Download Control and Integration of Metabolism

Control and Integration of
Metabolism
Arjumand Warsy
BCH 540
(1425-26)
Major Goals of Metabolism
To provide precursors for
synthesis of Macromolecules
i.e. proteins, polysaccharides
lipids, DNA, RNA, etc.
To extract energy (ATP) and
reducing power (NADPH, NADH)
from nutrients, for synthesis of
complex molecules
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Types of Metabolic Pathways
Anabolic pathways
Catabolic pathways
• Synthesise larger
molecules from
small molecules
• use energy NADPH,
NADH
• Degrade large
molecules to
smaller molecules.
• Produce ATP,
NADH, NADPH
e.g.
Glycolysis,
fatty acid
oxidation,
a.a.oxidation
Amphibolic pathways
• Both
catabolic and
anabolic in
nature
e.g.
Gluconeogenesis, Fatty
acid synthesis, Glycogen
synthesis, Lipid synthesis
Protein synthesis
ANABOLISM +
CATABOLISM =
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e.g.
T.C.A.
cycle
METABOLISM
(Endothermic reactions) (Exothermic Reactions)
Basal Metabolic Rate (BMR)
Defination: Amount of energy required to maintain basic
physiologic functions while at rest
• Higher in males than females
Calculation of BMR
Men
BMR (Kcal/hr/kg = 66 + (13.7 x W) + (5 x H) – (6.8 x A)
Women:
BMR (Kcal/hr/kg = 65.5 + (9.5 x W) + (1.8 x H) – (4.7 x A)
BMR During pregnancy and Lactation
2nd and 3rd Trimester : Additional 300 Kcal/24 hrs
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Lactation : Additional 500 Kcal/24 hrs
Energy from Main Constituents of Human
Nutrition
Macronutrient:
Energy content
Kcal/g
- Carbohydrate ………………….
- Protein ………………………….
- Fat ………………………………
4.1
4.1
9.3
Micronutrients:
-Vitamins ………………………..
- Minerals ……………………….
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Nil
Nil
Integration of Catabolism and Anabolism
by Energy Carriers
Catabolic Pathway
Anabolic Pathway
Metabolic Fuels
Cellular Components
Carbohydrates
Proteins
Fats
Polysaccharides
Nucleic acids
Proteins
Lipids
ADP + P
NAD+, NADP+
ATP,
NADH
NADPH
Energy carriers and reducing powers
small energy-depleted
metabolites
(CO2, H2O, NH3)
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Precursor molecules
(Monosaccharides,
Amino acids, Fatty acids,
Purines, Pyrimidines, etc.
Overview of Metabolism
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Regulation of Metabolism
ƒ Homeostatis is living organisms demands that
metabolism be carefully controlled.
ƒ Cells continuously adjust rate of metabolic pathways
to ensure adequate energy and building blocks are
available to carry out normal cellular functions.
ƒ Several mechanisms exist to control metabolism
through control of certain key enzymes, which play an
essential role in control of metabolism as their
concentration and activity may be altered. These are
Regulatory Enzymes. A.Warsy
Level of Control
ƒ The regulation of a metabolic pathway may occur at
several levels.
ƒ Since enzymes are the indispensable catalysts for all
reactions in the cell, the control of metabolism is
ultimately concerned with the regulation of enzyme
activity. An alteration in enzyme activity can be
achieved in two fundamental ways:
ƒ Coarse Control: control of the amount of an
enzyme. A slow process as it involves protein
synthesis.
ƒ Fine Control: control of the activity of the enzyme. A
fast process as it involves changing the activity of
enzyme already available
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Types of Regulatory Mechanisms
Coarse Control
Fine control
Control of amount of an
enzyme by ↑ or ↓ synthesis
(Protein synthesis (inducible
enzyme))
Control of Enzyme Activity
By noncovalent interaction
By covalent interaction
By Hormones,
Growth Factors
Other molecules
Substrate
Availability
e.g.
↑Substate →↑ product
↑ Glucose → ↑ Glycogen
↑ FA + Glycerol.3.p →↑ TG
↑ AA+ ↑ energy → ↑ protein
Allosteric
Regulation
e.g.
↑ ATP→inhibition of PFK
and hence glycolysis
(Feedback inhibition
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Protein-protein
interaction
e.g.
↑activity of many
regulatory proteins by
binding to calmodulin
↑(Ca++ binding protein)
Coarse Control
•In this central process a metabolite or
other controlling substance causes an
increase or decrease in the amount of
enzyme (protein) synthesized.
•This takes a long time to be completed as
it involves the couple machinery of protein
synthesis:
– - Transcription
– - Translation
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Fine Control
• In this process a metabolite causes increase or
decrease in the activity of the pre-existing enzyme.
