Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 431

Biochemistry

Fall 2008

Lecture Notes: 3 October

© R. Paselk 2008
 
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Introduction to Enzymes

Look at major aspects of enzyme study:

Enzyme Specificity

Models for Enzyme Specificity:

 

Enzyme Kinetics

CHEMICAL REACTION KINETICS

Gives information on dynamic systems.

Sets the parameters for catalytic mechanisms such as:

A C + X;

B + X D etc.

Review some Kinetics from General Chemistry:

We have now reviewed kinetics as tools. Before we go to enzymes a few comments:

Plots of vi = d[P]/dt vs. [S] for 0 - 3rd order

Look at simple, one-substrate enzymes:

For simple enzyme, S P get rectangular hyperbola type plot for vi vs [S] (text Figure 6-11), similar to Mb binding curve.


Let's look at a mathematical model and attempt to generate curve. This was first done by Michaelis and Menten for an equilibrium model. Better is the steady state model of Haldane and Briggs (more general), which we will derive.

Want to come up with a model in expermentally accessible terms, e.g. vi,

For S P assume[S], etc.

And for initial reaction conditions [P] = 0 & therefore k4 = 0, so have

Now vi = d[P]/dt = k3[ES] (Note that kcat is often used instead of k3);

Assume steady state (steady state assumption: d[ES]/dt= 0):

d[ES]/dt= 0; Thus: 0 = d[ES]/dt= k1[E][S] - k2[ES] - k3[ES].

Continuing we can now substitute for E (free enzyme), because hard to find experimentally, and gather constants:

[E] = [Et] - [ES]; then

d[ES]/dt= k1([Et] [S] - [ES][S]) - k2[ES] - k3[ES],

gathering constants: ,

Now define

Then , where KM is the Michaelis-Menten constant.

{Note that if k2 >> k3 (that is the equil. of E+S with ES is rapid compared to breakdown of ES to P), then M-M const = 1/(affinity)= the dissociation constant, but only in these special conditions.}


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Last modified 3 October 2008