Enzymes Flashcards
ENZYMES AS BIOLOGICAL CATALYSTS MECHANISMS OF ENZYME ACTIVITY ENYZMES KINETICS EFFECTS OF LOCAL CONDITIONS ON ENZYME ACTIVITY REGULATION OF ENZYME ACTIVITY
Mechnisms for reducing activation energy
Transient covalent bonds Reactant destabilization Adjusting substrate proximity Microenvironment adjustments Transition state stabilization
Enzyme classification
- Oxidoreductases
- Catalyze oxidation-reduction reactions
- Often have cofactor (NAD+ etc)
- Reductant: electron donor
- Oxidant: electron acceptor- Transferases
- Catalyze movement of functional groups (i.e aminotransferase, kinases)
- Hydrolases
- Catalyze the breaking of a compound using water (i.e phosphatase, peptidase, nuclease, lipase)
- Lyases
- Catalyze the breaking of a compound without water
- Catalyze synthesis of two small compounds into one, w/o energy input (i.e synthases)
- Isomerases
- Catalyze rearrangement of bonds within a molecule
- used in stereoisomers and constitutional isomers
- Ligases
- Catalyze addition or synthesis of two large compounds
- Transferases
Enzyme type theories
- Lock and key theory
- The active site conformation is fixed
- Substrate either fits or doesn’t- Induced fit Model
- More accepted than “lock and key”
- Posits that enzymes active site and substrate changes conformation before, during and after catalysis
- Requires energy to conform (endergonic)
- Induced fit Model
What are cofactors
INORGANIC molecules needed for protein function
Types of cofactors
Free metal cations
polyatomic ions
what is Holoenzyme
An enzyme with all necessary coenzymes and cofactors present.
what is Apoenzyme
Enzyme without cofactors or coenzymes.
what is coenzyme
Extrinsic and organic molecules needed for protein function derived from adenine and vitamins
Prosthetic group
Tightly bound (covalently) coenzymes
impact of enzymes
Reduce activation energy (Ea),
Increase reaction rate
No effect thermodynamics
Requires energy
Purpose of Michaelis Constant (Km)
Measure and compare monomeric enzyme-substrate affinity of two or more monomeric enzymes
*note: an enzyme that is fast acting will have a high Vmax and substrate affinity but a low Km value. A slow acting enzyme will have a low Vmax and substrate affinity but a high Km value in order to reach half of its maximum potential rate.
For a given reaction using enzyme A, Vmax is found to be 2.3 mmol/sec. When enzyme B is introduced to the substrate instead, Vmax is 1.4 mmol/sec. Which statement is true regarding this situation?
1) Enzyme A has a higher Km value.
2) Enzyme A has a higher enzyme substrate affinity than Enzyme B.
3) Enzyme B is the faster enzyme.
Answer is: 2. Enzyme A has a higher enzyme substrate affinity than Enzyme B.
Purpose of Lineweaver-Burk plots
For understanding enzyme regulation with inhibitors. Can distinguish inhibition types.
When comparing two Lineweaver-Burk plots, the enzyme that produces the graph with the steepest slope will have:
the largest Km value.
what structural factor is known for Cooperative binding
Substrate binding at one subunit induces affinity change at others.
affected by inhibitors and not the changes in substrate concentration.
Km
The four subunits of hemoglobin exhibit cooperative binding. As PO2 of the blood changes entering the pulmonary veins, the KM of hemoglobin towards this substrate:
Remains unchanged
A man has high blood pressure partially due to a high-salt diet. What effect does the increased salt concentration in his blood have on enzyme activity?
Enzyme activity is not changed.
Increasing salt concentration (in vitro) may cause denaturation of what
Enzymes
Which pH would best support an enzyme involved in the initial steps of chemical protein digestion?
pH of 2
because chemical digestion occurs in the stomach in which the pH is 2
Enzyme A has a Vmax of 4 moles / second. A competitive inhibitor is introduced that lowers the rate of product formation to 2 moles / second. Excess substrate is then added. The new Vmax could be?
4moles/ second
*note: an increase in substrate will overcome the presence of inhibitor so therefore the rate will change.
True or false:
A noncompetitive inhibitor can bind an enzyme if the substrate is bound or not.,
TRUE
Enzyme A has a Vmax of 4 moles / second. A noncompetitive inhibitor is introduced that lowers the rate of product formation to 2 moles / second. Excess substrate is then added. The new Vmax could be?
2moles/ second
which inhibitor has affinity for both enzyme and substrate-enzyme complex
Noncompetitive inhibitor
Which inhibitor has ONLY an affinity for enzyme-substrate complex
Uncompetitive inhibitor
This inhibitor has either an affinity for enzyme with no substrate or enzyme-substrate complex but not Both at the same time.
Mixed inhibitor
Why is irreversible inhibition more permanent than “reversible” noncompetitive inhibition.?
Removing noncompetitive inhibitors reverses their inhibition, but removing irreversible inhibitors does not.
True or False
There are allosteric activators and inhibitors
TRUE
What is the main reason why many enzymes are synthesized as inactive zymogens?
Inactive zymogens can be activated only if, when, and where they are needed.
An enzyme catalyzes the conversion of protein A to protein B. The rate of this conversion greatly diminishes after adding compound X, but this is only true when protein A is not in excess. Compound X is most likely
competitive inhibitor.
Consider a reaction catalyzed by enzyme A with a Km value of 5x10^-6M and Vmax of 20 mol/min. At a concentration of 5x10^-4M substrate, the rate of the reaction will be?
20mmom/min
The concentration is 100 times higher. At such high value the enzyme is at or near its Vmax.
How does the Temperature change with and without an enzyme catalyst for a reaction.
The temperature is generally lower with a catalyst than without.
in which case of enzyme activity does proximity induce change
induced fit model
If the enzyme-catalyzed reaction E + S ⇋ ES ⇋ E + P is proceeding at or near the Vmax of E, what can be deduced about the relative concentrations of S and ES
S is abundant, [ES] is at its highest point
What relative values of Km and kcat would describe an enzyme with a high catalytic efficiency
kcat is the turnover number, is equal to Vmax /[Et]
Catalytic efficiency is described by the specificity constant, which is equal to kcat /Km
An enzyme with a high catalytic efficiency could be described by having a Low Km and high kcat
For the enzyme-catalyzed reaction E + S ⇋ ES ⇋ E + P, what is the value of Km if [S]=25, and the initial reaction velocity is half of Vmax
The Michaelis-Menten equation states that Vnot = (Vmax [S])/(Km + [S]).
If the initial reaction velocity is half of Vmax ,then (Vmax/2) =(Vmax[S])/(Km +[S]).V, by Vmax
Therefore, Km+[S] = 2 [S], and Km = [S].K,
If [S]=25, and the initial reaction velocity is half of Vmax then Km= 25
With respect to the binding of regulatory compounds, what properties define an enzyme as being allosteric?
Reversible, noncovalent binding of regulatory compounds
Reasons for decreased enzyme activity
Decreased substrate level
Denaturation
Inhibition
Increase of enzymes concentration does what to a Michaelis-Menten plot and Lineweaver-Burk plot
Increase level of Vmax
High Km
Low affinity → slow reaction rate (V)
note: rate limiters are slow steps in a reactions
