Examples of Allosteric Proteins

We will consider two specific examples of allosteric proteins—one an enzyme and the other an oxygen transport protein. 

Relationship between the initial velocity (v) and the substrate concentration [S] for an allosteric enzyme that shows a heterotropic effect with constant A o.5 but with varying Fmax- Curve a is obtained in the absence of any modulator, curve b in the presence of a positive modulator, and curve c in the presence of a negative modulator. Regulation is achieved by modulation of Vmax without change in Ko.s- 

Aspartate transcarbamoylase (ATCase) is an allosteric enzyme of the bacterium Escherichia coli, which has been extensively studied. This enzyme catalyzes the transfer of the carbamoyl group from carbamoyl phosphate to the z-amino group of aspartate  

Catalytic Regulatory Native cluster cluster enzyme 

ATCase can assume different conformations depending on whether it is active or inactive. In the presence of the substrate or positive modulator, it is in a catalytically more active conformation known as the relaxed state or R-state. In the presence of the negative modulator, the enzyme is in less active conformation known as the taut state or T-state. 

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