Enzymes as Biological Catalysts

What enzymes do

Your cells carry out thousands of chemical reactions every second. At body temperature, most of them would be far too slow without help. The helpers are enzymes.

Why are reactions slow to begin with? Because reactants must get over an energy "hill" before they turn into products. That hill is the activation energy.

An enzyme lowers the activation energy. The reaction still goes from reactants to products, and the energy of the reactants and products does not change. It just gets there much faster.

Because enzymes are not used up, one enzyme molecule can work again and again. A cell needs only a small amount of each enzyme.

Active site and specificity

The molecule an enzyme acts on is its substrate. The substrate binds to a small pocket on the enzyme called the active site. The enzyme and substrate form a temporary enzyme–substrate complex, the reaction happens, and the products are released. The enzyme is unchanged and ready for another substrate.

Each enzyme has an active site with a specific 3-D shape and chemistry. That shape comes from the way the protein folds. Only substrates that fit can bind. This is substrate specificity.

Two models describe the fit:

• Lock-and-key: the substrate fits the active site like a key in a lock.

• Induced fit: the active site changes shape slightly to grip the substrate, like a glove around a hand.

Temperature and pH

An enzyme works only if its protein keeps the right 3-D shape. Conditions that change the shape change the activity.

Temperature. At low temperatures molecules move slowly, so collisions between enzyme and substrate are rare and the rate is low. As temperature rises, collisions become more frequent and energetic, and the rate rises. Past the optimum temperature, the protein unfolds, the active site loses its shape, and the rate drops sharply. This is denaturation. Human enzymes work best near 37 °C.

pH. Changes in pH alter the charges on amino acids, which disrupts the bonds that hold the protein in shape. Each enzyme has an optimum pH. Pepsin works best at about pH 2 in the stomach. Trypsin works best at about pH 8 in the small intestine.

Substrate concentration and inhibitors

Suppose you keep the amount of enzyme the same and add more substrate. At first, the rate climbs: more substrate means more chances that a molecule finds an empty active site. Then the rate levels off. Every active site is busy, so the enzyme is saturated. Adding more substrate cannot speed things up.

To raise the maximum rate, you need more enzyme (more active sites), not more substrate.

A competitive inhibitor has a shape similar to the substrate and blocks the active site. It slows the reaction at low substrate concentration, but a lot of substrate can outcompete it, so the rates come close together.

To compare rates, use percent decrease: $\dfrac{\text{rate without} - \text{rate with}}{\text{rate without}} \times 100\%$. If a rate drops from 20 to 8, the decrease is $\dfrac{12}{20} \times 100\% = 60\%$.

Summary

Back to the start: liver and potato contain catalase. It lowers the activation energy for breaking down hydrogen peroxide, so the reaction fizzes. Without it, the reaction is far slower.

• Enzymes are mostly proteins that act as catalysts. They lower activation energy and are not used up.

• The substrate binds to the active site. The fit gives substrate specificity (lock-and-key, induced fit).

• Heat and extreme pH denature enzymes, changing the active site. Each enzyme has an optimum temperature and pH.

• More substrate raises the rate until the enzyme is saturated. More enzyme raises the maximum.

• A competitive inhibitor blocks the active site, and extra substrate reduces its effect.