Chapter 9

Regulation of Metabolism

Metabolism:

Catabolism: to generate energy

Anabolism: to use eneragy

Metabolism in the living organism contains many

pathways.

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 Oxidative phosphrylation Mitochondrion

Urea synthesis Mit, Cytosol Protein synthesis ER, Cytosol

Glycolysis Cytosol DNA synthesis Nucleus

P.P.P Cytosol mRNA synthesis Nucleus

Glycogenolysis Cytosol tRNA synthesis Nucleoplasma

Glycogenesis Cytosol rRNA synthesis Nucleus

Gluconeogenesis Cytosol Ch synthesis ER, Cytosol

FA β-oxidation Mitochondrion PL synthesis ER

FA synthesis Cytosol Heme synthesis Cytosol, Mit

Respiratory chain Mitochondrion Hydrolytic enzymes Lysosome

ⅱ. Multienzyme System and Multifunctional Enzyme

Monomeric enzyme

Oligameric enzyme

Multienzyme System:

Pyruvate dehydrogenase complex

Multifunctional Enzyme:

FA synthase system

ⅲ. IsoenzymeIsoenzymes (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:Glycogen phos

phorylaseGlucogenolysis : Gn G1P G6P G

(—)

(+)Glycogen synthase

UDPG

ⅱ. Substrate Cycle

In a metabolism pathway, the direction of reversible reaction is controlled by different enzyme.

F-1,6-2P

F-6-P

ADP ATP

Pi

FPK-1

Fructose biposphatase-1

AMP F-2,6-2P

(+)

(+)

(-)

(-)

In a chain reaction, when an enzyme is

activated, other enzymes are activated in turn

to bring primal signal amplifying.

ⅲ. Cascade Reaction

Adenyly cyclase （ inactive ）

hormones （ glucagon 、 epinephrine ） + receptor

cAMP

PKA(inactive)

Phosphorylase b kinase

PKA(active)

Phosphorylase b Phosphorylase a-P

Phosphorylase b kinase-P

Adenyly cyclase（ active ）

ATP

inactive 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 effectors

• allosteric enzymeallosteric enzyme

• allosteric siteallosteric site Allosteric activator

Allosteric inhibitor

Table 3 Some allosteric enzymes and effectors in enzyme systems of metabolism pathways

Metabolism Allosteric Activator Inhibitor pathway enzymes Glycolysis HK AMP, ADP, FBP, Pi G6P PFK-1 FBP Citrate PK FBP ATP, Acetyl CoA Cictric acid cycle Citrate synthase AMP ATP, long-chain fatty acyl CoA Isocitrate DHase AMP, ADP ATP Gluconeogenesis Pyr carboxylase Acetyl CoA, ATP FBPase Citrate AMP Glycogenolysis Glycogen phosphorylase AMP, G1P, Pi ATP, G6P Glycogenesis Glycogen synthase G6P FA synthesis Acetyl CoA carboxylase Citrate, Isocitrate long-chain Cholesterol synthesis HMG CoA reductase CholesterolAmino acid metabolism GLDH ADP, Leu, Met ATP, GTP, NADH

fatty acyl CoA

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 si

gmoidal-shaped curve. The binding of allosteric enzyme with an effector will induce a conformational change

General Properties of Allosteric Enzymes

T state R state(high activity) (low activity)

FDP

FDP

FDP

FDPFDP

FDP

FDP

FDP

AMP

AMP

AMP

AMP

(allosteric inhibitor)AMP

Glyceraldehydes-3-phosphate

Fatty acid –carrier proteinCitrate(allosteric activator)

Allosteric effect of fructose-1,6-biphosphatase

2). Covalent Modification(rapid regulation ）

It means the reversible covalent attachment of a chemical.

Types of Covalent Modification:Types of Covalent Modification: phosphorylation / dephosphorylation adenylylation/deadenylylation methylation/demethylation acetylation/deacetylation －－ SH / SH / －－ SS －－ S S , etc

Protein-OH

Protein-O-P=O

O-

O-

ATP

ADP

Protein kinase

H2O

Pi

Protein phosphatase

The reversible phosphorylation and dephosphorylation of an enzyme

Covalent Modification

Table 4 Regulation of covalent modification in enzyme activities

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

Enzyme Reactive type Effect

Change of a covalent bond

The most common is the phosphorylation or dephos

phorylation. Enzymes----protein kinases or phosphata

ses

The activity of an enzyme after the modification ca

n increase or decrease.

The modification is a rapid, reversible and effective

process.

