Page 209 - Essentials of physical chemistry

More Kinetics and Some Mechanisms 171

FIGURE 8.6 A drawing of o-diphenol oxidase where the polypeptide chain is shown as a heavy strand. The
active site of the enzyme is shielded but accessible in the center with a Cu–O–Cu moiety shown as large
spheres. (From the Brookhaven Protein Data Bank as 2P3X.pdb. Thanks to Prof. Glen E. Kellogg of the
Virginia Commonwealth University, Medical College of Virginia.)

with charged ion sites in specific regions. However, getting the key into the lock is only the first
step, it is necessary to turn the key. Thus, a second geometrical feature of the active site is that it
often encourages the conformation of the substrate molecule to assume the geometry of the
transition state. No wonder enzyme reactions can be both stereospecific and efficient. We see that
with catechol oxidase, although the specificity is lower than with some other enzymes but it is tuned
to oxidize an o-diphenol. This particular enzyme was isolated from grapes but catechol oxidase is
common in many fruits and vegetables. It is apparently part of a protective mechanism in that
whenever there is a physical injury to an apple, banana, or potato, catechol is released and the
enzyme converts it to benzoquinone, which is an antiseptic to bacteria and fungal infections. Later
the benzoquinone reacts further with oxygen to produce black spots. One the one hand, the plant is
protecting itself but on the other hand most people consider blackened fruit as spoiled, so two
strategies are used to keep the fruit from blackening: cooling to slow the reaction and packing in
a nitrogen atmosphere to keep oxygen away from the produce.

BASIC MICHAELIS–MENTEN EQUATION
The treatment of enzyme kinetics was given by Leonor Michaelis (1875–1940), a German bio-
chemist and physician, and Maud Menten (1879–1960), a Canadian biochemist and physician, and
is called the Michaelis–Menten [12] equation today.
k 1 k 2
The model for the reaction is E þ S Ð (E S) ƒƒƒƒ! P þ E (E ¼ enzyme, S ¼ substrate, and
k 1
P ¼ product), but a special consideration is given to the concentration of the free enzyme [E free ]
compared to the total amount of the enzyme in the solution [E tot ], so that we have
[E free ] ¼ [E tot ] (E S). We proceed to the rate step, which is susceptible to the steady-state
analysis:
d[E S]
¼ k 1 [E free ] [S] k 1 [E S] k 2 [E S] ffi 0,
dt

balancing creation=loss of the (E S) species.