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Flashcards in Enzymes In Metabolism Deck (12):

Why do metabolic pathways need to be controlled?

Metabolic pathways make essential products in the organsim. There are mechanisms that control the activity of enzymes and so control the metabolic pathways.


How can enzymes be turned on or off?

Some enzymes can be activated or inactivated by having molecular units added or removed. Covalent bonds are made or broken, so this is called covalent modification. This changes the enzymes shape so it is made either active or not. Either way, the concentration of active enzymes is changed, so the reaction rate is changed.


What can kinase enzymes do?

They can phosphorylate other enzymes. The covalent addition of a phosphate by a condensation reaction activates or inactivates the enzyme. Similarly, phosphatase enzymes can dephosphorylate (remove a phosphate by hydrolysis) other enzymes causing them to be activated or deactivated. A good example if covalent modification is the control if glycogen metabolism.


What happens when blood glucose concentration falls?

Two liver enzymes are phosphorylated by different kinases. One kinase enzyme adds a phosphate to glycogen phosphorylase; this activates to breakdown glycogen to release more glucose. At the same time, another kinase phosphorylates glycogen synthase so it is inactivated and stops synthesising glycogen. Obviously, when blood glucose concentration rises again, both these enzymes can be dephosphorylated to increase glycogen synthesis.


What is covalent modification?

When covalent bonds in the polypeptide chain are cut, so changing the shape of the enzyme. Pancreas cells make an inactive form of trypsin called trypsinogen. This is secreted into the small intestine where it is cut by a protease enzyme. This converts the inactive trypsinogen into the active form of enzyme, trypsin. As, the concentration of the active form increases, the reaction rate increases.


How can the rate of formation of products by a metabolic pathway be regulated?

By changing the affinity of one enzyme in the pathway for its substrate. This enzyme catalyses the rate-limiting step for the whole pathway and the affinity is changed by a modulator molecule that binds to the enzyme.


What happens when a modulator binds?

It changes the shape of the enzyme; the enzyme is called an allosteric enzyme. The modulator binds - through hydrogen bonds and ionic bonds - to the enzyme at the second binding site away form the active site. This is known as the allosteric site.


What does the binding of the modulator cause?

A change in the enzymes shape which, in turn, affects the shape of the active site and hence it's affinity for the substrate. The rate of reaction will be changed as the effectiveness of the enzyme has been changed.


What happens if the product if a metabolic pathway is in short supply?

Then a positive modulator (activator) can bind to the allosteric site and so increase the affinity of the enzyme for its substrate. This enzyme becomes more effective and makes more of the substrates for the next enzyme in the pathway, thereby increasing the rate of formation of all subsequent products in the pathway.


What happens when there is enough of the final product in the pathway?

A negative modulator (inhibitor) can bind to a different allosteric site and so reduce the affinity of the enzyme for its substrate. The enzyme becomes less effective, makes less substrate for the enzyme in the pathway and the whole pathway slows down.


What is end product inhibition?

When the product of the final reaction accumulates, it slows down the first enzyme of the pathway and so slows down its own synthesis. In some pathways the final product works as a competitive inhibitor of the first enzyme, while in other pathways the final product is a negative modulator of the enzyme.


Why does end-product inhibition happens.

So energy is not wasted producing products we don't need.