Introduction to Metabolism and Enzyme Function

Energy and Metabolism

Overview of Metabolism

Metabolism encompasses all the chemical reactions that occur within an organism to sustain life. These reactions are organized into metabolic pathways, which transform matter and energy according to the laws of thermodynamics.

• Metabolic Pathways: Series of chemical reactions catalyzed by enzymes, converting substrates into products.

• Catabolic Pathways: Break down molecules to release energy (e.g., cellular respiration).

• Anabolic Pathways: Build complex molecules from simpler ones, requiring energy (e.g., protein synthesis).

• Example: Glycolysis and the Krebs cycle are key metabolic pathways in cells.

Types of Metabolic Pathways

Chain	Cycle
Linear sequence of reactions (A → B → C → D → E → F → G)	Reactions form a closed loop, regenerating the starting molecule (e.g., Krebs cycle)

Forms and Transformation of Energy

Energy is the capacity to do work or cause change. It exists in various forms and can be transformed or transferred between objects and systems.

• Kinetic Energy: Energy of motion (e.g., heat, light).

• Potential Energy: Stored energy due to position or structure (e.g., chemical bonds).

• Thermal Energy: Random movement of atoms or molecules.

• Light Energy: Used in photosynthesis.

• Chemical Energy: Stored in molecular bonds, released during chemical reactions.

Thermodynamics in Biology

Biological systems obey the laws of thermodynamics, which govern energy transformations.

• First Law: Energy cannot be created or destroyed, only transformed.

• Second Law: Every energy transfer increases the entropy (disorder) of the universe.

• Implication: Living systems maintain order by increasing entropy elsewhere (e.g., heat loss).

Free Energy and Spontaneity of Reactions

Gibbs Free Energy ($G$)

Free energy determines whether a reaction will occur spontaneously. The change in free energy ($\Delta G$) predicts the direction of chemical reactions.

• Exergonic Reaction: Releases energy, $\Delta G < 0$, spontaneous.

• Endergonic Reaction: Requires energy input, $\Delta G > 0$, non-spontaneous.

• Equation: $\Delta G = \Delta H - T\Delta S$

• Example: Cellular respiration is exergonic; photosynthesis is endergonic.

ATP: The Energy Currency of the Cell

Structure and Function of ATP

Adenosine triphosphate (ATP) stores and transfers energy within cells. Hydrolysis of ATP releases energy used to power cellular work.

• ATP Hydrolysis: $\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{Energy}$

• Coupled Reactions: Energy from ATP hydrolysis drives endergonic reactions.

• Example: Muscle contraction, active transport, biosynthesis.

Enzymes and Catalysis

Role of Enzymes

Enzymes are biological catalysts that speed up chemical reactions by lowering the activation energy required. They are highly specific for their substrates.

• Activation Energy ($E_a$): Energy needed to start a reaction.

• Enzyme-Substrate Complex: Substrate binds to the enzyme's active site, forming a temporary complex.

• Optimal Conditions: Each enzyme has an optimal temperature and pH for activity.

• Example: Catalase breaks down hydrogen peroxide into water and oxygen.

Enzyme Action: Energy Profile

With Enzyme	Without Enzyme
Lower activation energy, faster reaction	Higher activation energy, slower reaction

Factors Affecting Enzyme Activity

• Temperature: Increases reaction rate up to an optimal point; excessive heat denatures enzymes.

• pH: Each enzyme has an optimal pH; extremes can denature the enzyme.

• Substrate Concentration: Rate increases with substrate concentration until saturation is reached.

• Enzyme Concentration: More enzyme increases reaction rate until all substrate is used.

Enzyme Inhibition

Types of Inhibitors

• Competitive Inhibitors: Bind to the active site, blocking substrate binding.

• Noncompetitive Inhibitors: Bind elsewhere, changing enzyme shape and reducing activity.

• Irreversible Inhibitors: Permanently inactivate enzymes (e.g., toxins, heavy metals).

Comparison of Inhibition Types

Type	Binding Site	Effect on Reaction Rate
Competitive	Active site	Can be overcome by increasing substrate
Noncompetitive	Allosteric site	Cannot be overcome by substrate increase

Feedback Inhibition

Feedback inhibition regulates metabolic pathways by using the end product to inhibit an earlier step, preventing overproduction.

• Allosteric Site: Site other than the active site where regulatory molecules bind.

• Example: Threonine deaminase inhibited by isoleucine in amino acid synthesis.

Tables of Enzyme Inhibitors

Competitive Inhibitors

Enzyme	Substrate	Inhibitor	Application
Alcohol dehydrogenase	ethylene glycol	Fomepizole	Prevents poisoning by antifreeze ingestion
Phosphodiesterase type 5 (PDE5)	cGMP	Sildenafil (Viagra)	Treats erectile dysfunction
Acetylcholinesterase	Acetylcholine	Sarin	Nerve agent, inhibits neurotransmission

Noncompetitive Inhibitors

Enzyme	Substrate	Inhibitor	Application
Cytochrome oxidase	Oxygen	Cyanide	Blocks cellular respiration
Enzymes inhibited by mercury	Hydrogen peroxide (H2O2)	Mercury	Mercury poisoning

Summary of Learning Objectives

• Explain how metabolism transforms matter and energy.

• Describe factors affecting enzyme shape and function.

• Distinguish between catabolic and anabolic pathways.

• Interpret the role of ATP in cellular energy transfer.

• Analyze enzyme inhibition and feedback regulation in metabolic pathways.