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Lecture 31: 11/08/06 Enzyme kinetics

Bimolecular Reactions and Allosteric Enzymes

40 % of the enzymatic reactions in our bodyinvolve only one substrate.

60 % of the enzymatic reactions in our body therefore require involve two or more

substrates (bisubstrate reactions) that most often yield 2 or more products.

Examples of bisubstrate reactions are:

Transferase reactions that catalyse the transfer of specific functional group,

X from one substrate to another.

To represent these bi substrate reactions W.Cleland introduced a nomenclature that

takes into account the order of the reaction of substrate with the enzyme and the

order of the release of the respective products.

The above is an example of a random addition of two substrates to one enzyme and

two products produced in random order. Therefore this reaction is termed a

Random/Unordered Bi-Bi reaction

In the Ordered Bi-Bi reaction shown below, the substrates are added and products

are released in a non-random fashion.

Allosteric enzymes

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Whenever a substrate molecule binds to the enzyme a conformational change, i.e. a

change in the 3-D structure of the enzyme, usually occurs.

This conformational change is caused by the interaction between the substrate and

the enzyme molecule in response to substrate binding to amino acids in the protein.

All the enzymatic reactions that we know of have some degree of conformational

change associated with substrate binding.

If the binding of a molecule cause a significant change in the activity of an enzyme,

the molecule is termed an allosteric effector.

There are two types of allosterism:

Positive allosterism- when a molecule binds to the enzyme causing a

conformational change in the enzyme that results in an increase in

activity of enzyme and by promoting binding of a substrate molecule to

that conformationally altered protein.

Negative allosterism- when a molecule binds to the enzyme causing a

conformational change in the enzyme that results in a decrease in activity

of the enzyme and a reduced binding of substrate molecule to theconformationally altered protein.

Allosteric regulation is one of the important and most widely used methods to

regulate enzyme reactions in biological systems.

A classic example of allosterism is binding of Oxygen to hemoglobin.

http://en.wikipedia.org/wiki/Hemoglobin