What will you learn

• How do enzymes work
• What is the role of the active site of an enzyme
• Factors that affect enzyme activity

[Figure 1]

Do cells have one enzyme with lots of functions, or many enzymes, each with just one function?

Enzymes. Vital proteins necessary for life. So how do enzymes work? How do they catalyze just one specific biochemical reaction? In a puzzle, only two pieces will fit together properly. Understanding that is one of the main steps in understanding how enzymes work.

Enzyme Function

How do enzymes speed up biochemical reactions so dramatically? Like all catalysts, enzymes work by lowering the activation energy of chemical reactions. Activation energy is the energy needed to start a chemical reaction. This is illustrated in the Figure below. The biochemical reaction shown in the figure requires about three times as much activation energy without the enzyme as it does with the enzyme.

[Figure 2]

Enzymes generally lower activation energy by reducing the energy needed for reactants to come together and react. For example:

• Enzymes bring reactants together so they don’t have to expend energy moving about until they collide at random. Enzymes bind both reactant molecules (called the substrate), tightly and specifically, at a site on the enzyme molecule called the active site.
• By binding reactants at the active site, enzymes also position reactants correctly, so they do not have to overcome intermolecular forces that would otherwise push them apart. This allows the molecules to interact with less energy.
• Enzymes may also allow reactions to occur by different pathways that have lower activation energy.

The active site is specific for the reactants of the biochemical reaction the enzyme catalyzes. Similar to puzzle pieces fitting together, the active site can only bind certain substrates.

The activities of enzymes also depend on the surrounding conditions such as the temperature and the pH. For example, some enzymes work best at acidic pHs, while others work best in neutral environments.

• Digestive enzymes secreted in the acidic environment (low pH) of the stomach help break down proteins into smaller molecules. The main digestive enzyme in the stomach is pepsin, which works best at a pH of about 1.5. These enzymes would not work optimally at other pHs. Trypsin is another enzyme in the digestive system, which breaks protein chains in food into smaller parts. Trypsin works in the small intestine, which is not an acidic environment. Trypsin’s optimum pH is about 8.  
• Biochemical reactions are optimal at physiological temperatures. For example, most biochemical reactions work best at the normal body temperature of 98.6˚F. Many enzymes lose function at lower and higher temperatures compared to their optimum temperature. At higher temperatures, an enzyme’s shape deteriorates, also known as denaturing. Only when the temperature comes back to normal does the enzyme regain its shape and normal activity.

Explore the effect of pH on the activity of the pepsin enzyme

Summary

• Enzymes work by lowering the activation energy needed to start biochemical reactions.
• The activities of enzymes depend on the temperature and the pH of the surroundings.

Review

1. How do enzymes speed up biochemical reactions?
2. Where is the active site located? Explain the role of the active site?
3. Complete this sentence: The activities of enzymes depends on the __________ and the __________ of the surroundings.
4. Distinguish between the conditions needed for the proper functioning of pepsin and trypsin.

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