Structural Biochemistry/Enzyme/Transition State Analogs

From Wikibooks, open books for an open world
< Structural Biochemistry‎ | Enzyme
Jump to: navigation, search

Introduction[edit]

The basis of enzyme catalysis is the lowering of the enzyme activation energy to create a faster rate of reactants turning into products. Enzymes do this by actually stabilizing the “middle” state in which reactants have to undergo before turning into products, called a transition state. These transition states are in the highest energy state in the reaction making it the most unstable. Regulation of an enzyme is however required in physiological processes in order to prevent damaging results.

One way it can do this is by an irreversible method in which a modified substrate can bind to the enzyme 

and forever deactivate it. However, knowing the fact that enzyme-substrate complexes undergo a transition states, we can therefore conclude that inhibition by a modified transition state is also possible; we call

this transition state analog inhibition.

Inhibition Process[edit]

As a substrate binds to its enzyme we know that it undergoes chemical and geometric shifts attaining an intermediate state. In this situation, a transition state analog, one exhibiting the same properties such as shape and charge of the original transition molecule, may come in and bind. Although the analog displays similar properties as the original transition molecule, because it is still slightly different it will not result to a product and will ultimately deactivate and inhibit the enzyme and prevent it from binding to a substrate. The transition state analog is able to bind to the enzyme with ease because of the great affinity for it. The transition state is the most unstable condition throughout the entire catalysis so the enzyme complex will seek out any molecule that will help stabilize it. This is why when a mimic comes in it is fooled into believing that it is binding to the right molecule.

Example[edit]

Transition-state analogs are compounds that resemble the transition state of a catalyzed reaction. These compounds make for especially potent enzyme inhibitors that offer insight into an enzyme's mechanistic behavior. One example of such a compound is pyrrole-2-carboxylate, a transition-state analog and consequent inhibitor of proline racemase. The mechanism for proline racemization is given below:

Isomerization of L-proline to D-proline via proline racemase

The racemization of L-proline to D-proline proceeds through a planar transition state, where the tetrahedral α-carbon becomes trigonal as a proton leaves the L-proline. The transition-state analog for this step is pyrrole 2-carboxylic acid.

Transition-state analog

The α-carbon of this transition-state analog, like that of the planar transition state, is trigonal. The resemblance of the transition state rather than the substrate allows transition-state analogs to be highly potent and specific inhibitors of enzymes.

Antibody[edit]

Transition-state analogs are also ideal for generating catalytic antibodies (a bzymes). Antibodies (immunoglobins) may be created to recognize transition states, and thus function as catalysts for the reaction. The transition-state analog acts as an antigen (immunogen) to generate the antibody. An example of this process is the production of an antibody that catalyzes the insertion of an iron ion into the porphyrin plane, which must be bent in order to allow the iron to enter. Normally, this step is catalyzed by ferrochelatase, the final enzyme in the production of heme. N-methylprotoporphyrin was found to resemble the transition state because N-alkylation bends the porphyrin, much like the ferrochelatase enzyme. Therefore, an antibody catalyst was produced by using an N-alkylporphyrin as the immunogen. The produced antibody is able to distort the porphyrin in order to facilitate the entry of ferrous iron. Using a similar technique, antibodies that catalyze ester and amide hydrolysis, transesterification, and photoinduced cleavage, among other reactions, have been developed.

File:N-methylprotoporphyrin.png
N-methylprotoporphyrin used to create an antibody to catalyze the insertion of an iron ion into a porphyrin ring


Importance of Transition State Analogs[edit]

Importance of Transition State analogs: • They are able to act as very powerful inhibitors


• Transition state analogs are very important in understanding the kinetics and inner workings of enzyme catalysis. The analogs can “function as antimetabolites” [1.] One of their most important functions in biochemistry too is the role they play in identifying the mechanism a substrate undergoes during catalysis. Since the analog is a structure intermediate, if it plays a part on a researched reaction, then determination of the actual structure and actual transformation of the original substrate is actually possible.

• Transition State analogs also help in identifying the “binding determinants at the active site” [1.]

• Transition State analogs are able to generate immunogens which displays catalytic nature.

Sources[edit]

1. http://arjournals.annualreviews.org/doi/pdf/10.1146/annurev.bb.05.060176.001415?cookieSet=1


2. http://arjournals.annualreviews.org/doi/pdf/10.1146/annurev.biochem.67.1.693