BAHAN KULIAH BIOKIMIA POWER POINT BAGIAN 1 /BIOCHEMISTRY POWER POINT LECTURES PART 1 | Karya Tulis Ilmiah

Enzyme Kinetics and Catalysis II

Kinetics of Enzymes

Enzymes follow zero order kinetics when substrate

concentrations are high. Zero order means there is no increase in the rate of the reaction when more substrate is added.

Given the following breakdown of sucrose to glucose and fructose

Sucrose + H₂0 Glucose + Fructose

P

E

ES

S

E

2 1 1 -k k k



 







E = Enzyme S = Substrate P = Product

ES = Enzyme-Substrate complex

k₁ rate constant for the forward reaction

k_-1 = rate constant for the breakdown of the ES to substrate

When the substrate concentration becomes large enough to force the equilibrium to form completely all ES the second step in the reaction becomes rate limiting because no more ES can be made and the enzyme-substrate complex is at its maximum value.

 

 

ES

P

2

k

dt

d

v





[ES] is the difference between the rates of ES formation minus the rates of its disappearance.

 

ES

  

E

S

 

ES

 

ES

2 1

1

k

k

Assumption of equilibrium

k-1>>k2 the formation of product is so much slower than the formation of the ES complex.

That we can assume:

  

 

ES

S

E

1

1







k

k

K

_s

Assumption of steady state

Transient phase where in the course of a reaction the concentration of ES does not change

 

0

ES



dt

d

 

E

T



   

E



ES

₃

Combining 1 + 2 + 3

   



E

-

ES



 

S



k

k



 

ES

k

_{1 T}



_-1



₂

 

ES



k

_-1



k

₂



k

₁

 

S





k

₁

   

E

_T

S

 

   

 

S

K

S

E

ES

T





M 1 2 1

-k

k

k

K

_M





rearranging

 

 

   

 

S

K

S

E

ES

P

_{2 T}

2 0





















 M t o

k

k

dt

d

v

v_o is the initial velocity when the reaction is just starting out.

And is the maximum velocity

V

_max



k

₂

 

E

_T

 

 

S

K

S

V

max





M

o

The K_M widely varies among different enzymes

The K_M

can be expressed as:

1 2 1 2 1 1 _K K k k k k k k s

M     

As Ks decreases, the affinity for the substrate

There are a wide range of K_M, Vmax , and efficiency seen in enzymes

The double reciprocal plot

 

_max

Lineweaver-Burk plot: slope = K

_M

/Vmax,

1/v

_o

intercept is equal to 1/Vmax

the extrapolated x intercept is equal to -1/K

_M

For small errors in at low [S] leads to large errors in 1/vo

 

E

max

T

V



cat

k

k_cat is how many reactions an enzyme can catalyze per second

For Michaelis -Menton kinetics k₂= k_cat

When [S] << K_M very little ES is formed and [E] = [E]_T

and

   

  

E

S

K

k

S

E

K

k

M cat T M 2





o

v

What is catalytic perfection?

When k₂>>k_-1 or the ratio

2 1 2 1

k

k

k

k



 is maximum Then 1 M

K

k

k

_cat



Or when every substrate that hits the enzyme causes a reaction to take place. This is catalytic

perfection.

Diffusion-controlled limit- diffusion rate of a substrate is in the range of 108 to 109 M-1s-1. An enzyme lowers

Reaction Mechanisms

A: Sequential Reactions

Ping-Pong Reactions

• Group transfer reactions

Kinetic data cannot unambiguously

establish a reaction mechanism.

Although a phenomenological description can be obtained the nature of the reaction intermediates remain indeterminate and other independent

Inhibition kinetics

There are three types of inhibition kinetics competitive, mixed and uncompetitive.

 

 

S

K

S

V

M

max







o

v

 

















I

K

I

1



  

 

EI

I

E

Mixed inhibition is when the inhibitor binds to the enzyme at a location distinct from the substrate

binding site. The binding of the inhibitor will either alter the K_M or V_max or both.

  

 

EI

I

E

K

_I



  





ESI

I

ES

K



I



 

 

S

K

S

V

M

max











o

v

 

