Aspartate transcarbamoylase (ATCase) catalyzes the formation of N-carbamoyl aspa

Question

Aspartate transcarbamoylase (ATCase) catalyzes the formation of N-carbamoyl aspartate from carbamoyl phosphate and aspartate, a step in the multienzyme process that synthesizes cytidine triphosphate (CTP).

Carbamoyl phosphate + aspartate = N-carbamoylaspartate ===UMP== UTP =CTP

Kinetic studies of ATCase activity as a function of aspartate concentration yield the results shown in the graph.

NOTE: from graph: WITH ATP; Aspartame (nM) Has a higher % relative activity.

   NO CTP OR ATP: the line is between both the above and bottom lines.

   with CTP: Aspartame (nM) Has the lowest % relative activity.

a) Is ATCase an allosteric enzyme? How do you know?

b) What kind of an effector is CTP? Explain. What is the biological significance of CTP’s effect on ATCase?

c) What kind of an effort is ATP? Explain. What is the biological significance of ATP’s effect on ATCase?

Explanation / Answer

a) Allosteric enzymes are enzymes which change their confirmation upon binding of the effector molecule. The confirmational change may leads to the increased activity or decreased activity of the enzyme which affect the product formation. When the level of ATP is increased it leads to the activity of the enzyme whereas CTP will inhibit the enzyme activity.

b) CTP is an allosteric inhibitor. Enzyme has two subunits such as c unit - catalytic unit and r unit - regulatory unit. The increased cytidine triphosphate level causes the binding of it to the regulatory unit and causes a confirmational change known as the "T" state means less active state.

c) ATP is an allosteric activator. It binds to the high affinity site and causes a confirmational change known as the "R" state means high active state.

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