"Molecular & Cellular Princ Med Enzymes as Biological Catalysts Patricia Kanepopp" GABY

Question 1

Why do we say that enzymes play a central role in biology?

Answer 1

1. Carry out almost all chemistry required by living systems
2. Permit a wide range of chemical reactions in a narrow set of conditions
3. Allow rapid, efficient adjustments to environmental conditions

Question 2

Medical Aspects of Enzymology

Answer 2

1. Diseases traceable to enzymatic defects
2. Use of enzyme activities in diagnosis
3. Drugs as enzyme inhibitors
4. Therapeutic administration of enzymes
5. Design of therapeutic enzymes and enzyme inhibitors

Question 3

General Characteristics of Enzymes

Answer 3

1. Tremendous enhancement of reaction rate
2. Reactions occur under relatively mild conditions
3. Extreme specificity
4. Capacity for regulation

Question 4

Why are enzyme kinetics important?

Answer 4

It serves as an important tool in understanding enzyme mechanism and manipulating enzyme function

Question 5

Before Vmax is achieved, the initial rate of an enzymatic reaction depends on ___

Answer 5

substrate concentration

Question 6

After Vmax is achieved, the initial rate of an enzymatic reaction does not depend on ___

Answer 6

substrate concentration

Question 7

What happens to the enzyme-substrate complex (ES) after it is formed?

Answer 7

It can either dissociate into E+S
OR
it can go on to form product
E+P

Question 8

Must an ES complex form for the reaction to take place?

Answer 8

YES

Question 9

At low substrate concentrations, ___ limits how much ES can form, so v is ___ on [S]

Answer 9

[S]; dependent

Question 10

At high substrate concentrations, ___ limits how much ES can form, so v is ___ of [S]

Answer 10

[E]; independent

Question 11

What do k1, k2 and k3 mean?

Answer 11

They are kinetic constants

k1 of the –> E+S=ES reaction

k2 of the –> ES=E+S reaction

k3 of the –> ES=E+P reaction

Question 12

Is it possible to determine the kinetic constants relative values?

Answer 12

Yes, by making some assumptions

Question 13

Michaelis-Menten equation

Answer 13

v=(Vmax)X[S]/([S] + Km)

Vmax = k3 [E]t 
Km = (k2 + k3)/k1

Question 14

Can Km and Vmax be determined experimentally?

Answer 14

You can estimate Km and Vmax from a v vs [S] plot, but they are more accurately determined from a linearized form of the Michaelis-Menten equation (Lineweaver-Burk plot)

Question 15

What does the Km mean?

Answer 15

Km = [S] at which v = 1/2 Vmax

Question 16

Km will always have units of ___

Answer 16

CONCENTRATION!

Question 17

The catalytic rate of an enzyme is MOST SENSITIVE to [S] when ___

Answer 17

[S] &laquo_space;Km

Question 18

Knowing the Km allows you to calculate what fraction of enzyme molecules are bound to substrate at any [S]

Answer 18

fES=(v/Vmax)= [S]/([S] + Km)

Question 19

When k2»k3, i.e. the substrate binds and dissociates many times before it goes onto form product…

Answer 19

Km is approximately the same as the dissociation constant (Kd) for the ES complex under these conditions

Question 20

Km is approximately equal to Kd when ___

Answer 20

k2»k3

*Km = k2/k1 = Kd for ES complex

Question 21

T or F: Enzymes with low vs. high Km are adapted to different physiological functions

Answer 21

True

Examples: hexokinase (low Km) & glucokinase (high Km)

Question 22

What does the Vmax mean?

Answer 22

Vmax = maximal velocity achievable for a specific concentration of enzyme

*Note: Vmax will have units of velocity

Question 23

What is kcat?

Answer 23

A constant that is independent of enzyme concentration

kcat = Vmax/ [E]t
*[E]t = total enzyme conc.

Question 24

What does kcat/Km allow?

Answer 24

Direct comparison of enzyme efficiencies

Question 25

Why does higher kcat/Km imply higher efficiency?

Answer 25

kcat–determines how quickly ES complex is used

Km–describes how much ES complex is available

Question 26

Why can we determine [E]t from reaction rate under Vmax conditions?

Answer 26

Because under Vmax conditions (enzyme saturated with substrate) v is directly proportional to the concentration of enzyme present

Question 27

Lineweaver-Burk plot equation

Answer 27

1/v = 1/Vmax + (Km/Vmax)(1/[S])

*slope=Km/Vmax

Question 28

Irreversible enzyme inhibitors

Answer 28

Bind very tightly to an enzyme and inactivate an essential functional group on the enzyme

Question 29

Reversible enzyme inhibitors

Answer 29

Bind reversibly to enzyme and give temporary inactivation while they are bound

Question 30

What types of reversible enzyme inhibitors are there?

Answer 30

1. Competitive
2. Non-competitive
3. Uncompetitive

Question 31

Competitive Inhibitors

Answer 31

• compete with substrate for binding to active site
• inhibition can be overcome by increasing [S]
• Km appears to increase in presence of inhibitor; Vmax does not change

Question 32

Noncompetitive Inhibitors

Answer 32

• bind to a site on the enzyme other than the active site
• inhibition cannot be overcome by increasing [S]
• Vmax appears to decrease in the presence of inhibitor; Km does not change

Question 33

What does the Ki mean?

Answer 33

1. It quantifies the strength with which an inhibitor binds to an enzyme
2. It is the dissociation constant for the enzyme-inhibitor complex

Question 34

T or F: Higher Ki implies weaker inhibition

Answer 34

True

Question 35

A graph of 1/[S] (x axis) and 1/v (y axis) and several [I] (inhibitors) plotted, will intersect at the y axis if I is a ___ inhibitor

Answer 35

competitive

Question 36

A graph of 1/[S] (x axis) and 1/v (y axis) and several [I] (inhibitors) plotted, will intersect at the x axis if I is a ___ inhibitor

Answer 36

non-competitive

Question 37

T or F: Doubling enzyme (and thus Vmax) changes the Km

Answer 37

False

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