Enzymes: Biological Catalysts, Classification, and Mechanisms (Chapter 16 Study Notes)

Enzymes: Biological Catalysts

Definition and Function

Enzymes are specialized proteins that act as biological catalysts, accelerating chemical reactions in living organisms. They are essential for nearly all biochemical processes in cells.

• Biological catalyst: A substance that increases the rate of a chemical reaction without being consumed.

• Enzymes lower the activation energy required for reactions, making them proceed faster.

• Enzyme-catalyzed reactions are vital for metabolism and cellular function.

Example: The breakdown of hydrogen peroxide in cells is catalyzed by the enzyme catalase, which speeds up the reaction and prevents cellular damage.

Equation:

$\text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{HCO}_3^- + \text{H}^+$

Additional info: The energy diagram shows that the catalyzed reaction has a lower activation energy than the uncatalyzed reaction.

Enzyme Names

Naming Conventions

Enzyme names are systematic and often indicate the substrate or type of reaction they catalyze.

• Most enzyme names end with the suffix -ase.

• Names may identify the substrate (e.g., sucrase acts on sucrose).

• Names may describe the reaction (e.g., oxidase catalyzes oxidation).

• Some enzymes have common names, especially digestive enzymes (e.g., pepsin, trypsin).

Example: Lactase catalyzes the hydrolysis of lactose into glucose and galactose.

Classification of Enzymes

Major Classes and Their Functions

Enzymes are classified based on the type of reaction they catalyze. There are six main classes:

Class	Type of Reactions Catalyzed
Oxidoreductases	Oxidation–reduction
Transferases	Transfer groups of atoms
Hydrolases	Hydrolysis
Lyases	Add or remove atoms to or from a double bond
Isomerases	Rearrange atoms
Ligases	Use ATP to combine small molecules

Example: Hydrolases catalyze the hydrolysis of bonds, such as the breakdown of proteins by proteases.

Enzyme Mechanism: Active Site and Substrate Binding

Active Site Structure and Function

The active site is a specific region on the enzyme where the substrate binds and the reaction occurs.

• The active site fits the shape of the substrate molecule.

• Contains amino acid R groups that interact with the substrate.

• After the reaction, products are released and the enzyme is free to catalyze another reaction.

Example: The enzyme lactase has an active site that binds lactose, facilitating its hydrolysis.

Enzyme-Catalyzed Reaction Process

Steps in Enzyme Action

Enzyme-catalyzed reactions follow a specific sequence:

• Substrate attaches to the enzyme's active site.

• An enzyme-substrate (ES) complex forms.

• The reaction occurs, producing products.

• The enzyme is regenerated and can be reused.

General equation:

$E + S \rightarrow (ES) \rightarrow E + P$

Where E is enzyme, S is substrate, P is product.

Example: Hydrolysis of Lactose by Lactase

Mechanism and Products

In the hydrolysis of lactose, the enzyme lactase binds lactose at its active site, forming an ES complex. The glycosidic bond is positioned for hydrolysis, and the enzyme's R groups catalyze the reaction.

• Products glucose and galactose are released.

• Lactase is not consumed and can catalyze further reactions.

Equation:

$\text{Lactase} + \text{Lactose} \rightarrow \text{Lactase–Lactose Complex} \rightarrow \text{Lactase} + \text{Glucose} + \text{Galactose}$

Models of Enzyme Action

Lock-and-Key Model

The lock-and-key model proposes that the active site has a rigid shape, and only substrates with a matching shape can bind.

• Active site is nonflexible.

• Substrate fits exactly, like a key in a lock.

Limitation: Does not account for enzyme flexibility.

Induced-Fit Model

The induced-fit model suggests that the enzyme's active site is flexible and adapts to the shape of the substrate upon binding.

• Enzyme structure adjusts to fit the substrate.

• Increases substrate specificity and catalytic efficiency.

• Shape changes lower the activation energy of the reaction.

Example: Lactase adapts its active site to bind lactose, facilitating hydrolysis.

Isoenzymes

Definition and Biological Role

Isoenzymes are different forms of an enzyme that catalyze the same reaction but are found in different tissues or cells.

• Consist of quaternary structures with slight variations in amino acid sequences.

• Allow for tissue-specific regulation of biochemical reactions.

Example: Five isoenzymes of lactate dehydrogenase (LDH) catalyze the conversion between lactate and pyruvate in various tissues.

Learning Checks

Classification and Mechanism Review

• Hydrolases: Catalyze hydrolysis reactions.

• Isomerases: Rearrange atoms in a molecule to form isomers.

• Transferases: Transfer groups between compounds.

• Ligases: Form bonds between molecules using ATP energy.

Active Site: The active site is a section of the enzyme where substrate binding and catalysis occur.

Induced-Fit Model: The enzyme adapts its shape to fit the substrate during binding.