5.3  Metabolic Pathways and Enzymes

The reactants, or substrates, are the chemicals that enter the metabolic pathway

Enzymes are protein molecules that function as organic catalysts to speed up a chemical reaction.

Enzymes are the cell's chemical tools


Energy of Activation

Substrates often must be activated before a chemical reaction can occur.

The energy needed to cause a substrate to react with another molecule is called the energy of activation (Ea)

Enzymes help catalyze reactions by lowering the energy of activation for a reaction.

Imagine the amount of energy that it would take for you to loosen a nut with the correct wrench verses with only a pair of pliers. It would be easier to use the wrench because the pliers would require extra energy to squeeze them hard enough the prevent the nut from slipping as you tried to loosen it. Enzymes work in the same sort of way, the shape of the enzyme helps the reaction to occur more easily.



Graph of energy of activation with and without enzymes

An Enzyme's Active Site

The active site of an enzyme is the point where a substrate attaches tightly- like a key in a lock, or like the end of a wrench where a bolt would fit. Its called the active site because that's where the activity will take place.

Once the substrate is attached to the active site, the enzyme helps to convert it into a product.

The product is then released from the active site.

Some enzymes break large molecules apart into smaller pieces (example below), while other enzymes connect two smaller pieces together.



How an enzyme works



Enzyme Inhibition

Enzyme inhibition occurs when an active enzyme is prevented from attaching to a substrate by an inhibitor.  This keeps the enzyme from doing its job and so the normal products are not made.

In the example below, E1 is the first of 3 enzymes that will convert the substrate (S) into the product (P). Notice how there is a groove on the side of the enzyme that is not red. If there is enough of the product that has already been made. The product can begin attaching to this groove and causing the enzyme to change shape. When the enzyme's shape has changed, its normal substrate will no longer fit into the distorted active site and it can no longer do its job.

How is this a good system? It slows enzymes down when they have already done enough work (made enough products) to last a cell for awhile.



Feedback inhibition

Some inhibitors are poisonous to living organisms.

Cyanide is an inhibitor that stops ATP synthesis. If a cell can't recharge the batteries that it needs to use to keep it alive (ATP) then it will die.

Penicillin inhibits a specific bacterial enzyme that makes the bacterial cell wall. If a bacterium can't make more cell wall, then it can't grow or reproduce.


 Toggle open/close quiz question

If you changed the shape of an enzyme:
    a.It would no longer work properly because the substrate would no longer fit in its active site
    b.It would work just as well
    c.It would lower the energy of activation even more



Extra Help


More about enzymes