Enzymes

Question 1

Mechnisms for reducing activation energy

Answer 1

Transient covalent bonds
Reactant destabilization
Adjusting substrate proximity
Microenvironment adjustments
Transition state stabilization

Question 2

Enzyme classification

Answer 2

• 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

Question 3

Enzyme type theories

Answer 3

• 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)

Question 4

What are cofactors

Answer 4

INORGANIC molecules needed for protein function

Question 5

Types of cofactors

Answer 5

Free metal cations

polyatomic ions

Question 6

what is Holoenzyme

Answer 6

An enzyme with all necessary coenzymes and cofactors present.

Question 7

what is Apoenzyme

Answer 7

Enzyme without cofactors or coenzymes.

Question 8

what is coenzyme

Answer 8

Extrinsic and organic molecules needed for protein function derived from adenine and vitamins

Question 9

Prosthetic group

Answer 9

Tightly bound (covalently) coenzymes

Question 10

impact of enzymes

Answer 10

Reduce activation energy (Ea),
Increase reaction rate
No effect thermodynamics
Requires energy

Question 11

Purpose of Michaelis Constant (Km)

Answer 11

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.

Question 12

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 12

Answer is: 2. Enzyme A has a higher enzyme substrate affinity than Enzyme B.

Question 13

Purpose of Lineweaver-Burk plots

Answer 13

For understanding enzyme regulation with inhibitors. Can distinguish inhibition types.

Question 14

When comparing two Lineweaver-Burk plots, the enzyme that produces the graph with the steepest slope will have:

Answer 14

the largest Km value.

Question 15

what structural factor is known for Cooperative binding

Answer 15

Substrate binding at one subunit induces affinity change at others.

Question 16

affected by inhibitors and not the changes in substrate concentration.

Answer 16

Km

Question 17

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:

Answer 17

Remains unchanged

Question 18

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?

Answer 18

Enzyme activity is not changed.

Question 19

Increasing salt concentration (in vitro) may cause denaturation of what

Answer 19

Enzymes

Question 20

Which pH would best support an enzyme involved in the initial steps of chemical protein digestion?

Answer 20

pH of 2

because chemical digestion occurs in the stomach in which the pH is 2

Question 21

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?

Answer 21

4moles/ second

*note: an increase in substrate will overcome the presence of inhibitor so therefore the rate will change.

Question 22

True or false:

A noncompetitive inhibitor can bind an enzyme if the substrate is bound or not.,

Answer 22

TRUE

Question 23

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?

Answer 23

2moles/ second

Question 24

which inhibitor has affinity for both enzyme and substrate-enzyme complex

Answer 24

Noncompetitive inhibitor

Question 25

Which inhibitor has ONLY an affinity for enzyme-substrate complex

Answer 25

Uncompetitive inhibitor

Question 26

This inhibitor has either an affinity for enzyme with no substrate or enzyme-substrate complex but not Both at the same time.

Answer 26

Mixed inhibitor

Question 27

Why is irreversible inhibition more permanent than “reversible” noncompetitive inhibition.?

Answer 27

Removing noncompetitive inhibitors reverses their inhibition, but removing irreversible inhibitors does not.

Question 28

True or False

There are allosteric activators and inhibitors

Answer 28

TRUE

Question 29

What is the main reason why many enzymes are synthesized as inactive zymogens?

Answer 29

Inactive zymogens can be activated only if, when, and where they are needed.

Question 30

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

Answer 30

competitive inhibitor.

Question 31

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?

Answer 31

20mmom/min

The concentration is 100 times higher. At such high value the enzyme is at or near its Vmax.

Question 32

How does the Temperature change with and without an enzyme catalyst for a reaction.

Answer 32

The temperature is generally lower with a catalyst than without.

Question 33

in which case of enzyme activity does proximity induce change

Answer 33

induced fit model

Question 34

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

Answer 34

S is abundant, [ES] is at its highest point

Question 35

What relative values of Km and kcat would describe an enzyme with a high catalytic efficiency

Answer 35

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
​

Question 36

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

Answer 36

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

Question 37

With respect to the binding of regulatory compounds, what properties define an enzyme as being allosteric?

Answer 37

Reversible, noncovalent binding of regulatory compounds

Question 38

Reasons for decreased enzyme activity

Answer 38

Decreased substrate level
Denaturation
Inhibition

Question 39

Increase of enzymes concentration does what to a Michaelis-Menten plot and Lineweaver-Burk plot

Answer 39

Increase level of Vmax

Question 40

High Km

Answer 40

Low affinity → slow reaction rate (V)

note: rate limiters are slow steps in a reactions