Biochemical reactions in cells are controlled by enzymes

Introduction: The Biological Accelerators

Welcome to one of the most exciting parts of Biology! Have you ever wondered how your body manages to break down a sandwich or copy your DNA in seconds? Left to themselves, these chemical reactions would happen so slowly that life would be impossible.

In this chapter, we explore enzymes—the biological catalysts that make life happen at high speed. We’ll look at how they are built, how they work, and what happens when things get too hot or too acidic for them to handle.

3.1.3.1 Enzymes and Enzyme Action

What is an Enzyme?

An enzyme is a protein that acts as a biological catalyst. A catalyst is something that speeds up a chemical reaction without being used up itself. This means a single enzyme molecule can be used over and over again!

The secret to an enzyme's power lies in its shape. Because enzymes are proteins, they have a specific tertiary structure. This 3D shape creates a special "pocket" or "groove" called the active site.

• Only a specific molecule, called a substrate, can fit into this active site.
• The substrate and the active site have complementary shapes (like two matching puzzle pieces).
• When they join together, they form an enzyme-substrate complex.

Lowering Activation Energy

Every chemical reaction needs a little "push" of energy to get started. This is called activation energy.
Analogy: Imagine you are trying to push a heavy boulder over a hill. The hill is the "activation energy." Enzymes work by making that hill much shorter, so it’s much easier and faster to get the boulder to the other side.

By forming an enzyme-substrate complex, the enzyme puts stress on the bonds of the substrate, making it easier for them to break or join, which lowers the activation energy.

Models of Enzyme Action

Our understanding of enzymes has improved over time. There are two main models you need to know:

1. The Lock and Key Model: This is the older model. It suggests that the active site is a rigid shape that perfectly fits the substrate, just like a key fits a specific lock.
2. The Induced Fit Model: This is the more modern (and accurate) version. It suggests that the active site is not perfectly rigid. Instead, as the substrate gets close, the active site changes shape slightly to wrap tightly around it.
Analogy: Think of a glove. The glove has a general shape for a hand, but as you put your hand in, the glove stretches and molds to fit your fingers perfectly.

Quick Takeaway: Enzymes are proteins with a specific 3D shape. They speed up reactions by lowering activation energy through the formation of enzyme-substrate complexes.

3.1.3.2 The Properties of Enzymes

The Importance of Collisions

For an enzyme to work, it must physically "bump into" a substrate molecule. This is called a collision. The more successful collisions that happen per second, the faster the rate of reaction.

Factors Affecting Enzyme Activity

Several factors can change how well an enzyme works by affecting either the number of collisions or the shape of the active site:

1. Temperature
• Low temperatures: Molecules move slowly. There are fewer collisions, so the reaction is slow.
• Optimum temperature: The "perfect" temperature where the rate is highest because molecules have more kinetic energy.
• High temperatures: If it gets too hot, the enzyme's molecules vibrate so much that the bonds holding the tertiary structure together break. The active site changes shape, and the substrate can no longer fit. The enzyme is now denatured.

2. pH (Acidity)
Every enzyme has an optimum pH. If the pH moves too far away from this, the charges on the amino acids in the active site change. This breaks the hydrogen and ionic bonds, causing the enzyme to denature.

3. Enzyme and Substrate Concentration
• If you add more enzyme, there are more active sites available, so the rate increases (as long as there is enough substrate).
• If you add more substrate, the rate increases because there are more molecules to collide with the enzymes. However, eventually, all the active sites become full (saturated). At this point, adding more substrate won't speed up the reaction anymore.

Common Mistake to Avoid: Never say an enzyme is "killed" by heat or pH. Enzymes are not alive! Use the term denatured instead.

Enzyme Inhibitors

Inhibitors are substances that slow down or stop enzyme activity. There are two types:

Competitive Inhibitors:
These have a similar shape to the substrate. They compete for the active site and "block" it.
Trick: You can overcome competitive inhibition by adding more substrate. If there are 100 substrates and only 1 inhibitor, the substrate is much more likely to "win" the race to the active site.

Non-competitive Inhibitors:
These bind to the enzyme away from the active site (at an allosteric site). This causes the entire enzyme to change shape, which changes the shape of the active site so the substrate no longer fits.
Note: Adding more substrate does not help here, because the active sites are permanently broken or changed.

Key Takeaway: Temperature and pH can denature enzymes. Competitive inhibitors block the active site, while non-competitive inhibitors change the active site's shape from a distance.

Required Practical 1: Investigating Enzyme Rates

In your practical work, you will often measure the rate of an enzyme-controlled reaction. You can calculate the rate using this simple formula:

\( \text{Rate} = \frac{1}{\text{time}} \)

Or, if you are measuring a product (like oxygen gas from the breakdown of hydrogen peroxide by catalase):

\( \text{Rate} = \frac{\text{Volume of product}}{\text{Time}} \)

Don't worry if this seems tricky at first! Just remember that "rate" simply means "how much happened in a certain amount of time." When you draw a graph of these experiments, the gradient (steepness) of the line tells you the rate. A steeper line means a faster reaction.

Did you know? The enzyme carbonic anhydrase in your blood can process 600,000 molecules of substrate every single second! That is some serious speed!

Chapter Summary Review

• Enzymes are protein catalysts with specific tertiary structures.
• They work by lowering activation energy.
• The Induced Fit model explains how the active site molds around the substrate.
• Denaturation happens when bonds in the enzyme break, changing the active site shape.
• Competitive inhibitors bind to the active site; non-competitive inhibitors bind elsewhere.