Do Now 12/112 /1 Enzyme Inhibitors notes 22

Briefly sketch three graphs that show the initial reaction rates of enzyme catalyzed reactions with varying temperature, enzyme concentration, and substrate concentration. Label each graph, and compare the shape of each curve.

Enzyme InhibitorsINB Pg 22

Inhibition• Active site of enzyme fits perfectly to substrate• However, it is possible for another molecule to bind to an

enzymes active site if it is very similar in shape to the enzyme’s substrate

• This would inhibit the enzyme’s function

Substrate

Enzyme

Inhibitor

Competitive inhibition

• A competitive inhibitor is any compound which closely resembles the chemical structure and molecular geometry of the substrate. • The inhibitor competes for

the same active site as the substrate molecule.

Competitive inhibitors• If a competitive inhibitor binds only briefly to the active

site, there is competition between it and the substrate for the site• Which one “wins” depends on which has a higher

concentration• http://www.youtube.com/watch?v=duN73LFWNlo

Competitive inhibition• Usually reversible, because the inhibitor does not

permanently bind to the enzyme• Inhibition can be reversed by increasing concentration of

substrate

Competitive inhibition example

• Methanol poisoning occurs because methanol is oxidized to formaldehyde and formic acid which attack the optic nerve causing blindness. • Ethanol is given as an antidote for methanol poisoning

because ethanol competitively inhibits the oxidation of methanol. • Ethanol is oxidized in preference to methanol and

consequently, the oxidation of methanol is slowed down so that the toxic by-products do not have a chance to accumulate.

Non-competitive inhibition

• Inhibition does not depend on substrate concentration• Inhibitor will block enzyme function at low and

high concentration of substrate• Two main types• Irreversible inhibition• Allosteric inhibition

Non-competitive irreversible inhibition

• Sometimes, inhibitor can remain permanently bonded with the active site and therefore will cause an irreversible block to the substrate

• No competition occurs because no matter how much substrate is present, the active sites will be permanently occupied by the inhibitor

• http://www.youtube.com/watch?v=PILzvT3spCQ

Non-competitive irreversible inhibition

• Penicillin works by permanently occupying the active site of an enzyme that is essential for the synthesis of bacterial cell walls

• Penicillin and other β-lactam antibiotics act by inhibiting penicillin-binding proteins, which normally catalyze cross-linking of bacterial cell walls.

Non-competitive allosteric inhibition

• If a molecule can bind to another site on the enzyme (besides active site) and stop enzyme function, it is an allosteric inhibitor

• Can disrupt the 3D shape of enzyme molecule so active site cannot accept substrate

• Can be reversible or irreversible

End-product inhibition• As an enzyme converts substrate to product, it is slowed down

because the end product binds to another part of the enzyme and slows down its function• Called negative feedback inhibition• The more product = slower reaction• Controls metabolic processes to stop enzyme from “running wild”

End product inhibition

E

EE

E

S

S P

I

I

Enzyme modeling– 10 pts• Using a sheet of computer paper and provided construction

paper, design a poster that demonstrates:• Competitive inhibition• Irreversible non competitive inhibition• Allosteric inhibition• End-product (negative feedback) inhibition

• Be sure to label the substrate, enzyme, and inhibitor!• On the back of each picture, briefly describe (1-2 sent.) the

mechanism of inhibition• Due Wednesday @ end of class (will have time to work with

partners in beginning before quiz)

Chapter 3 quizConcepts to know:• lock and key hypothesis• Induced fit hypothesis• vocab: substrate, enzyme, active site, activation energy• reaction curves• effects of temperature, and pH, enzyme concentration,

and substrate concentration on enzyme controlled reactions• Types of enzyme inhibition