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08 Allosteric enzymes

08 Allosteric enzymes - Levels of Control There are two...

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Levels of Control There are two levels of control of metabolism: Acute Control is the second by second adjustment of a pathway’s activity to the exact requirements of the cell at that point in time. It requires changes in enzyme activity. This can be brought about by enzyme inhibition or activation. Chronic (long term) control is the adjustment of pathway capacity to meet needs dictated by long term changes in environmental conditions. This requires changes in enzyme amount. It is brought about by enzyme synthesis or degradation. In any control system both mechanisms will work but will be responsible for different aspects of the control mechanism.
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Changing Enzyme Activity Enzyme activity can be changed in three fundamentally different ways. All work in complex organisms but only the final one is a feature of prokaryotes. 1. Proteins synthesised as inactive pro-enzymes can be activated by proteolytic cleavage. 2. Co-valent modification of enzymes. Requires the addition or removal of a non protein group by another enzyme. Usually involves phosphorylation or dephosphorylation of the –OH of a serine, threonine or tyrosine side chain. 3. Non-covalent binding of activating or inhibiting ligands changes the conformational state of the protein. This is allosteric regulation.
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Kinetics of Allosteric Enzymes The V/S plot of an allosterically regulated enzyme is not hyperbolic as most enzyme V/S plots are, but is sigmoid sigmoid (S – shaped).
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In the presence of an inhibitor the curve is shifted to the right ie. a higher
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