Metabolism, Energy, and Enzymes: Foundations of Cellular Function

Metabolism: The Chemical Processes of Life

Definition and Overview

Metabolism refers to the total of all chemical reactions carried out by an organism. These reactions are essential for maintaining life, enabling cells to grow, reproduce, maintain their structures, and respond to environmental changes.

• Anabolism: The set of metabolic pathways that construct molecules from smaller units. These reactions require energy input. Example: Synthesis of proteins from amino acids.

• Catabolism: The set of metabolic pathways that break down molecules into smaller units, releasing energy. Example: Breakdown of glucose during cellular respiration.

Metabolic pathways are often organized into multienzyme complexes, where the product of one enzyme becomes the substrate for the next, increasing efficiency and regulation.

The Flow of Energy in Living Systems

What is Energy?

Energy is the capacity to do work or cause change. In biological systems, energy is required for processes such as movement, synthesis of molecules, and active transport.

• Kinetic energy: The energy of motion. Example: A child sliding down a slide.

• Potential energy: Stored energy due to position or structure. Example: A child at the top of a slide.

• Other forms: Mechanical, thermal, chemical, electrical, sound, and nuclear energy.

The Laws of Thermodynamics and Free Energy

First Law of Thermodynamics

The First Law of Thermodynamics states that energy cannot be created or destroyed, only transformed from one form to another. The total amount of energy in the universe remains constant.

Second Law of Thermodynamics

The Second Law of Thermodynamics states that the disorder (entropy) of the universe tends to increase. Energy transformations are not 100% efficient; some energy is always lost as heat, increasing entropy.

• Spontaneous processes increase the disorder of a system.

• Ordered forms of energy are converted to less ordered forms.

Free Energy (Gibbs Free Energy)

Free energy (G) is the energy available to do work in a system. The change in free energy ($\Delta G$) determines whether a reaction is spontaneous.

• Formula:

$\Delta G = \Delta H - T\Delta S$

• $\Delta H$ = change in enthalpy (total energy in chemical bonds)

• $T$ = absolute temperature (in Kelvin)

• $\Delta S$ = change in entropy (disorder)

• Exergonic reactions: $\Delta G < 0$; release free energy; spontaneous.

• Endergonic reactions: $\Delta G > 0$; require input of energy; not spontaneous.

Redox Reactions: Electron Transfer and Energy Flow

Oxidation-Reduction (Redox) Reactions

Redox reactions involve the transfer of electrons between molecules, which is a key mechanism for energy flow in biological systems.

• Oxidation: Loss of electrons from a molecule (often releases energy).

• Reduction: Gain of electrons by a molecule (often gains energy).

• In a redox reaction, the reduced molecule gains energy.

ATP: The Energy Currency of Cells

Structure and Function of ATP

ATP (Adenosine Triphosphate) is the primary energy carrier in cells. It consists of:

• Ribose (a five-carbon sugar)

• Adenine (a nitrogenous base)

• Three phosphate groups (linked by high-energy bonds)

ATP is not suitable for long-term energy storage but is ideal for short-term, immediate energy needs. Cells typically store only a few seconds' worth of ATP.

ATP Hydrolysis and Energy Coupling

• Hydrolysis of ATP (breaking a phosphate bond) releases energy:

$\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

• This energy is used to drive endergonic (energy-requiring) cellular processes.

• ATP has more free energy than ADP or AMP.

Enzymes: Biological Catalysts

Role and Mechanism of Enzymes

Enzymes are biological catalysts, usually proteins (some are RNA), that speed up chemical reactions by lowering the activation energy required. They are not consumed or permanently changed by the reaction.

• Enzymes do not affect the overall free energy change ($\Delta G$) of a reaction.

• They increase the rate of reaction by stabilizing the transition state.

• Enzyme activity can be affected by substrate concentration, temperature, and pH.

Enzyme Structure and Function

• Active site: The region of the enzyme where the substrate binds.

• Induced fit: The enzyme changes shape slightly to better fit the substrate, applying stress to specific bonds and lowering activation energy.

Enzyme Regulation

• Allosteric regulation: Enzymes can be regulated by molecules that bind to sites other than the active site (allosteric sites), causing conformational changes that increase or decrease activity.

• Allosteric activators: Increase enzyme activity.

• Allosteric inhibitors: Decrease enzyme activity.

• Competitive inhibition: Inhibitor binds to the active site, blocking substrate binding.

• Noncompetitive inhibition: Inhibitor binds to an allosteric site, changing the enzyme's shape so the substrate cannot bind.

Cofactors and Coenzymes

• Cofactors: Non-protein chemical compounds (often metal ions) that assist enzyme function.

• Coenzymes: Organic molecules (often derived from vitamins) that assist in enzyme activity, often by transferring electrons or functional groups.

Organization and Regulation of Metabolic Pathways

Pathway Structure

Metabolic reactions are often organized into pathways, with each step catalyzed by a specific enzyme. Multienzyme complexes allow for efficient substrate channeling and regulation.

Feedback Inhibition

Feedback inhibition is a regulatory mechanism in which the end product of a metabolic pathway inhibits an earlier step, often by binding to an allosteric site on an enzyme. This prevents the overaccumulation of products and ensures efficient use of resources.

Summary Table: Types of Enzyme Regulation

Type	Mechanism	Effect	Example
Allosteric Activation	Activator binds to allosteric site	Increases enzyme activity	Phosphofructokinase in glycolysis
Allosteric Inhibition	Inhibitor binds to allosteric site	Decreases enzyme activity	Feedback inhibition by ATP
Competitive Inhibition	Inhibitor binds to active site	Blocks substrate binding	Sulfa drugs inhibiting folate synthesis
Noncompetitive Inhibition	Inhibitor binds elsewhere, changes enzyme shape	Prevents substrate binding	Heavy metal poisoning

Clinical Relevance

Defects in metabolic pathways can lead to the accumulation of intermediates and disease. For example, a nonfunctional enzyme in a pathway can cause a buildup of its substrate, which may be toxic or interfere with normal cellular function.

Example: Inherited metabolic disorders such as phenylketonuria (PKU) result from enzyme deficiencies that disrupt normal metabolism.