Enzymes: The Catalysts of Life – Comprehensive Study Notes

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Enzymes: The Catalysts of Life

Introduction to Enzymes

Enzymes are biological catalysts that accelerate chemical reactions in cells, making the difference between reactions that can occur and those that actually do occur under physiological conditions. They are essential for life, as they enable reactions to proceed at rates compatible with cellular function.

Definition: Enzymes are proteins (and some RNA molecules) that catalyze biochemical reactions by lowering the activation energy required.
Example: Hydrolysis of ATP is thermodynamically favorable but occurs slowly without enzymes.

Activation Energy and the Metastable State

Many cellular reactions are thermodynamically feasible but do not proceed at appreciable rates due to high activation energy barriers.

Activation Energy (EA): The energy required to initiate a reaction.
Metastable State: Reactants remain stable until sufficient energy is provided.
Example: ATP hydrolysis has ΔG = –7.3 kcal/mol, but ATP is stable in water for days.

Free energy diagram for uncatalyzed ATP hydrolysis Normal distribution of kinetic energy and thermal activation

Increasing Reaction Rates: Temperature vs. Catalysis

Heat Input: Raising temperature increases kinetic energy, allowing more molecules to reach activation energy. However, cells are isothermal and cannot use heat for this purpose.
Catalysts: Catalysts provide a surface for reactants to interact, lowering EA without being consumed.

Free energy diagram showing catalyzed vs. uncatalyzed reaction Catalytic activation lowers activation energy

Properties of Catalysts and Enzymes

All catalysts, including enzymes, share three basic properties:

Increase reaction rates by lowering activation energy (EA).
Form transient, reversible complexes with substrates.
Change the rate at which equilibrium is achieved, not the equilibrium position.

Organic Catalysts: Most enzymes are proteins, but some RNA molecules (ribozymes) also have catalytic activity.
Example: Ribonuclease P and peptidyl transferase activity in ribosomes.

Major Classes of Enzymes

Enzymes are classified into six major classes based on the type of reaction they catalyze.

Class	Reaction Type	Example	Reaction Catalyzed
Oxidoreductases	Oxidation-reduction reactions (electron transfer)	Alcohol dehydrogenase	Oxidation of ethanol to acetaldehyde as NAD+ is reduced to NADH
Transferases	Transfer of functional groups from one molecule to another	Glucose-6-phosphatase	Phosphorylation of glucose to glucose-6-phosphate using ATP
Hydrolases	Hydrolytic cleavage of bonds	Glucose-6-phosphatase	Hydrolysis of glucose-6-phosphate into glucose and phosphate
Lyases	Removal or addition of groups to form double bonds	Pyruvate decarboxylase	Removal of a carboxyl group from pyruvate to produce acetaldehyde and CO2
Isomerases	Movement of functional groups within a molecule	Maleate isomerase	Isomerization of maleate to fumarate
Ligases	Joining of two molecules to form a single molecule	Pyruvate carboxylase	Addition of CO2 to pyruvate to produce oxaloacetate

Table of major enzyme classes

The Active Site

The active site is a specific region of the enzyme formed by a cluster of amino acids, resulting from the three-dimensional folding of the protein. This site binds substrates with high affinity and is where catalysis occurs.

Structure: Usually a groove or pocket accommodating the substrate.
Function: Substrate binding and catalysis.

Unfolded and folded lysozyme showing active site

Cofactors and Coenzymes

Some enzymes require nonprotein cofactors for catalytic activity.

Prosthetic Groups: Metal ions or small organic molecules tightly bound to the enzyme.
Coenzymes: Organic cofactors, often derived from vitamins.
Importance: Explains the cellular requirement for trace vitamins and minerals.

Enzyme with cofactor Enzyme with coenzyme

Enzyme Specificity

Enzymes exhibit high substrate specificity due to the shape and chemistry of their active sites.

Specificity: Only substrates with the correct shape and chemical properties can bind and be catalyzed.
Example: Succinate dehydrogenase acts specifically on succinate.

Succinate dehydrogenase substrate specificity Fumarate to succinate reaction Fumarate and maleate structures

The Induced-Fit Model

The induced-fit model describes how enzyme-substrate binding causes a conformational change in the enzyme, optimizing the active site for catalysis.

Conformational Change: Brings necessary amino acid side chains into the active site.
Binding: Substrate is held by noncovalent interactions, distinguishing it from similar molecules.
Evidence: X-ray diffraction shows different enzyme shapes with and without substrate.

Induced-fit model with substrate binding

Mechanisms of Substrate Activation

Enzymes activate substrates through three common mechanisms:

Bond Distortion: Makes bonds more susceptible to catalytic attack.
Proton Transfer: Increases substrate reactivity.
Electron Transfer: Forms temporary covalent bonds between enzyme and substrate.

Enzyme Inhibition

Enzyme activity can be inhibited by molecules that interfere with substrate binding or catalysis.

Irreversible Inhibitors: Bind covalently, causing permanent loss of activity (often toxic).
Reversible Inhibitors: Bind noncovalently and can dissociate; include competitive and noncompetitive inhibitors.

Competitive inhibition diagram Noncompetitive inhibition diagram

Enzyme Regulation

Enzyme rates are continuously adjusted to meet cellular needs.

Substrate-Level Regulation: Reaction rates increase with substrate concentration and decrease with product concentration.
Allosteric Regulation: Enzymes are regulated by molecules binding to sites other than the active site, causing conformational changes.
Allosteric Enzymes: Have two conformations: one with high substrate affinity and one with low affinity.

Allosteric enzyme regulation Allosteric inhibition and activation

Feedback Inhibition

Feedback inhibition is a regulatory mechanism where the end product of a pathway inhibits an earlier step, preventing overproduction.

Importance: Maintains metabolic balance and prevents wasteful reactions.

Feedback inhibition pathway

Covalent Modification of Enzymes

Enzyme activity can be regulated by the addition or removal of chemical groups.

Phosphorylation/Dephosphorylation: Addition/removal of phosphate groups by kinases/phosphatases.
Methylation/Demethylation: Addition/removal of methyl groups.
Acetylation/Deacetylation: Addition/removal of acetyl groups.

Phosphorylation and dephosphorylation regulation

Irreversible Covalent Activation

Some enzymes are activated irreversibly by proteolytic cleavage, removing part of the polypeptide chain.

Zymogens: Inactive enzyme precursors activated by proteolytic cleavage.
Examples: Proteolytic enzymes in the pancreas, coagulation cascade, complement cascade.

Summary Table: Key Concepts

Enzymes: Biological catalysts, mostly proteins, some RNA (ribozymes).
Activation Energy: Lowered by enzymes, enabling reactions to proceed rapidly.
Specificity: High substrate specificity due to active site structure.
Regulation: Substrate-level, allosteric, feedback inhibition, covalent modification, proteolytic activation.

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