In our last instruction, we learned that the proteins in our cells do many things. One of the most important things they do is to act as enzymes.

What’s an enzyme? An enzyme is a catalyst.  What’s a catalyst?

A catalyst is something that changes things without being changed itself. A catalyst gets things going. It makes things happen. Think of a street fight. If somebody gets other people to fight but doesn’t fight themselves, they’re a catalyst.

Enzymes are the catalysts for chemical reactions. They either make them happen or speed them up. Chemical reactions keep our bodies going -- without them, we’d die.

So enzymes are very, very important. There are enzymes in all the tissues and fluids of our bodies and these enzymes are very specific. That means that each enzyme will only work on certain specific substances.  The substances they work on are called substrates.

Enzymes and Substrates
http://en.wikipedia.org/wiki/Enzyme

As we said, each enzyme will only work on certain specific substrates. It’s like a keyhole and a key. Only certain keys fit certain keyholes. When the right key fits into the right keyhole, the door opens. When the right enzyme connects with the right substrate, a reaction happens.

Here’s an example.

Have you ever cut yourself and put Hydrogen Peroxide on the cut while it was still bleeding? Remember how it bubbles and fizzes? That’s because the Hydrogen Peroxide is decomposing into hydrogen and oxygen and reacting with an enzyme in your blood called catalase. Here is the chemical formula for what’s happening:

2 H₂O₂ = 2 H₂O + O₂ and a protein enzyme to help the reaction

That’s what we mean by specific. If you put Hydrogen Peroxide on your skin where there’s no blood – no reaction!

Action / Reaction
 http://www.spartechsoftware.com/reeko/

The place where this all fits together (where the right enzyme binds itself to the right substrate) is called the “active site” of the enzyme. Here’s a simple description of how enzymes cause the kind of chemical reactions we’ve been talking about:

1. The enzyme attaches itself to the substrate at its active site.
2. A reaction begins
3. The substrate is changed.
4. The reaction stops.
5. The enzyme lets go. It is regenerated (which means it returns to normal and is ready to cause another reaction). But the substrate has been changed forever.
6. It is now called the product.

When does the reaction stop? When the reactants (the enzyme and the substrate) reach what is called “equilibrium.”

Equilibrium
http://www.chem1.com/acad/webtext/chemeq/

This may be a little difficult to follow, but stick with us -- because most chemical reactions aren’t quite as simple as we just described.

When the initial chemical reaction has been completed (chemists call it the forward reaction) and has resulted in a product, it often wants to reverse itself (chemists call this the backward reaction). This means it wants to turn itself back into its original chemicals.  There is a point where these two reactions meet and cancel each other out.
At this point, the rate of the forward and backward reactions are equal. The product becomes stable. This is called the point of equilibrium.

Simple and Complex Enzymes
http://dwb.unl.edu/Teacher/NSF/C11/C11Links/web.indstate.edu/thcme/mwking/enzyme-kinetics.html

Most enzymes require the presence of other compounds – cofactors – before they can act as catalysts.  There are two different kinds of enzymes.

All enzymes are mostly protein, but many are only protein. These enzymes are known as simple enzymes.

Enzymes that contain other things, “complex enzymes,” are called holoenzymes.

A holoenzyme has two parts. The protein part is called the apoenzyme. The other part is called the coenzyme. However, simple and complex enzymes work in much the same way:

There are many things that affect their performance. Some of these things are:

• Temperature Proteins change shape as their temperature changes. This affects the “lock and key” fit we described earlier. So if the enzyme changes its shape, it can’t work.
   
• pH Levels (pH means how acid something is). For example, if you did an experiment where you changed the pH level from 7 to 6 and more product was formed, that would prove that the enzyme was a more active conformation (worked better) at pH6.
   
• Activators Sometimes you need an enzyme to work faster and your body creates something to make it do so – it activates it. That’s an activator. It’s like eating a candy bar to give yourself energy.
   
• Inhibitors are very important-- they are the opposite of activators. They slow the chemical reaction down or even stop it altogether. As we said earlier, the chemical reactions that enzymes create are what keep us alive. Unfortunately, they can also make us sick. But if those reactions can be stopped or slowed, we get well. That’s why the whole area of “enzyme inhibitors” is where the action is in the development of new drugs by pharmaceutical companies.
   

Enzyme Inhibitors
http://www.worthington-biochem.com/introBiochem/inhibitors.html

Does your grandmother have arthritis? Maybe she takes Celebrex. ™ Celebrex is an enzyme inhibitor. It inhibits the work of the Cox II enzyme -- the enzyme that causes the chemical reaction that causes much of the pain of arthritis.

 Ever had a bacterial infection? Almost everyone has taken penicillin in one form or another. Antibiotics can also work by inhibiting enzymes. Penicillin doesn’t allow bacteria to make the cell wall. No cell wall means it can’t thrive in the host (You). This doesn’t hurt you because humans don’t need that enzyme anyway.

There are two broad classes of enzyme inhibitors:

1. Irreversible enzyme inhibitors (like cyanide and other poisons) – these are life threatening and do not pose much opportunity for beneficial drug research.
    
2. Reversible enzyme inhibitors – these offer great promise. The hallmark of all reversible enzyme inhibitors is this: when the inhibitor concentration drops, enzyme activity is regenerated. These reversible enzyme inhibitors are divided into two groups:
    
   1. competitive – these enzyme inhibitors are called competitive because they compete with the substrate for binding at the active site of the enzyme. Most enzyme inhibitor drugs (like aspirin or Celebrex) are competitive enzyme inhibitors.
       
   2. noncompetitive - these inhibitors bind at places other than the active site of the enzyme so they don’t compete with the substrate.
       

If you add more of a noncompetitive inhibitor to an enzyme reaction, product formation decreases. This makes the exploration of these inhibitors less attractive for drug research.

Experiments for Home and Classroom

Why do apples turn brown when they are cut and exposed to the air? It's because an enzyme in the apple (tyrosinase) reacts with the oxygen in the air. In this experiment, students see how this happens for themselves and are also invited to inject a number of variables into the experiment. Click:
http://chemistry.about.com/od/demonstrationsexperiments/ss/appleenzyme.htm

These experiments are primarily designed for classroom use since they require beakers and other simple laboratory equipment. Also, they need to be done in pairs. They could also be done at home with adult supervision (as we said, each experiment requires two participants). The purpose of the experiments is to test the factors that affect enzyme reactions. Click:
http://educ.queensu.ca/~science/main/concept/chem/c01/C01LACG2.htm

In this experiment, students examine and report on various enzyme reactions. This experiment requires Pdf. Click:
http://www.ccmr.cornell.edu/education/modules/documents/DiscoveringEnzymes.pdf