Chem 431 

Fall 2008 
Lecture Notes: 6 October 


PREVIOUS 
Last time we stopped with:
K_{M} = [S]([E_{t}]  [ES]) / [ES] , where K_{M} is the MichaelisMenten constant.
Substituting for [ES]: v_{i}/k_{3} = [E_{t}] [S] / (K_{M} + [S]),
and v_{i}= k_{3}[E_{t}] [S] / (K_{M} + [S]),
The rate constant (First order) for the breakdown of the [ES] complex, k_{cat} (k_{3}), is also known as the turnover number, that is the maximum number of substrate molecules processed/active site (moles substrate/mole active site): k_{cat}=V_{max} / [E]_{total}. Note that this is best determined under saturating conditions. (text Table 608) At very low concentrations of [S] can find the secondorder rate constant for the conversion of E + S E + P: v_{o} = (k_{cat} / K_{M})([E][S].
Double Reciprocal or LineweaverBurke Plot: Need in form: y = ax + b , so take reciprocals of both sides and have
Other linear plots are also available, and are better in terms of statistics (LB one of worst, best quality points [high concentration] have least influence on slope, while low precision points [low concentration] are more spread out, and have a large moment, with a strong influence on the slope and the K_{M} intercept this is not as much of a concern now with computer statistical packages, but you still have to understand the statistics). We will see others in the laboratory discussion.
FYI  The EadieHofstee PlotOne common plot is shown below. Note that the data points are distributed much more evenly over the plot giving better statistics for the slope. In addition the value of K_{M} is obtained from the slope, giving better precision.

In Competitive inhibition S & I are mutually exclusive, E can bind to one OR the other.
Plots: (text Box 62 figure 1)
We can model this inhibition with chemical equations, keeping in mind that S & I are mutually exclusive, E can bind to one OR the other: (text Figure 615a)
Classically assume binding to same site, but other possibilities also.
 steric hindrance between S & I in different sites.
 overlapping sites for S & I.
 Partial sharing of sites.
 Conformational change of enzyme with binding of either such that other can not bind.
Pathway Diagrams 
