How Enzymes Work

• Activation Energy ($E_a$): Every chemical reaction requires an initial input of energy to break existing bonds and reach an unstable transition state. Without a catalyst, this energy barrier is often too high for reactions to occur at body temperature.

• Lowering the Barrier: Enzymes work by providing an alternative reaction pathway with a lower activation energy. By stabilizing the transition state, they allow the reaction to proceed much faster at lower temperatures.

• Energy Changes: While enzymes lower the $E_a$, they do not change the overall free energy change ($\Delta G$) of the reaction. The energy levels of the reactants and products remain the same; only the 'hump' in the middle is reduced.

Models of Binding

Types of Reactions

• Catabolic Reactions: These involve breaking down complex molecules into simpler ones (e.g., digestion). A single substrate enters the active site and is split into multiple products.
• Anabolic Reactions: These involve building complex molecules from simpler ones (e.g., protein synthesis). Multiple substrates enter the active site and are joined into a single product.

• Graph Interpretation: When looking at reaction progress graphs, always identify the peak. The difference between the reactant energy level and the peak is the Activation Energy. If the peak is lower, a catalyst is present.

• Specificity Keywords: Use terms like 'complementary shape' and 'tertiary structure' when explaining why an enzyme only works with one substrate. Avoid saying the enzyme 'is the same shape' as the substrate; say the active site is complementary.

• Verification: If asked about the effect of an enzyme on equilibrium, remember that enzymes speed up both the forward and reverse reactions equally, so they do not change the final position of equilibrium.

• 'Used Up': A common error is thinking enzymes are consumed like reactants. They are catalysts and emerge from the reaction ready to be reused immediately.

• Denaturation vs. Inhibition: Denaturation is a permanent change in the enzyme's tertiary structure (usually due to heat or pH), while inhibition is often a temporary blockage of the active site by another molecule.

• Temperature Misconception: While increasing temperature increases kinetic energy and collision frequency, it also increases the vibration of the protein chains. Beyond an optimum, these vibrations break the hydrogen bonds holding the enzyme's shape, leading to denaturation.