• This is a rapid control and is easily switched ‘On or
Off’.
• Several factors affect the enzyme activity:
- S availability
- Cofactor availability
- Product removal
- Feedback inhibition
- Compartmentation
- Covalent modification of enzyme activity
- Hormonal control
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(a) Substrate Availability
• Any metabolic pathway could be regulated by availability of
substrate.
• A reduction in substrate conc. will decrease the activity of a
enzyme (provided it is not saturated with substrate) and this
could result in a decreased flux through the pathway.
• An increase in substrate concentration could stimulate the
pathway.
• For some metabolites such as blood glucose and intracellular
glycogen several factors play a part in order to regulate their
concentration.
• The concentration of plasma fatty acids appears to play a
fundamentally important role in the regulation of their oxidation
by various tissues, and in turn their oxidation can modify the
rate of carbohydrate utilization by the animals. In such
situations, if the substrate conc. is shown to be regulatory the
emphasis must also lie on the factors that are responsible for
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changes in substrate conc. e.g.
conc. of plasma FA seems to
play a role in the regulation of their oxidation.
• Other factors play a part in control of plasma FFA conc. e.g.
plasma FA conc. ↑ (due to lipolysis) during:
•
(a)
Stress (caused due to relese of adrenaline
and noradrenaline) due to stimulation of
adenyl cyclase
∴ ↑ in cAMP in adipose tissue
∴ causing ↑ in release of FFA from adipose
tissue
∴ ↑ in plasma FFA TAG → FFA + Glycerol
(b)
Starvation causes ↑ in FFA in serum, due to
decreased esterification of FFA.
So FFA + Glycerol ↓→ TAG
and also TAG → FAA + Glycerol ↑.
Several hormones may be involved in this during starvation
e.g. Growth hormone stimulates lipolysis. Lack of Insulin :
During starvation there seem to be fact in plasma insulin levels
and so the lipolysis hormones causes lipolysis (as insulin
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inhibits lipolysis).
Conti..
• (d) Glucagon simulator lipolysis (due to glucagon ↑ in
fasting subjects).
• (e) Other factors: e.g. TSH, ACTH is also elevation
plasma FFA levels.
• (f) Prolonged muscular activity alters FFA levels.
•
↑ FFA due to mobilization of FFA.
•
↑ in concentration causes ↑ in FFA oxidation by
various tissues (so a good fuel alternate + glucose)
and so glucose utilization ↓ in muscles and possibly
other tissues). ↑ oxidation of FFA, ↑ in KB in liver.
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(b). Cofactors Availability
• Cofactors play important part in control of a pathway.
Inhibition of enzyme activity can be achieved by ↓ the
concentration of its cofactor e.g. FA oxidation can be
controlled by the concentration of carnitine.
• So theoretically it is possible that the concentration of
carnitine could regulate the rate of FA oxidation.
• Another example of the control by cofactor availability
is the regulation of Electron transport chain and
oxidative phosphorylation in the introduction and
presence of adenine nucleotides is necessary for
synthesis of ATP. ↓ ADP causes ↓ in the oxidation
phosphorylator and ↓ in ATP causes an increase.
•
ADP
ATP
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Examples of cofactors
• Availability of Ca++ for:
– blood clotting
– Muscle contraction
– Transmission of nervous impulses
• Availability of Mg++ for:
– Replication
– Transcription
– Translation
– Activity of all kinases
• Availability of ATP or ADP
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• Availability of carnitine
in lipid metabolism
Role of Ca++ in muscle contraction
• In muscle:
ATPase
ATP
ADP
(Regulated by Ca++ )
• Absence of Ca ++ in the sarcoplasmic space during
resting stage decreases enzyme inactive- no muscle
contraction.
• Muscle stimulation → Ca++ is released from
sarcoplasmic reticulum and activates ATPase. So
ATP is broken down and muscle contraction occurs
Ca++ pump then removes the Ca++ and enzymes
become inactive and resting
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(c) Product Removal
• The removal of a product of metabolic pathway can
control the rate of its formation from the substrate.
This is not so important for the major metabolic
pathways but does play part in some of the minor
pathway.
e.g. pyruvate → lactate in muscle goes to lactate in
blood. As the product is removed from muscle more
pyruvate could change to lactate.
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(d) Regulatory enzymes
• Some enzymes possess properties that specifically
endow them with regulatory roles in metabolism. Such
more highly specialized forms are called Regulatory
enzymes. Two types of regulatory enzymes:
• (a) Allosteric enzymes: Whose catalytic activity is
modulated through the non-covalent binding of a
specific metabolite at a site as the protein other than
the catalytic site, and
• (b) Covalently modulated enzymes, which are
interconverted between active and inactive forms by
the action of other enzymes. Some of the enzymes in
the second class also respond to noncovalent
allosteric modulators. A.Warsy
Regulation of metabolism by regulatory
enzymes
• These two types of regulatory enzymes are
responsive to alterations in the metabolic state
of a cell or tissue on a relatively short time– allosteric enzymes within seconds and
– covalently regulated enzymes within minutes.
