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3.1 Enzymes

Keywords

English Term 中文翻译 Definition & Explanation
Enzyme 酶 A macromolecule (usually a protein) serving as a biological catalyst, speeding up chemical reactions without being consumed.
Catalyst 催化剂 A substance that increases the rate of a chemical reaction by lowering the activation energy.
Activation Energy (\(E_A\)) 活化能 The initial energy required to start a chemical reaction by breaking the bonds of the reactants.
Substrate 底物 The specific reactant molecule that an enzyme acts on.
Active Site 活性位点 The specific region on an enzyme where the substrate binds and where catalysis occurs.
Enzyme-Substrate Complex 酶-底物复合物 A temporary complex formed when an enzyme binds tightly to its substrate(s).

1. Enzymes as Biological Catalysts

Living systems rely on highly ordered, regulated, and efficient chemical reactions. Enzymes, which are primarily proteins, serve as biological catalysts that make these reactions happen fast enough to sustain life.

Core Concept: Lowering Activation Energy

Every chemical reaction involves breaking and making bonds, which requires an initial investment of energy called Activation Energy (\(E_A\)).

  • How enzymes work: Enzymes speed up metabolic reactions by lowering the activation energy barrier. They make it easier for reactant molecules to reach the transition state.
  • What enzymes DO NOT do: Enzymes do not change the overall free energy (\(\Delta G\)) of the reaction. They do not make an endergonic reaction exergonic; they only speed up reactions that would occur eventually anyway.
Activation energy graph with and without enzyme

2. Specificity and The Enzyme-Substrate Complex

Enzymes are highly specific. An enzyme that digests proteins will not digest carbohydrates. This specificity is a direct result of the enzyme's complex 3D shape, which dictates the structure of its active site.

When the substrate binds to the active site, the enzyme and substrate form an enzyme-substrate complex.

Crucial Detail: Shape and Charge Compatibility

For an enzyme-mediated chemical reaction to occur, the substrate must fit perfectly into the enzyme's active site. This compatibility depends on two main factors:

  1. Shape: The physical contour of the substrate must match the pocket or groove of the active site (often described by the Induced Fit model, where the enzyme slightly changes shape to hug the substrate).
  2. Charge: The chemical properties (charge, polarity, hydrophobicity) of the amino acids lining the active site must be compatible with the substrate to allow binding via weak interactions (like hydrogen bonds or ionic bonds).
Enzyme-substrate complex model

Quiz

Source: Campbell Biology Practice Test - Chapter 8 (An Introduction to Metabolism)