Key points:

PP

PP

P

P

2ATP 2ADP

2Pi

Phosphorylase b kinase

phosphatase

Phosphorylase b(dimer)

Inactivity

Phosphorylase a(dimer)

High activity

Phosphorylase a(tetramer)

Activity

Covalent modification of phosphorylase

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:

Structural geneOperator gene Promotor

repressor gene

I

NH2

Some repressor readily bind to the operator and block transcription: lac operon

Z Y

repressor protein

mRNA

A

mRNA

When no lactose:

RNA polymeras

e

lactose

Structural gene

repressor gene

I

NH2

Z Y

repressor protein

mRNA

A

mRNA

When there is lactose:

P O

RNA polymeras

e

NH2

NH2

ZYA

mRNA

OPtrpR

Some repressor can not bind to the operator directly : Trp operon

Structural geneWhen Trp

RNA polymeras

e

repressor protein

mRNA

OPtrpR

Structural gene

When Trp

Trp ( corepressor )

RNA polymeras

e

repressor protein

ⅱ. ActivatorsActivators promote the transcription of mRNA.

OP

Structural gene

RNA polymeras

e

activator-binding site

Activator is an allosteric protein which can not bind to the activator-binding site normally.

activator

When no inducer:

mRNA

OP

Structural gene

RNA polymeras

e

activator-binding site

activator

When inducer:

mRNA

inducer

ⅲ. Bacteria also Use Translational

Control of Enzyme Synthesis The bacteric produces antisense RNA tha

t is complementary to the mRNA coding for t

he enzyme.

When the antisense RNA binds to the m

RNA by complementary base paring, the mR

NA 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 receptorsHormones 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

RＲ

H

AC

γαβ

GDPαGTP

βγ

腺苷酸环化酶

AC

ATP

cAMP

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 th

e environment, the body regulates metabolism

through neurohumoral pathways to satisfy ene

rgy needs and maintain homeostasis of the inte

rnal environment.

1. Metabolism Regulation in Stress

injury

pain

frostbite

oxygen deficiency

toxicosis

infection

out-of-control rage

Excitation of sympathetic nerves

Adrenal medullary/cortical hormones

Epinephrine, glucagons, wrowth hormone

Insulin

Metabolism of carbohydrates

lipids change

proteoins

Effect:Stimulus

Catabolism Anabolism

Stress is a tense state of an organism in response to unusual stimulus.

1). Change of Carbohydrate MetabolismHyperglycemi

a catecholamine glucagon growth hormone corticosteroid

Insulin

GluconeogenesisGlycogenolysis

Blood glucoseIf exceeds renal thre

shold of glucose (8.96 mmol/L)

GlucosuriaStress hyperglycemia Str

ess glucosuria

In the beginning phase of stress:

Liver

Muscle

Glycogens

Gluconeogenesis

Glycogens

Most tissue utilizes glucose

In brain, it utilizes glucose normally.

2). Change of Triacylglycerol Metabolism

AdrenalineNoradrenalineGlucagon

Fatty acidKetone bodies

Fat mobilization

Tissue utilize FA as energy

3). Change of Protein Metabolism

Protein hydrolysis

Amino acid: as material for Gluconeogenesis

Urea synthesis

Equilibrium of negative nitrogen

Stress

Sympathetic excitation

Adrenal cortex/ medulla hormone

TAG hydrolysisLipocyte

Liver

Gluconeogenesis

glucose

Glycerophosphate

Glycogenolysis

Ketogenesis Pyruvate Ureogenesis

FA LA Alanine NH3

Urea

Blood vessel

Kidney

Glucosuria

FA LA Glucose

Glycerophosphate

Alanine

Muscle

Muscle glycogenolysis

Protein degradation

2. Change of Metabolism in Starvation1) Starvation in Short-term (1-3 days)

Glycogen reserve

Blood Glucose

Insulin

glucagoncorticosteroid

a series of metabolic changes

ⅰ. Protein Metabolism

Protein

Amino acid gluconeogenesis

deamination

Pyruvatetransamination Alanine

Blood

degradation

Alanine

Pyruvate

Glucose

transamination

Protein degradation , Amino acid Glucose

Muscle Liver

ⅱ. Carbohydrate Metabolism

Gluconeogenesis

Liver : 80%

Renocortical : 20%

Lactic acid 30%

Glycerol 10%

Amino acids 40%

Tissue utilize glucose

In brain , glucose is still the

main fuel source.

Renal cortex

ⅲ. Triacylglycerol Metabolism

Fat mobilization

Fatty acidGKetone bodies

Heart Skeletal muscle

Part

Amino acid , but Glu deamination

2) Starvation in Long-term

ⅰ. Protein Metabolism

Muscle protein degradation

Urea

NH3 AcidismIn urine

( by ketosis )

ⅱ. Carbohydrate Metabolism

( almost equal to that in liver )

In kidney :

Gluconeogenesis

Lactic acidPyruvate

The main materials of gluconeogenesis in liver:

ⅲ. Triacylglycerol Metabolism

Fat mobilization

Fatty acidGKetone 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.