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Allosteric Enzymes
Activity of these enzymes may change in response to
small changes in environment.
ƒ These enzymes catalyse Rate Limiting Step or
Committed step i.e. once this step occurs the whole
pathway must go on.
ƒ They are Feed back inhibited by the end product.
ƒ These are allosteric enzymes.
ƒ Branched reactions
Multiple feedback loops provide addition control.
E F↔G↔H
E
E
A→B→C↔D↔E
Z
E X ↔ Y ↔A.Warsy
Allosteric Enzymes
• Control [regulatory] enzymes
• Have quaternary structure
• Have active site and modulatory site
– Active site binds substrate to give product
– Modulatory site binds +ve or – ve modulator to increase or
decrease the activity of the active site
• Catalyse an irreversible reaction
• Inhibited by end product
A B
C
D
E
F
G
H
Feedback inhibition
• Activated by substrate and other positive modulators
• Do not obey Michaelis Menten
Kinetics
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Heterotropic and Homotropic Enzymes
• Heterotropic and homotropic effect depends on the
nature of the modulating molecules.
• Heterotropic enzymes are stimulated or inhibited by
an effector or modulator molecule other than their
substrate e.g. modulator molecule other than their
substrate e.g. threonine dehydratase, the substrate is
threonine and the modulator is L-isolecuien.
• In Homotropic enzymes, the substrate also functions
as the modulator. Homotropic enzymes contain two or
more binding sites for the substrate- Modulation of
these enzymes depends on how many of the
substrate sites are occupied. Most allosteric enzymes
are of the mixed homotropic-heterotropic types in
which both the substrate and some other metabolite
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may function as modulators.
Kinetics of Allosteric Enzymes
• Allosteric enzyme do not show classical Michaelis
Menten Kinetics between substrate concentration.
(I.e.Vmax and Km).
• Their kinetic properties are greatly altered by
variations in the concentration of the allosteric
modulator.
• Many allosteric enzymes particularly homotropic
show a sigmoid curve relating initial velocity to
substrate concentration rather than the rectangular
hyperbolic curve.
• Sigmoid curve implies that binding of the first
substrate molecule to the E enhance the binding of
subsequent substrate molecules to the other
substrate site. I.e. Co-operative
effect
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• The activity of allosteric enzymes is modulated by low
MW allosteric effectors (modulators), which generally
have no or very little structural similarity to the
substrate or coenzymes for the regulatory enzymes.
• There are two types of modulators:
– Negative modulators
– Positive modulators
• Negative allosteric effector (feedback inhibitor) binds
to a allosteric site on the E and inhibits.
• Positive allosteric effector activates the activity of the
enzyme.
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Positive and negative modulator
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Feedback Regulation
• Feedback regulation of enzyme activity is the most
flexible and biologically widespread mechanism of
metabolic control.
• Feedback control in metabolic systems operates
solely through the regulation of the activity of enzymes
their catalyse non-equilibrium reaction.
A→B↔C
↔D
↔E
↔F
↔G
↔H
↔I
↔J
E
Feedback Inhibition
• The regulation of a pathway by control of the activity of
regulatory enzymes that catalyse non-equilibrium
reaction is of great significance.
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Structure of PFK
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Activation of PFK by ADP
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Pyruvate dehydrogenase complex
Control of TCA cycle
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Steps in
Glycolysis
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Contd…
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Regulation of Glycolysis
DHAP
Hexokinase*
Glucokinase
Glucose
Inhibited by
Glucose-6phosphate
Phosphofructokinase (PFK)*
G6P ↔F6P
G3p
F1,6.p2
Activated by
AMP &
Fructose 2,6,
diphosphate
Inhibited by
ATP &
Citrate
1,3,BPG ↔3 PG
2 PG
Pyruvate
PEP
kinase*
Activated by
F1, diphosphate
inhibited by
ATP & pyruvate
alanine
*Allosteric Enzyme
(Regulatory) catalyse irreversible reactions
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Formation of lactate from Pyruvate
↓O2
Pyruvate dehydrogenase
complex
↑O2
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Acetyl Co A
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Entry of Glycogen, Starch, Disaccharides & Hexoses into the
Pathway of Glycolysis
Lactose
Lactase
Trehalose
D-Galactose
Glycogen,
starch
Trehalase
D-Glucose
Phosphorylase(p)
ATP
Sucrose
Sucrose
D-Fructose
Fructokinase
G.I.P.
