Enzymes (College Board AP® Biology): Flashcards
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Define enzyme.
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Cards in this collection (34)
Define enzyme.
Enzyme is a globular protein that acts as a biological catalyst, speeding up the rate of a chemical reaction without being used up.
Define active site.
Active site is the region of an enzyme with a specific shape that is complementary to a specific substrate.
Define activation energy.
Activation energy is the amount of energy needed to make a substrate unstable enough for a reaction to occur and products to form.
How do enzymes speed up the rate of a chemical reaction?
They lower the activation energy of the reaction, providing an alternative energy pathway.
What must be compatible for an enzyme-mediated reaction to occur?
The shape and charge of the substrate must be compatible with the enzyme's active site.
When a substrate binds to an enzyme's active site, an complex is formed.
When a substrate binds to an enzyme's active site, an enzyme-substrate complex is formed.
Why are enzymes described as having high specificity?
Each enzyme's active site has a specific shape complementary to only one type of substrate, so it catalyzes only a specific reaction.
True or False?
Enzymes are used up in the reactions they catalyze.
False.
Enzymes are recyclable — they are not used up or changed by the reaction and can be used again.
True or False?
An enzyme raises the activation energy of the reaction it catalyzes.
False.
An enzyme lowers the activation energy, which is how it speeds up the reaction.
Why are enzymes essential for life?
Virtually every metabolic reaction in living organisms is catalyzed by an enzyme, so these reactions could not occur fast enough without them.
Define denaturation.
Denaturation is the change to an enzyme's tertiary structure that occurs when its hydrogen bonds are broken, permanently damaging the active site so the substrate can no longer bind.
How does denaturation stop an enzyme from working?
Breaking the hydrogen bonds changes the enzyme's tertiary structure, which alters the shape of the active site so the substrate can no longer bind and the reaction cannot be catalyzed.
True or False?
Denaturation of an enzyme is always permanent.
False.
In some cases denaturation is reversible, allowing the enzyme to regain activity.
Temperature and pH outside the optimal range disrupt an enzyme's structure, causing it to .
Temperature and pH outside the optimal range disrupt an enzyme's structure, causing it to denature.
Define optimum temperature.
Optimum temperature is the temperature at which an enzyme catalyzes a reaction at its maximum rate.
Why does increasing temperature (up to the optimum) increase the rate of an enzyme-catalyzed reaction?
Molecules move more quickly, giving a higher frequency of successful collisions between substrate and active site, so more enzyme-substrate complexes form.
Why do low temperatures slow down enzyme-catalyzed reactions?
Molecules move slowly, so there is a lower frequency of successful collisions and less energy, meaning fewer enzyme-substrate complexes form.
What happens to the reaction rate as temperature rises above the enzyme's optimum?
The rate drops sharply, because the enzyme begins to denature.
At extremes of pH, hydrogen and ionic bonds break and the shape of the changes so substrate can no longer bind.
At extremes of pH, hydrogen and ionic bonds break and the shape of the active site changes so substrate can no longer bind.
How does pH affect enzyme activity differently from temperature?
pH does not affect collision rate; it disrupts the substrate's ability to bind the enzyme, reducing successful collisions until the active site changes shape so much that none can occur.
Pepsin works in the stomach. What does this suggest about its optimum pH?
Its optimum pH is around pH 2, matching the acidic stomach environment created by hydrochloric acid.
Provided substrate is not limiting, how does increasing enzyme concentration affect the reaction rate?
The initial rate increases linearly, because more active sites are available, giving a greater chance of enzyme-substrate complex formation.
True or False?
With a fixed amount of enzyme, adding more and more substrate raises the reaction rate without limit.
False.
The rate rises then plateaus at a saturation point, where all active sites are occupied.
Define limiting factor.
Limiting factor is a factor whose shortage caps the reaction rate; e.g. when substrate is limited, adding more enzyme will not further increase the rate.
Define enzyme inhibitor.
Enzyme inhibitor is a molecule that slows down or stops enzyme activity, decreasing the rate of reaction.
Where does a competitive inhibitor bind, and why can it do so?
It binds to the enzyme's active site.
It can do this because it has a similar shape to the substrate, so it competes with the substrate for the active site.
Where do noncompetitive inhibitors bind, and what effect does this have?
They bind to an allosteric site (not the active site).
This alters the shape of the active site, preventing the substrate from binding.
A competitive inhibitor binds to the of the enzyme, while a noncompetitive inhibitor binds to an allosteric site.
A competitive inhibitor binds to the active site of the enzyme, while a noncompetitive inhibitor binds to an allosteric site.
Why can increasing substrate concentration overcome a competitive inhibitor but not a noncompetitive one?
Competitive: substrate and inhibitor compete for the active site, so more substrate outcompetes the inhibitor.
Noncompetitive: the inhibitor does not bind the active site, so adding substrate cannot displace it.
What is the difference between a reversible and an irreversible inhibitor?
A reversible inhibitor does not form covalent bonds with the enzyme, so its effect can be reduced or reversed.
An irreversible inhibitor forms a covalent bond, permanently inactivating the enzyme.
True or False?
Increasing the concentration of an inhibitor decreases the rate of reaction.
True.
As inhibitor concentration rises the rate falls, and if it keeps increasing the reaction can stop completely.
True or False?
A competitive inhibitor lowers the maximum rate of reaction that can be reached.
False.
A competitive inhibitor lowers the initial rate, but the same maximum rate can still be reached with enough substrate. A noncompetitive inhibitor lowers the maximum rate.
Define allosteric site.
Allosteric site is a site on an enzyme, away from the active site, where a noncompetitive inhibitor can bind.
How does an irreversible inhibitor differ from a reversible one in how its effect can be removed?
A reversible inhibitor's effect can be removed by taking the inhibitor away or adding substrate.
An irreversible inhibitor cannot be removed this way, because it is covalently bonded to the enzyme.
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