Enzyme and Proteins
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82 Terms
An enzyme is best described as what?
A biological catalyst that lowers activation energy without being consumed.
The molecule an enzyme acts upon.
Substrate
The location where the substrate binds an enzyme.
Active Site
The molecule produced after an enzyme
catalyzed reaction.
What is the primary function of enzymes?
Increase reaction rate by lowering activation energy.
Do enzymes change ΔG of a reaction?
No
Do enzymes change the equilibrium constant (Keq)?
No
Do enzymes change the overall products formed?
No
What do enzymes lower?
Activation Energy
What is activation energy?
The energy required to start a chemical reaction.
The tendency of an enzyme to bind only certain substrates.
Specificity
The ability of an enzyme to recognize only one or a few substrates is due to what?
Shape of the active site.
The model stating that enzyme and substrate fit exactly together.
Lock and Key Model
The model stating that enzyme shape changes upon substrate binding.
Induced Fit Model
Most enzymes are composed of what type of molecule?
Proteins
Enzymes ending in "ase" usually catalyze what?
the breakdown, modification, or synthesis of a specific substrate
Which model suggests enzymes change shape during substrate binding?
Induced Fit Model
Which model suggests enzymes already perfectly fit the substrate?
Lock and Key Model
Which quantity changes when enzymes are added: Activation Energy or ΔG?
Activation Energy
Which quantity does NOT change when enzymes are added: Activation Energy or Equilibrium Constant?
Equilibrium Constant
Which molecule binds the enzyme?
Substrate
Which molecule leaves the enzyme?
Product
A scientist adds an enzyme to a reaction. What happens to the activation energy?
It decreases.
A scientist adds an enzyme to a reaction. Does the equilibrium change?
No.
A scientist doubles the amount of enzyme while substrate remains abundant. What happens to reaction rate?
It increases.
A mutation changes the shape of an enzyme's active site. What is most likely affected?
Substrate binding.
A substrate no longer fits inside an enzyme's active site. What property has been altered?
Specificity.
An inorganic molecule required for enzyme function.
Cofactor
An organic molecule required for enzyme function.
Coenzyme
Many coenzymes are derived from what?
Vitamins
Metal ions commonly serving as cofactors.
Zn²⁺, Mg²⁺, Fe²⁺, Cu²⁺
Vitamin
derived electron carriers used as coenzymes.
A protein enzyme without its cofactor.
Apoenzyme
A complete functional enzyme with its cofactor bound.
Holoenzyme
Which is organic: Cofactor or Coenzyme?
Coenzyme
Which is usually a metal ion: Cofactor or Coenzyme?
Cofactor
Which enzyme is inactive: Apoenzyme or Holoenzyme?
Apoenzyme
Which enzyme is active: Apoenzyme or Holoenzyme?
Holoenzyme
A patient has severe zinc deficiency. Which type of enzyme helper is most likely missing?
Cofactor
A vitamin deficiency decreases enzyme activity. Which helper molecule is most likely affected?
Coenzyme
The maximum reaction rate when all enzyme active sites are occupied.
Vmax
The substrate concentration at which reaction velocity equals one half of Vmax.
Michaelis constant (Km)
A low Km indicates what property?
High substrate affinity
A high Km indicates what property?
Low substrate affinity
The graph describing enzyme velocity as substrate concentration increases.
Michaelis-Menten Plot
Which enzyme has greater substrate affinity: Low Km or High Km?
Low Km
Which enzyme binds substrate more tightly: Km = 2 μM or Km = 50 μM?
Km = 2 μM
What does Vmax represent: Binding affinity or Maximum reaction rate?
Maximum reaction rate
An enzyme reaches a point where adding more substrate no longer increases reaction rate. What has been reached?
Vmax
Two enzymes have identical Vmax values, but one has a lower Km. Which enzyme binds substrate better?
The enzyme with the lower Km.
A scientist observes that half maximal velocity occurs at 10 μM substrate concentration. What is the Km?
10 μM
An inhibitor that competes with substrate for the active site.
Competitive Inhibitor
An inhibitor that binds away from the active site and decreases enzyme activity.
Noncompetitive Inhibitor
An inhibitor that binds only the enzyme substrate complex.
Uncompetitive Inhibitior
An inhibitor that can bind either the free enzyme or enzyme substrate complex with different affinities.
Mixed Inhibitor
Which inhibitor binds the active site?
Competitive
Which inhibitor binds an allosteric site?
Noncompetitive
Which inhibitor binds only the enzyme substrate complex?
Uncompetitive Inhibitor
Which inhibitor can bind both enzyme and enzyme substrate complex?
Noncompetitive Inhibitor
Competitive inhibition changes what kinetic parameter?
Increases Km
Competitive inhibition changes Vmax how?
No Change
Why does competitive inhibition increase Km?
More substrate is required to outcompete the inhibitor.
Can increasing substrate overcome competitive inhibition?
Yes
Noncompetitive inhibition changes Km how?
No Change
Noncompetitive inhibition changes Vmax how?
Decreases
Can increasing substrate overcome noncompetitive inhibition?
No
Uncompetitive inhibition changes Km how?
Decreases
Uncompetitive inhibition changes Vmax how?
Decreases
Mixed inhibition changes Km how?
Usually changes (may increase or decrease)
Mixed inhibition changes Vmax how?
Decreases
A scientist adds a large amount of substrate and enzyme activity returns to normal. Which inhibitor was present?
Competitive Inhibitor
An inhibitor decreases Vmax but Km remains unchanged. What type of inhibitor is present?
Noncompetitive Inhibitor
An inhibitor decreases both Km and Vmax. What type of inhibitor is present?
Uncompetitive Inhibitor
A drug binds outside the active site and cannot be overcome by adding substrate. What type of inhibitor is it?
Noncompetitive Inhibitor
A mutation alters the active site of an enzyme. Which type of inhibitor will be most affected?
Competitive Inhibitor
A drug binds only after the substrate is already bound. What inhibitor is this?
Uncompetitive Inhibitor
Increasing enzyme concentration while substrate is abundant causes…
Increased reaction rate.
Increasing substrate concentration eventually causes…
The reaction to reach Vmax.
Increasing temperature above the optimum causes…
Enzyme denaturation and decreased activity.
Increasing pH far above the enzyme's optimum causes…
Decreased enzyme activity.
Denaturation causes what change?
Loss of protein structure and enzyme function.
Most enzymes function best under what conditions?
Their optimal temperature and pH.