Enzymes

Enzymes are essential because they increase reaction rates to levels necessary to sustain life.

The active site of an enzyme has a specific shape that is complementary to its substrate. The substrate binds to the active site to form an enzyme-substrate complex, resulting in the formation of products.

At very high temperatures, the enzyme's structure is irreversibly altered (denatured) by breaking the bonds that hold the enzyme together, causing the active site to change shape. This prevents the substrate from binding. An example is boiling an egg; the egg white proteins (enzymes) denature, causing it to solidify.

Changes in pH can disrupt the bonds holding the enzyme together, altering the shape of the active site and preventing substrate binding. Extreme pH values can denature the enzyme.

Enzymes are specific because their active site has a unique shape that only fits a specific substrate, like a lock and key. Only the correctly shaped substrate can bind to the active site and undergo a reaction.

Increasing temperature increases the kinetic energy of molecules, leading to more frequent and forceful collisions between enzyme and substrate. This increases the rate of reaction until the optimum temperature is reached. However, beyond this point, denaturation starts to occur.

Extreme pH levels can disrupt the ionic and hydrogen bonds that maintain the enzyme's 3D structure. This change in shape alters the active site, preventing the substrate from binding and leading to denaturation. An example is how changes in blood pH affect the activity of enzymes involved in respiration.

As substrate concentration increases, enzyme activity increases until all active sites are occupied (saturated). After this point, increasing substrate concentration no longer increases the reaction rate.

Above the optimum temperature, the enzyme molecules vibrate too much, breaking the weak bonds (hydrogen bonds) that hold the enzyme in its 3D shape. The active site changes shape — this is called denaturation.

Once denatured, the substrate can no longer fit into the active site (the shape is no longer complementary). This is a permanent change — the enzyme cannot regain its shape when cooled.

As temperature increases:
1. Both enzyme and substrate molecules gain more kinetic energy
2. They move faster and collide more frequently
3. More enzyme-substrate complexes form per unit time
4. The rate of reaction increases

This continues up to the optimum temperature (typically 37°C for human enzymes). The graph shows a steady increase in rate up to the optimum.

Each enzyme can only catalyse one specific reaction because the shape of its active site is complementary to only one substrate (like a lock and key).

Each enzyme has an optimum pH where it works fastest:
• Most human enzymes: pH 7 (neutral)
• Pepsin (stomach): pH 2 (acidic)
• Pancreatic enzymes: pH 8-9 (alkaline)

If the pH is too far from the optimum (too acidic or too alkaline), the charges on the amino acids in the active site change. This alters the shape of the active site, so the substrate no longer fits. At extreme pH values, the enzyme is denatured.

1. The substrate has a specific shape
2. The active site of the enzyme has a complementary shape — they fit together like a key fits a lock
3. The substrate binds to the active site, forming an enzyme-substrate complex
4. The enzyme catalyses the reaction (breaking down or joining molecules)
5. The products are released
6. The enzyme is unchanged and can be reused with another substrate molecule

Only one type of substrate can fit each enzyme — this explains enzyme specificity.

Without enzymes, the chemical reactions needed for life (metabolism) would occur too slowly to sustain life at body temperature.

Enzymes:
• Speed up reactions by lowering the activation energy
• Work at relatively low temperatures (body temperature ~37°C)
• Are highly specific — each controls one reaction, allowing precise metabolic control
• Are not used up — one enzyme molecule can catalyse thousands of reactions

Without enzymes, digestion, respiration, and protein synthesis could not happen fast enough.