruvate
ⅲ. Triacylglycerol Metabolism
Fatty acidG
Ketone bodies
Skeletal muscle: FA as an energy source to
ensure that adequate amounts of ketone bodies
are available in brain.
Fat mobilization
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.