Enzymes 3 - kinetics, activation and inhibition

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  • Created by: Ivana
  • Created on: 01-12-13 20:24

Course of a reaction

E + S  <->  ES  <->  E + P 

All stages of the enzyme catalysed reaction are REVERSIBLE and move towards equilibrium in a closed system. 

Equilibriums are rare because the products of one reaction often become to substrate of another reaction so it will not hang around for long. 

Product: rises
Substrate: declines
Enzyme: Declines and then becomes steady
ES: rises and then become steady because all active sites are being used up. 

INITIAL RATE OF REACTION: these are required for accurate measurements of catalytic activity. 
IT will obtain a rate of reaction for a particular substrate. e.g change in Abs min-1 or micromol of product form min-1.  

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Michaelis Menten Plot

V max = theoretical maximum rate of reaction

1st order rate: At low substrate concentrations the rate of reaction is directly proportional to the substrate concentration 

0 order rate: At high substrate concentrations the rate is independent of the substrate concentration.

DISADVANTAGES:
-difficult to accuratley measure Vmax because it goes off to infinity(http://web.campbell.edu/faculty/nemecz/323_lect/enzymes/images/mm_plot2.jpg)

 

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Michaelis menten equation

Vo = Vmax [So]
        Km + [So] 

Rate of reaction
 Maximum theoretical rate of reaction
Orginal substrate concentration 
Michaelis menten constant

Km = a measure of the affinity of a substrate for an enzyme
The lower the Km the the higher the affinity of the substrate for the enzyme

The equation gives us an important kinetic constant Km which is a measure of the specificity of a substrate for an enzyme.

It is impossible to reach the Vmax at any substrate concentration because all enzyme active sites have been used up.

Km is independent of enzyme concentration. So the Km will be the same if the Vmax is the same no matter what the enzyme conc. is.  

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Kcat

Kcat -> a measure of turnover (how quickly a product is produced) 
Can be measured by Vmax/Enzyme concentration
Reflects the maximum rate of production formation, how quickly the enzyme works.  

Specificity constant -> Kcat/Km
Better value of the specificity of a substrate
Theoretical max = 1 x 10'9 M-1 sec-1
Sometimes referred too as kinetic efficiency because the rate of reaction directly varies with the level of affinity of the subtrate to the enzyme (Km) and how efficiently they bind in solution.  

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Line weaver burke plot

-easier to determine Km and Vmax by inverting 1/Km and 1/Vmax
-all plots have some statistical problems and they all have supporters and detractors.  

(http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/L/Lineweaver_Burk.gif)

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