L9- Enzymes

Question 1

Symbiotic bacteria can fixate N2 from the atmosphere but what is the problem with this?

Answer 1

It require a huge amount of energy. 16 ATP.

Nitrogenase is used to accelerate the reaction.

Question 2

The greater the Ea the…

Answer 2

Slower reaction rate

Question 3

Are enzymes lock and key?

Answer 3

No. Induced fit. Full complimentarity is achieved after the substrate binds to the active site.

Question 4

What is the active site most complimentary to?

Answer 4

The transition state. The transition state is therefore stabilised and the energy barrier is lowered.

Question 5

What are the residues at the active site involved in?

Answer 5

Catalysis as well as binding energy

Question 6

WHat do the residues at the active site do in catalysis?

Answer 6

1. Be proton donors/acceptors
2. Form temporal covalent bonds to substrate
3. Hold metal cofactors
   (Asp and Glu for most, Cys and His for heavy ones)

Question 7

Extra functional groups are provided by small molecules that may bind near the active site. What are they called and what can tehy be classified into?

Answer 7

Cofactors.

1. Essential ions- loosely or tightly bound in metalloenzymes
2. Coenzymes- carriers of functional groups or atoms.
   - loosely bound=cosubstrates
   - covalently bound = prosthetic groups e.g. haem

Question 8

What does the graph look like in a reaction, time against total amount of product?

Answer 8

The first part of the curve is linear, and corresponds to the inital rate of reaction. Over time the curve levels off because substrate is being used up.

Question 9

If we plot initial rate of reaction versus the substrate concentration, what do we get?

Answer 9

We obtain a straight line. Its slope is k1.

Question 10

If we plot initial rate versus the substrate concentration for a catalyzed reaction what do we get?

Answer 10

First order at first then zero order

Question 11

What’s the formula for velocity?

Answer 11

V= d[P]/dt = Vmax[S]/ Km+[S]

Question 12

How many steps to enzymatic reactions have and which is usually the time-limiting step?

Answer 12

• Enzymatic reactions have at least 2 steps
• The second is usually the time-limiting step

E+S <-> ES -> E + P

Question 13

What is the Michaelis constant (Km) defined as?

Answer 13

K-1 +K2/ K1.

K2 is negligable because it’s so slow. K-1/K1.

K1 is the first reaction from E+S to ES. K-1 is the backwards one to this. K2 is the second reaction from ES to E + P.

Question 14

What is Km a measure of?

Answer 14

Km is a measure of the affinity of an enzyme for its substrate. The lower the Km, the higher the affinity and the tighter the substrance is bound.

Question 15

What is the formula for kcat?

Answer 15

Kcat = Vmax / [Et]

[Et] = total enzyme concentration i.e. number of active sites.

Question 16

What is Kcat a measure of?

Answer 16

The number of molecules of substrate converted to product per second per active site.

Question 17

The higher the Kcat/Km the ….?

Answer 17

Better the enzyme

Means it has higher affinity and the more substrate converted to product

Question 18

What are the 6 classes of enzymes?

Answer 18

1. Oxidoreductases (dehydrogenases)- catalyze oxidation- reduction reactions
2. Transferases- catalyzes group transfer reactions
3. Hydrolases- hydrolysis rxns
4. Lyases- lysis
5. Isomerases- isomerism reactions
6. Ligases- ligation (joining) of 2 substrates

Question 19

What is the michaelis menten equation?

Answer 19

V= vmax[S]/Km+[S]

V= reaction rate
Vmax= max. rate of rxn
Km= conc. at which 1/2 vmax
[S]= conc. substrate

Question 20

What does the michaelis menten equation relate?

Answer 20

Reaction rate to concentration of substrate

Question 21

What is Vmax?

Answer 21

The max initial velocity when the enzyme is saturated with substrate. It’s proportional to [E]. (conc of enzymes)

Question 22

What is vmax relation to kcat?

Answer 22

vmax= Kcat[E]t

Kcat= catalytic constant or turnover number

Kcat= k2 for simple reactions and represents the number of substrate molecules converted into products

Question 23

What do we plot the lineweaver- burk plot?

Answer 23

1/Vo versus 1/[S]

Question 24

What can we find from a lineweaver-burk plot?

Answer 24

Km and Vmax

Where the line meets the X-axis or the 1/[S] axis the value is -1/Km

Where the line meets the Y axis or the 1/Vi axis, the value is 1/Vmax

Question 25

What is 1/V??

Answer 25

Flip over the michaelis menten equation

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

= (Km/ vmax)(1/[S]) + 1/vmax

gives a lineweaver burk graph equation

Question 26

In the lineweaver-Burk plot. What is the equation of the line in the form y=ax+b?

Answer 26

1/V= (km/vmax) x (1/[s]) + (1/vmax)

y= 1/v

x = 1/[S]

a= km/vmax

b= 1/vmax

which can be derived from turning the michaeles menten equation upside down