Enzymes: active sites, activation energy and substrate specificity

What the exam asks

• Concept Explanation (SP 1): Explain how an enzyme’s structure determines its specificity and how the enzyme–substrate complex lowers activation energy.
• Visual Representations (SP 2): Compare catalyzed and uncatalyzed reaction energy curves, or interpret models of substrates binding to active sites.
• Questions and Methods (SP 3): Identify the independent variable, dependent variable, and negative/positive controls in an enzyme-catalyzed assay (such as measuring product formation or substrate depletion over time).

Core ideas

Active sites and substrate specificity

Enzymes are biological catalysts—almost always proteins—that speed up chemical reactions without being consumed or permanently altered.

• An enzyme’s tertiary (and sometimes quaternary) structure folds to create a specific 3D pocket called the active site.
• Substrate specificity requires two things to match between the substrate (reactant) and the amino acid R-groups lining the active site:
  1. Shape (conformation): The substrate must physically fit into the active site.
  2. Chemical properties (charge and polarity): Positively charged R-groups attract negatively charged regions of the substrate; hydrophobic R-groups interact with nonpolar regions of the substrate.

When the substrate enters the active site, weak interactions (hydrogen bonds, ionic bonds, hydrophobic interactions) cause the enzyme to shift shape slightly to grip the substrate tightly. This is induced fit, forming the temporary enzyme–substrate complex.

Lowering activation energy ($E_A$)

Every chemical reaction requires an initial input of energy—the activation energy ($E_A$)—to reach the unstable transition state where bonds are strained and ready to break or form.

Enzymes speed up reactions by lowering the activation energy barrier through the enzyme–substrate complex:

• Bringing two substrates together in the exact orientation needed to react.
• Bending or straining chemical bonds in the substrate so they break more easily.
• Providing a favorable microenvironment (e.g., donating or accepting protons via acidic or basic R-groups).

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