Enzymes

Question 1

What is Km?

Answer 1

The Michaelis Menten constant. It is the concentration of substrate at which the reaction rate is at half maximum. When Km = [S], Vmax = 1/2

Question 2

What would a low Km indicate?

Answer 2

High affinity of substrate for enzyme

Question 3

What would a low Km indicate?

Answer 3

Low affinity of substrate for enzyme

Question 4

What is graphed in a Michaelis Menten plot?

Answer 4

Velocity of reaction (product concentration/time) vs substrate concentration

Question 5

What is the Michaelis Menten equation?

Answer 5

Question 6

What affect does cool temperate have on an enzyme?

Answer 6

results in low molecular motion and therefore slows catalysis

Question 7

What affect does heat have on an enzyme?

Answer 7

It causes denaturation of the enzyme due to excessive molecular motion and therefore catalysis stops.

Question 8

What affect does pH have on enzymes?

Answer 8

Enzymes have specific ionization states in their active sites. Altering the ion concentration can alter the ionization state and therefore denature the enzyme

Question 9

What is enthalpy?

Answer 9

The measure of types and number of chemical bonds.

Represented by the letter, H

Question 10

What is entropy?

Answer 10

The measure of disorder.

Represented by the letter S

Question 11

What is the second law of thermodynamics?

Answer 11

Entropy of the universe is ever increasing

Question 12

What does a - ΔG reaction indicate?

Answer 12

It is an exergonic reaction - Free energy is released be the reaction and therefore it will proceed spontaneously towards product formation.

Question 13

What does a reaction with + ΔG indicate?

Answer 13

It is an endergonic reaction - Free energy input is required in order for the reaction to proceed toward product formation. Without free enery input, the reaction favors substrate formation.

Question 14

What is energy coupling?

Answer 14

Combining a exergonic reaction with an endergonic reaction to overcome energy barriers

Question 15

What does a large K_eq indicate?

Answer 15

most of the substrate has been converted to product and therefore the reaction is more likely to go to completion

Question 16

What does a small K_eq indicate?

Answer 16

Little substrate has been converted to product therefore ther reaction is less likely to go to completion.

Question 17

What is the equilibrium constant?

Answer 17

The constant that reflects the molar concentration of product/substrate at equilibrium

Question 18

At equilibrium, what happens to potential energy?

Answer 18

It is balanced

Question 19

What does an unbalanced potential energy create?

Answer 19

A driving force towards equilibrium

Question 20

The magnitude of ΔG is dependent on what?

Answer 20

how far from equilibrium the system is

Question 21

What does ΔGº reflect?

Answer 21

Free energy change of reaction under standard conditions: pH = 7, 1 atmosphere and no influence of water

Question 22

What equation quantifies the likelihood that substrate will proceed to product?

Answer 22

ΔGº =-RTlnK_eq

Question 23

What proves that ES exists before release of product and enzyme?

Answer 23

1. high specificity of enzyme for substrate
2. ES can be isolated
3. physical properties of enzyme change with binding
4. EM and X-ray crystalography confirm it

Question 24

What does ΔG^¥ reflect?

Answer 24

Activation energy/energy barrier - the amount of free energy required to convert E + S to ES complex

Question 25

What affect do enzymes have on ΔG^¥?

Answer 25

Substrate binding to the active site of an enzyme lowers ΔG^¥.

Question 26

How is ΔG^¥ reduced by enzymes?

Answer 26

multiple weak non-covalent bonds release energy upon substrate:active site binding and induced fit.

Question 27

How does the ES complex stability compair to other steps of an enzymatic reaction?

Answer 27

It is more stable that the initial E + S encounter because the enzyme is most complementary to the transition state. It is less stable that product, making ES transitory and favoring product formation

Question 28

What roles to non-covalent bonds play in enzymes?

Answer 28

They give rise to enzyme specificity and are the driving force of catalysis

Question 29

What is a "super polar" residue?

Answer 29

It is a polar residue in a non-polar microenvironment of the active site, giving it special catalytic and binding properties

Question 30

What is the significance of the 3D structure of enzymes and unsequencial AA active sites?

