AP Biology : Understand competitive and noncompetitive inhibition

Study concepts, example questions & explanations for AP Biology

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Example Questions

Example Question #1 : Enzymes

Which of the following is true regarding competitive and noncompetitive inhibition?

   I. Both can be overcome by increasing the substrate concentration

   II. Competitive inhibition induces changes to the active site

   III. Noncompetitive inhibition has no effect on the enzyme affinity for substrates

Possible Answers:

II and III

II only

I only

III only

Correct answer:

III only

Explanation:

Statement I is false because increasing the substrate concentration will only help overcome competitive inhibition. Noncompetitive inhibition can only be overcome if the inhibitor is removed from the enzyme.

Statement II is also false because competitive inhibitors do not change the active site. They bind to the active site and prevent substrates from binding. Noncompetitive inhibitors bind elsewhere on the enzyme and alter the shape of the active site, thereby preventing substrate binding.

Statement III is true because noncompetitive inhibition does not affect the enzyme affinity for substrates. The enzyme still has the same affinity, but the substrates can’t bind because of the altered active site.

Example Question #1 : Enzymes

An antibiotic binds an enzyme, causing it to produce substrate C of a metabolic pathway instead of substrate A of the same pathway. Substrate C ultimately inhibits the enzyme in the normal course of the pathway.

In this metabolic pathway, Substrate C is acting as a(n) __________.

Possible Answers:

transcription factor

negative feedback inhibitor

noncompetitive inhibitor

nucleic acid

competitive inhibitor

Correct answer:

negative feedback inhibitor

Explanation:

Negative feedback interrupts a metabolic pathways by producing a substrate that inhibits enzymes in the beginning steps of the metabolic cycle. If a chemical is "mimicking" substrate C  or causing Substrate C to be produced before other steps in a cycle, the enzyme is inhibited by the excess of substrate C thus the pathway can not continue. Most such molecules are proteins that interact with enzymes. 

Example Question #1 : Enzymes

Substrates formed downstream in a metabolic pathway that act to increase the progression of that metabolic pathway are said to exhibit a __________ mechanism.

Possible Answers:

hormonal

negative feedback

competitive inhibition

noncompetitive inhibition

positive feedback

Correct answer:

positive feedback

Explanation:

A substrate that acts as a "positive motivator" of, or to enhance a metabolic pathway, is also known as a positive feedback regulator or a substance that has a positive feedback mechanism.

Example Question #1 : Enzymes

If an antibiotic binds the active site of an enzyme but does not change the structure of that enzyme, once removed, the enzyme returns to normal function. In this case, the antibiotic is acting via what enzyme interaction?

Possible Answers:

Denaturation

Noncompetitive inhibition

Positive feedback

Competitive inhibition

Negative feedback

Correct answer:

Competitive inhibition

Explanation:

Competitive inhibition occurs when an substrate or inhibitor compete with the normal substrate for binding the active sight of an enzyme. The proper functioning of the enzyme depends on the concentration ratio of inhibitor to enzyme or substrate to enzyme. The competitive inhibition of the enzyme in this case by the antibiotic has potentially bactericidal or bacteriostatic effect on the bacteria until that antibiotic concentration decreases. Negative feedback involves the product of a set of metabolic reactions inhibiting the formation of a precursor of that metabolic pathway, thereby decreasing its own production.

Example Question #1 : Enzymes

You are reading about the functions of a unique chemical compound. This compound works on enzymes throughout the body by altering the shape of the enzyme without blocking the active site. This compound functions via which mechanism?

Possible Answers:

Noncompetitive inhibition

Neutral inhibition

Positive inhibition

Competitive inhibition

Feedback inhibition

Correct answer:

Noncompetitive inhibition

Explanation:

Noncompetitive inhibition is a type of enzymatic alteration that results in changes to enzymatic function without alterations to the active site. If the active site was to be blocked, this compound would function via competitive inhibition. The other terms do not describe any type of enzymatic inhibition process in the human body. Be able to distinguish the difference between competitive and noncompetitive inhibition.

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