Chem 438 - Introductory Biochemistry - Spring 2013

Lecture Notes:

Enzyme Kinetics & Inhibition, cont.

Noncompetitive Inhibition [Figure 5.8D]

the inhibitor can bind to either E or ES. S & I do not bind to the same sites! [Figure 5.11a]

Model:

; and

Note that will have two inhibitor binding constants, they may be the same, as in the equation above, or could be different, leading to more complex behavior.

Plots for classic, simple situation: [Figure 5.11b]

L-B Plot of Non-competitive Inhibition showing itersection of lines at -1/Km

FYI - Uncompetitive Inhibition

In uncompetitive inhibition the inhibitor binds ONLY to the ES complex. [Figure 5.10]

Model:

; and

For double reciprocal plots get parallel lines! This is not generally found for single substrate enzymes, but is found in multi-substrate systems.

Kinetic Mechanism **[Figure 5.7a, b]**

Ordered Sequential Bi Bi

Random Sequential Bi Bi

Ping pong Bi Bi

Look at inhibition with multisubstrate enzyme: Note that in the Ping Pong and Ordered sequential mechanisms the enzyme forms before & after are the same and thus Q is a competitive inhibitor of A (that is A can displace Q) where as in the random sequential P & Q will be noncompetitive since neither can be disiplaced by substrate because of the different forms allowed by the random binding. Such inhibition differences can be used to distinguish between the different mechanisms.

Temperature and pH Effects on Enzymes (and Proteins)

Temperature Effects on Enzymes

Temperature profile reflects two underlying phenomena:

The effects of temperature on rate due to the fact that as temp. increases more molecules will have the energy required to achieve activation energy, E_a. (Review: Arrhenius equation: , and the Maxwell-Boltzman Distribution.)
The effects of temperature on protein stability: at high temperatures the protein denatures.

Together these effects lead to the plot below where the rising leg is due to activation energy effects (increasing rate) and the falling leg is due to protein denaturation.

Plot of v vs. T showing upward slope as T increases untilk protein dnaturation causes a rapid falloff

pH Effects on Enzyme Rate

Papain: inflection at pH 4.2 for cys-25 and at pH 8.2 for his-159. [Figure 6.5]

"bell-shaped" curve for activity vs. pH for enzyme rate. pH's also shown for the two pKa values of the upward and downward titration curves giving the bell

Note that the two legs represent two pH titration curves (rotate the left leg 90 deg. then flip; rotate the right leg 90 deg. counter clockwise and you can see them), with pK's equal to 4.2 and 8.2 respectively. This is a typical example for an enzyme with titratable groups in the active site. Can also have non-symmetrical curves with only one group. And of course can have curves due to denaturation by titration of charged surface and interior side chains

Enzyme Catalysis

We will look at catalysis in two types of systems:

Model systems in organic chemistry to elucidate probable mechanisms of chemical catalysis.
Example enzymes to demonstrate these mechanisms in enzyme catalysis.

Mechanisms of Chemical Catalysis

Look at some examples of catalysis in model systems (organic chemistry) and how they might operate in enzymes. Recall, catalysts reduce the activation energy. [Figure 6.1, 6.2]

Types of Catalysis:

Acid/Base
- Specific (H⁺ & OH^- in water)
- General (Bronsted acid definition: proton donor/acceptor pairs; buffers.)
Covalent
- nucleophilic
- electrophilic
Proximity/orientation
Stabilization of Transition State Conformation (Strain/distortion; Charge neutralization)
Metal/Metal ion.