Answer 30

gives active site more range of motion for orientation with substrate

Question 31

What is a zero order reaction?

Answer 31

A reaction whose rate is independent of substrate -usually when [substrate] \>\>\>\> [enzyme]

Question 32

What is the steady state assumption?

Answer 32

1. consider only the forward reaction ES⇒P, not ES ⇒E+S 2. measure rate from initial introduction of enzyme, t=0 3. Enzyme concentration is constant 4. [substrate] \>\>\> [enzyme] creating zero order reaction 5. ES concentration is constant 6. V_max cannot be obtained because of diffusion, temp, pressure, etc.

Question 33

How would high enzyme concentration affect the steady state assumption and enzyme kinetics?

Answer 33

The rate of product formation would appear faster.

Question 34

What is the rate limiting step of enzymatic reactions?

Answer 34

The formation of product due to slower reaction rate of ES to P, therefore overall reaction rate is proportional to the concentration of ES

Question 35

In MM kinetics, when K_M = [S], what does V_max equal?

Answer 35

1/2

Question 36

Lineweaver-Burk equation

Answer 36

Question 37

What does the Y intercept of a Lineweaver Burk Plot show?

Answer 37

1/V_max

Question 38

What does the X-intercept of a Lineweaver Burk plot show?

Answer 38

-1/K_M

Question 39

What is the slope of the Lineweaver Burk plot?

Answer 39

K_M/V_max

Question 40

What does K_M reflect in MM vs LB kinetics?

Answer 40

In MM kinetics, it reflects affinity of an enzyme for substrate In LB kinetics, is a measurement of when catalysis will occur because it is a numerical representation of 1/2 V_max

Question 41

What is k_cat?

Answer 41

a measure of the overall rate of catalysis, turnover number it is useful in enzyme kinetics with different rate limiting step

Question 42

What is k_cat/K_M?

Answer 42

measure of catalytic efficiency (rate of catalysis/affinity) -used in comparing enzyme preference for substrate

Question 43

How would a heterotropic stimulator affect V_max?

Answer 43

1/2 V_max would be achieved with less substrate

Question 44

How would a heterotropic inhibitor affect V_max?

Answer 44

more substrate would be required to reach 1/2 V_max

Question 45

Describe phosphorylation

Answer 45

A chemical modification of an enzyme - transfer of phosphoryl group from ATP or another neucleotide to Ser, Thr, Tyr or His via protein kinase. This can have a stimulatory or inhibitory affect on enzymes. Phosphatases reverse the reaction

Question 46

Inhibitor that binds to the active site of an enzyme?

Answer 46

competitive inhibitor

Question 47

Characteristics of competitive inhibitors?

Answer 47

- resemble the substrate - reaction slows because no product forms while inhibitor is bound - V_max is unchanged because increasing [substrate] will overcome inhibition - K_M has an apparent increase although affinity of substrate for enzyme has not changed, the reaction is just slowed by inhibitor

Question 48

What kind of inhibition is represented here?

Answer 48

competitive inhibition - V_max unchanged - apparent K_M increase

Question 49

What kind of inhibition is represented here?

Answer 49

uncompetitive inhibition -V_max and K_M decrease proportionally

Question 50

What kind of inhibition is represented here?

Answer 50

non competitive inhibition - V_max decreases - K_M unchanged

Question 51

Characteristics of non competitive inhibition?

Answer 51

- inhibitor and substrate bind at different sites - inhibitor can bind E or ES - V_max decreases because fuctional enzyme is depleted - K_M is unchanged because active site is unaltered

Question 52

Characteristics of uncompetitive inhibition?

Answer 52

- inhibitor binds ES complex only - inhibitor binds at site distinct from active site - V_max decreases because functional enzyme is depleated - K_M has apparent increase less active enzyme is available

Question 53

A compoud that binds to the active site of an enzyme and form a irreversible bond?

Answer 53

Suicide inhibitor

Question 54

Why do allosteric enzymes not obey MM kinetics?

Answer 54

- multiple binding sites - show cooperative binding and structure change - bind in non-hyperbolic pattern