Chapter 6.3: Enzyme Inhibition

CHEM 7784

Biochemistry

Professor Bensley

CHAPTER 6.3 Enzyme Inhibition

– Various types of enzyme inhibition

Today’s Objectives: (To learn and understand the)

Enzyme Inhibition

Inhibitors are compounds that decrease enzyme’s activity

• Irreversible inhibitors (inactivators) react with the enzyme

• Reversible inhibitors bind to, and can dissociate from the enzyme

• Reversible inhibitor can bind:

– To the free enzyme and prevent the binding of the substrate

– To the enzyme-substrate complex and prevent the reaction

Competitive Inhibition

Lines intersect at the y-axis

Uncompetitive InhibitionLines are parallel

Mixed InhibitionLines intersect left of the y-axis

MAKE SURE YOU DISTINGUISH THIS FROM NON-COMPETITIVE INHIBITION!!!!

Viagra (Sildenafil Citrate)

Medicinal Inhibitor Example

Irreversible Inhibitors

H3C O P

O

S C

C

H

O

O CH2CH3

C O CH2CH3

O

S

CH3

CH2

H C

CH3

O

CH3

P

F

O

O C

CH3

H

CH3

Diisopropyl fluorophosphate(nerve gas)

H3C O P

O

S

S

CH3

NO2

parathion

malathion

•Organophosphates•Inhibit serine hydrolases•Acetylcholinesterase inhibitors

Kinetics of Multisubstrate Reactions

E + A + B <-> E + P + Q

• Sequential Reactions (Ternary Complex

formed)

a) Random

b) Ordered

• Ping-Pong Reactions

Sequential Reactions

E EA (EAB) (EPQ) EQ E

A B P Q

AB PQ

A B

E

EA

EB

(EAB)(EPQ)

P Q

EQ

EP

E

Ordered

Random

Ping-Pong Reactions

E (EA)(FP) (F) (FB)(EQ) E

A BP Q

•In Ping-Pong rxns first product released before second substrate binds

•When E binds A, E changes to F

•When F binds B, F changes back to E

Lineweaver-Burke Plot of Multisubstrate Reactions

Increasing [B]

Increasing [B]

Sequential Ping-Pong

Vmax doesn’t changeKm changes

Both Vmax & Km change

1/Vo

1/[S]

1/Vo

1/[S]