Question

What is allosteric repression? Give an example and explain how the repression takes place

Answer 1

Allosteric proteins(enzymes are regulatory proteins which possess an active site (catalytic) and an allosteric site (effector). In an allosteric enzyme the substrate binds to the active site to produce product. When an effector molecule binds to the allosteric site, it changes the configuration of the active site. The shape change of the active site inhibits the binding of the substrate making the enzyme inactive.

All allosteric proteins are not enzymes. Repressors are allosteric proteins that possess binding sites for specific molecules. These molecules are called corepressors. Repressors 9in gene regulation) by itself are inactive and cannot bind to the operator(located downstream to promoter).When corepressor binds to the repressor (active corepressor complex) the complex binds to the operator preventing mRNA synthesis.

An example is the trp operon in E coli

The bacteria E. coli requires amino acid tryptophan for survival. They can either utilize tryptophan available in their environment or they are capable to synthesize tryptophan on their own.

Allosteric repression in trp operon gene regulation:

1. When tryptophan is present, the trp operon is turned off (gene is not expressed)

2. When tryptophan is absent, the trp operon is turned on (gene expressed) to synthesize tryptophan.

trp operon is a group of 5 genes (trpE, trp D,trp C,trp B, trp A) that encodes for 5 enzymes essential for the synthesis of trptophan along with a promoter region which is the binding site of RNA polymerase to initiate transcription and an operator region which is the binding site of repressor.

The reulatory protein is called trp repressor and the corepressor molecule is tryptophan itself.

trp repressor is inactive by itself and is only active as repressor-corepressor complex.

When tryptophan is present

The corepressor tryptophan binds to the active site in trp repressor changing its configuration. It enables the binding of the complex to the operator thereby blocking the path of RNA polymerase. Therefore inhibiting the expression of trp operon when tryptophan is present.

When tryptophan is absent

The corepressor is absent. Therefore trp repressor cannot bind to the operator region. RNA polymerase binds to the promoter region and initiates transcription. trp operon is expressed to synthesize tryptophan.