Lecture 19 Flashcards by Chanmi Park

What is Vmax

measure of the catalytic effectiveness of the enzyme

higher the Vmax, more product formed per minute

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What is Km (Michaelis constant)?

concentration of a substrate necessary to produce half of Vmax

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At high concentration of substrate, what is the order of reaction and why?

Zero order, because reaction is independent of substrate concentration ([s]»>Km)

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At low concentration of substrate, what is the order of reaction and why?

First order, because reaction is dependent on substrate concentration. ([s]<

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What does it mean if there is increase in Km?

The enzyme reflects a low affinity of enzyme for the substrate.

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How is Km related to affinity of enzyme for substrate?

Smaller the Km, larger the affinity of enzyme.

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Michaelis and Menten equation

V = [Vmax * [S]] / Km+[S]

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Lineweaver and Burk equation

1/V = [Km + [S]] / Vmax * [S]

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What does y-intercept mean in Linewevaer and Burk graph?

1/Vmax

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What does X-intercept mean in Linewevaer and Burk graph?

-1/Km

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2 types of inhibition

irreversible and reversible

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What bond is formed for irreversible inhibition?

covalent bond

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3 types of reversible inhibition

Competitive, noncompetitive, uncompetitive

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How can competitive inhibition overcome?

increase substrate concentration

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Example of competitive inhibitor

Statin (Lovastatin)

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Use and Mechanism of statin

Statin is used to decrease blood cholesterol level. Statin blocks HMG-CoA reductase, which make cholesterol. So since cholesterol is not synthesized in liver cell, cells draws cholesterol from blood, lowering blood cholesterol level.

What is the kinetic effect of competitive inhibitor

Unchanged Vmax, increased Km

Effect of competitive inhibitor on Lineweaver-Burk plot

steeper curve with same y intercept (Same 1/Vmax)

Role of non-competitive inhibitor

interact with functional groups other than those at active site, so interact with both enzyme and ES complex.

Can non-competitive inhibitor overcome by increase in [S]?

Kinetic effect of noncompetitive inhibitor

Vmax decreased, Km unchaged

Effect of noncompetitive inhibitor on Lineweaver-Burk plot

increased y intercept (1/Vmax), Same x intercept

As the concentration of noncompetitive inhibitor increase, graph gets steeper but Km stays the same.

Example of irreversible inhibitors (4)

Heavy metals

PB
Organophosphates
Cyanide, sulfide
Aspirin

Target enzyme of organophosphate

acetylcholinesterase

Reaction of acetylcholinesterase

acetylcholine > choline + acetate

Target enzyme of aspirin

Cyclooxygenase (COX) (prostaglandin synthase)

Symptoms of organophosphates inhibition

continuous muscle contractions such as seizure, respiratory arrest

What is the name of active ingredient of organophosphate pesticide?

DFP, which bind to serine residue of acetylcholinesterase, forming covalent bond

Role of penicillin

inhibits irreversibly bacterial glycoprotein transpeptidase (a serine protease), which creates cell wall.

What is the role of thromboxane and where is it synthesized?

Promotes platelet aggregation, vasoconstriction, bronchoconstriction, lymphotic proliferation. Synthesized in platelet

Synthesis of thromboxanes (2)

1. Arachidonate is turned into prostaglandin H2 by cyclooxygenase 2. Then prostalglandin H2 is turned into thromboxane by thromboxane synthetase

Mechanism of NASAIDs on COX

NSAIDs such as aspirin Inhibits COX 1 and 2 via covalent acetylation of Ser which lies along the wall of a tunnel inside the enzyme. As a result acetylation blocks the tunnel so that arachidonic acid is unable to migrate to active site.

What is the effect of baby aspirin?

81mg daily, it irreversibly inhibits COX1 in platelets, preventing platelet aggregation in blood. Improves vascular health.

What is suicide inhibitor?

Substrate analogue that is converted enzymically to a reactive specieis that combines irreversibly with the same enzyme.

Mechanism of hypoxanthine

Normally hypoxanthine is converted to xanthine by xanthine oxidase then to uric acid by the same enzyme which builds up in joints . When uric acid is crystallized at low temperature, it causes inflammation and pain.

What dies allopurinol do ?

It is suicidal enzyme, an analogue to hypoxantine. It binds with xanthine oxidase to be converted to the active enzyme, alloxanthine. Then alloxanthine binds to xanthine oxidase irreversibly which inhibits production of uric acid.

What disease does allopurinol treat?

Gouts

Why does hypoxanthine and xanthine produce less inflammatory response than uric aicd?

They are more water soluble than uric aicd