Enzimas

Enzyme Inhibition
• Many pharmaceuticals are enzyme inhibitors
• Inhibitors bind reversibly, inactivators - irreversibly
• Mode of action can be informative
– Competitive inhibitors interferewith substrate binding
• Free enzyme concentration is reduced E –> EI
• High substrate concentrations can overcome inhibition Vmax
unchanged

– Uncompetitive inhibitors bind to ES complex
• ESintermediate concentration is reduced ES –>ESI
• Vmax and Km are reduced by the same factor

– Mixed inhibitors bind to both E and ES

Inhibitor Binding Equilibria
• Inhibitor binding is assumed tobe at equilibrium

k 1

E + S ←→

k −1
+
Icomp
cK I
EI

ES

k 2→ E + P


+
Iuncomp
cK I '
ESI

• Mixed
Inhibitors
bind to both
E and ES

• ET is reduced; similar ineffect to inactivators
• Km effects depend on relative affinities of I for E and ES

Competitive inhibition

• Conservation
[E]T = [E] + [ES] + [ESI]

• Conservation

Slope increases
As(1+[I]/ KI )

[E]T = [E] + [ES] + [EI]

1/v

KM=[E][S]/[ES]
[E] = [E]T - [ES] - [EI]
[E] = [E]T - [ES] - [E][I]/ KI
[E](1+[I]/ KI ) = [E]T - [ES]

[E]T = [E] + [EI] +[ES] + [ESI]

Interceptincreases
As (1+[I]/ KI’ )

1/[S]

Mixed
inhibition

1/[S]

Regulation
• Through variation in enzyme levels

KI = [E][I]/[EI]
KI ‘= [ES][I]/[ESI]
In general, KI KI ‘

Both Slope

–Long term adjustments in synthesis, degradation and
transport rates

• Through variation in enzyme activity
1/v

– Homotropic and heterotrobic allosteric effectors
– Reversible covalentmodification

KM=[E][S]/[ES]
[E] (1+[I]/ KI ) =
[E]T - [ES] (1+[I]/ KI’ )

1/v

Intercept
unchanged

• Equilibrium

• Steady State

Slope unchanged

KI ‘= [ES][I]/[ESI]
KM=[E][S]/[ES]
[E] =[E]T - [ES] - [ESI]
[E] = [E]T - [ES] - [ES][I]/ KI‘
[E] = [E]T - [ES] (1+[I]/ KI’ )

KI = [E][I]/[EI]

• Conservation

• Equilibrium
• Steady State

• Equilibrium
• Steady State...