mutase
HK
ATP
ATP
HK
Fructose-1-P
Fructose-1-P
aldolase
Glyceraldehyde * Dihydroxy acetone P
Triose
kinase
UDP-Gal
UDP-Glu
Manoase
Glucose-6
P
ATP
HK
Mannose-6-P
Fructose-6
P
Triose
phosphate
isomerase
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Fructose 1,6
dip
Phospho-mannose
Isomerase
Glyceraldehyde
3-P
Pyruvate
Lactic Acidosis and Pyruvate Dehydrogenase (PDH)
Deficiency
Def. of PDH
(Acquired or
inherited)
Lactic acid in blood
Lactic acidosis
Acquired in:
ƒ Chronic alcoholism and
ƒ Thiamine deficiency
ƒ Arsenate and mercurial poisoning
Pyruvate kinase deficiency – inherited.
Haemolytic anaemia due to insufficient glycolysis and
hence ↓ ATP in RBCs.
Hexokinase deficiency – Inherited
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Hemolytic anaemia due to ↓ ATP in RBCs.
Tricarboxylic Acid Cycle
Catabolism of
acetyl CoA → 2CO2
+ 3NADH
FADH2
+GTP
Amphibolic pathway
Anabolic –
Synthesis of a.a.,
glucose; fatty acids
Heme from
Intermediates of TCA cycle
Deficiency of vitamins: Niacin, Thiamine, Panthothenic acid, Riboflavin, result in
decrease oxidation of Acetyl CoA and ↑ formation of lactic acid
∴ Lactic acidosis.
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The electron
transport chain
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Pentose Phosphate Pathway (Hexose Monophosphate Shunt)
<10% glucose oxidised by PPP. Glucose converted to riboses with production of NADPH.
(C6)
(C5)
Function:
• Synthesis of riboses for nucleic acid synthesis.
• Synthesis of NADPH for biosynthesis of FA, cholesterol etc.
Glucose
H-K
G-6-P
NADP+
G-6-PD
NADPH + H+
6-phosphogluconate
CO2
Ribulose-5-P
C3
C4
C5
C6
C7
F6P
Intermediates
PPP
• No ATP used or produced.
• Occurs in all cells. Rich in liver, adipose, lactating mammary glands
• NADPH protects the red cells from oxidative stress of drugs, metabolites.
Glucose-6-phosphate dehydrogenase deficiency
• Most common enzymopathy.
• XR
• Hemolytic anaemia under oxidative stress. A.Warsy
(Favism), Drugs e.g. sulfonamides antimalarial drugs; nitrofurans.
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Covalent Modification
• Covalently modulated enzymes, are
interconverted between active and inactive
forms by the action of other enzymes. Some of
the enzymes in the second class also respond
to noncovalent allosteric modulators.
• These two types of regulatory enzymes are
responsive to alterations in the metabolic state
of a cell or tissue on a relatively short timeallosteric enzymes within seconds and
covalently regulated enzymes within minutes.
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Types of covalent modifications
By covalently modifying enzymes and
hence its activity
By covalent interaction
Phosphorylation and
dephosphorylation protein
Protein kinase
O
E-O-P-O
O
E
Protein phosphatase
Limited Proteolysis
e.g. ↑ activity
• Glycogen phosphorylase
• Lipase
↓ Activity
• Glycogen synthetase
• Lipid synthetase
Protease
Zymogen
(inactive)
Active E + Peptide
e.g.
Prothrombin
Trypsinogen
Proelastase
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Thrombin
Trypsin
Elastase
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(e) Compartmentation:
An Animal Cell
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• Eucaryotic cells have many compartments, separated
from each other by a semi-permeable membrane.
• Compartmentation plays an important role in control of
metabolic activities. e.g:
– lysosomes contain hydrolytic enzymes such as proteases,
ribonucleases, phosphatases – Harmful for cell. But safely
compartmented in lysozymes. Released under certain
conditions so that they degrade macromolecules.
• Compartmentation also makes it possible to have
same processes going on at the same time in the
same cell e.g.
F.A. oxidation → Acetyl CoA in mitochondrial
matrix
F.A. synthesis from Acetyl CoA in cytoplasm.
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Pathways in different cell compartments
Acetyl CoA
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Interconversion of Metabolic Fuel
Storage:
Liver
Muscles
Adipocytes
Glycogen
Proteins
Triglycerides
↓ ↑
Amino Acids
↓ ↑
Fatty acids
Glucose
Amino Acids
Fatty acids
Glucose
Amino Acids
↑ ↓
Glucose
Transport:
Tissue
Metabolism
Pyruvate
Fatty Acids
Acetyl CoA
Cholesterol
Lactate
Ketoones
Lactate
CO2
Ketones
ATP Synthesis
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