Enzymes: The Biological Catalysts

  Metabolic reactions always take place inside our bodies. For instance, digestion is an important process that takes place inside us. Often reactions such as digestion are accompanied by the use of catalysts. These biological catalysts are known as enzymes. Enzymes are responsible for speeding up a reaction without being consumed by the reaction itself. This is an important property of enzyme because they can be used over and over again.

  There thousands of different kinds of enzymes. Amylase, protease and lipase are just some of the common enzymes that one might have heard of. Every reaction needs an initial investment of energy for it to start. This energy is known as the activation energy.  The activation energy acts a barrier, which determines the total amount of energy required in order to start a reaction. Most of the time this activation energy is too high and as a result it would take a long time to finish the reaction. This is when enzymes become useful. An enzyme catalyzes a reaction by lowering the activation energy, enabling the reactant molecules to absorb enough energy to reach the transition state even at moderate temperatures. Since enzymes are specific, they determine which chemical process will be going on in a cell at any particular time.

How enzyme lowers the activation energy

 

  The reactant an enzyme acts on is referred to as the enzyme’s substrate. When a substrate and an enzyme bind they form the enzyme-substrate complex.  If you can recall, I mentioned that enzymes are specific. The specificity of enzymes results from the different kinds of amino sequences. The active site is a region in which the substrate binds to the enzyme. Induced fit allows chemical groups of the active site into positions that enhance their ability to catalyze the chemical reaction. Substrates are held at the the active site by weak bonds. These bonds can range from hydrogen bonds to ionic bonds. The active site lowers the activation energy either by providing a favorable environment or by participating directly in the reaction. The active site can also behave as a template for the substrates. The substrates are then converted to products before being released. The process described above is commonly known as the enzyme cycle.

   There are many factors that influence enzyme activity. The environment an enzyme works in is crucial for the enzyme’s activity.  The optimal temperature for a typical human enzyme is around 37 degrees Celsius. Enzymes at this temperature work well because substrates collide with active sites more frequently when the molecules move rapidly. Higher temperature does not however, guarantee increased enzyme activity. Higher temperatures have the ability to break hydrogen bonds, ionic bonds and other weak interactions that stabilize the shape of an enzyme.

How temperature influences enzyme activity

 

Similarly, the pH of the environment can influence enzyme activity. Some enzymes work best in acidic conditions while others work well in basic environment. For instance the enzyme pepsin is best suited to a pH level of around 2, whereas trypsin works diligently in a pH of 8.

How pH levels influence enzyme activity

    

 Many chemicals inhibit the action of specific enzymes. Competitive inhibitors mimic the substrate and compete for the active site. This way the productivity of enzymes is reduced since the inhibitors block substrates from entering the active sites. This kind of inhibition can be overcome if more substrates are produced so that when active sites are available, the substrates can quickly go attach to the enzymes’ active sites before the inhibitors. Conversely, noncompetitive inhibitors bind to the enzyme away from the active site and as a result change the shape of the enzyme.

  Cells regulate enzymatic reactions in order to foster necessary reactions and inhibit unnecessary ones. One way of regulating enzyme action is allosteric regulation. In allosteric regulation, a molecule binds to an an enzyme at a site other than the active site. This changes the enzyme activity. One example of allosteric regulation is the binding of an inhibitor. When an inhibitor binds, it causes the active site to change its original shape. This modification hinders substrates from binding. When the inhibitor gets released again, the enzyme goes back to its original form. In contrast allosteric activators increase the productivity of an enzyme for its substrate.

 

How allosteric inhibition and allosteric activation 

For this post, I used the AP version of my 8th Edition Biology Campbell book exclusively. It is a really useful book if you want to have a better interpretation of the content I discussed in this post.

Here are some of the links I used to help me with my post: 

http://bcs.whfreeman.com/thelifewire/content/chp06/0602002.html

http://www.worthington-biochem.com/introbiochem/default.html