Allosteric Enzymes - Biochemistry - Lecture Slides, Slides of Biochemistry

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Allosteric enzymes

AllostericAllosteric EnzymesEnzymes

Regulatoryenzymes

Have active siteand modulatorysite

Do not obey Michaelis

Activated by substrate and other positive modulators

EnzymesEnzymes

Catalyse anirreversiblereaction

Inhibited byend product

Michaelis Menten Kinetics

substrate and other positive modulators

Normally composedof multiple subunits(identical/ different)

Allosteric Enzymes^ •^ Molecules that bind to allostericsites are called

effectors

or

modulators • Binding of a substance to one binding site increases or

EE

Active site

binding site increases or decrease the binding to anothersite^



Cooperative binding • Effectors may be positive ornegative • Effectors may be homotropic orheterotropic

Allosteric site

Substrate cannot fitinto the

active site Inhibitor fits into^ allosteric site Reversible inhibition^ (not competitive )(not competitive ) [inhibitor ]

^ the enzyme’s conformation changes back to itsactive form

-^ There are two conformational forms,

T^ (taut or

tight) and

R^ (relaxed) , which are in equilibrium

Properties of allosteric enzymes^ ▫^

T state: lower affinity for substrate ▫ R state: higher affinity for substrate • Modulators and substrates can bind to the

R

form^

only; the inhibitors can bind to the

T

form.

Allosteric T to R transition Concerted

model

Sequential model (The binding of substrate induces a conformational

E-I^ T

ET^

ER^

E-SR

I^ I

S S

Concerted

model (All subunits must be insame conformation, T or R(they all change together))

induces a conformational change from the T form tothe R form increasing thesites available to S)^



^ Cooperativity

The Sequential Model for AllostericRegulation(a) S binding can, by induced fit,cause a conformation changein the subunit to which itbinds.^ (b)^ If^ subunit interactions are^ (b)^ If^ subunit interactions are^ tightly coupled, binding of S toone subunit may cause theother subunit to assume aconformation having a greateror lesser affinity for S. That is,the ligand-inducedconformational change in onesubunit can affect the adjoiningsubunit.

The Sequential Model for AllostericRegulation

-^ Ligands such as S are

positive homotropic

effectors:

binding of one equivalent enhances the binding of additional equivalents of S to the same protein molecule

(substrate =

effector

Heterotropic and homotropiceffectors^ same protein molecule

(substrate =

effector

vo [S]

Mechanism and Example of Allosteric Effect

S^

S R

R

S^ R

A

R = Relax(active)

Allosteric site

Homotropic (+) Concerted Allosteric site

Kinetics

Cooperation

Models

(+)^ T R^ T T

S A S^ I

T^

(+) vo [S] vo^ (-)^ [S]

X^

X

X^ T = Tense(inactive)

Heterotropic (+) SequentialHeterotropic (-) Concerted (+)^ (-)

ATCase displays sigmoidal kinetics •^ Substrate binding to oneactive site converts enzymeto R state ,increasing their^ activity:

active sites show activity:

active sites show cooperativity (Homotrophic effect)

Aspartate Transcarbamoylase as anallosteric enzyme^ •^ CTP (

the end product)

is a negative modulator and

ATP is a positive modulator^ ▫ feedback inhibition

-^ Catalytic subunit consists of 3 chains (c

) and 3

regulatory subunit consists of 2 chains (r

Phosphofructokinase as an allostericenzyme^ CHOPO^2

2 −^3

CHOH^2 O 6

1

CHOPO^2

2 −^3

CHOPO^2

2 − 3 O 6

1

Phosphofructokinase^2 O H H OH^

H H^ O HO 5 4

(^23)

(^2) OH 3

H OH^

H H^ O HO 5 4

(^23)

ATP^ ADP^2 +Mg

fructose-6-phosphate

fructose-1,6-bisphosphate