Metabolism and Energy Transformations

Overview of Metabolism

Metabolism encompasses all chemical reactions occurring within an organism. These reactions are organized into metabolic pathways, which can be classified as either anabolic (building up) or catabolic (breaking down).

• Anabolic pathways: Synthesize complex molecules from simpler ones; require energy input (e.g., protein synthesis from amino acids).

• Catabolic pathways: Break down complex molecules into simpler ones; release energy (e.g., cellular respiration).

Metabolism is regulated by enzymes, which catalyze each step of the pathway.

Types of Energy

Energy is the capacity to cause change or do work. It exists in various forms:

• Kinetic energy: Energy of motion (e.g., water flowing over a dam).

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

Energy can be converted from one form to another but cannot be created or destroyed (First Law of Thermodynamics).

Thermodynamics in Biology

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

• Second Law of Thermodynamics: Every energy transfer increases the entropy (disorder) of the universe; some energy is always lost as heat.

• Equilibrium: At equilibrium, no net energy flow occurs, and no work can be performed.

Free Energy and Spontaneity of Reactions

Free Energy (Gibbs Free Energy)

Free energy (G) is the portion of a system's energy available to do work. The change in free energy () during a reaction determines whether the reaction is spontaneous.

• Spontaneous process: Occurs without input of energy; is negative.

• Nonspontaneous process: Requires energy input; is positive.

Equation:

• If , the reaction releases free energy (exergonic).

• If , the reaction requires energy input (endergonic).

Examples

• Cellular respiration: Exergonic reaction; is negative; releases energy.

• Photosynthesis: Endergonic reaction; is positive; requires energy input (sunlight).

ATP: The Cell's Energy Currency

Structure and Function of ATP

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

• Adenine (a nitrogenous base)

• Ribose (a five-carbon sugar)

• Three phosphate groups

The terminal phosphate bond is unstable and can be broken by hydrolysis, releasing energy:

Typical for ATP hydrolysis: (or )

ATP in Cellular Work

ATP powers cellular work by coupling exergonic reactions (energy-releasing) to endergonic reactions (energy-consuming). The phosphate group from ATP is often transferred to another molecule, creating a phosphorylated intermediate that is more reactive.

• Chemical work: Driving endergonic reactions (e.g., synthesis of macromolecules)

• Transport work: Pumping substances across membranes against gradients

• Mechanical work: Muscle contraction, movement of chromosomes

Table: Types of Cellular Work and Energy Use

Enzymes and Metabolic Reactions

Enzyme Structure and Function

Enzymes are biological catalysts that speed up metabolic reactions by lowering the activation energy (Ea) required for a reaction to occur. They are not consumed in the reaction.

• Active site: Region on the enzyme where the substrate binds.

• Substrate: The reactant molecule acted upon by the enzyme.

• Product: The molecule(s) produced by the reaction.

Mechanisms of Enzyme Action

• Orienting substrates correctly in the active site

• Straining substrate bonds to make them easier to break

• Providing a favorable microenvironment

• Directly participating in the reaction (e.g., forming temporary covalent bonds)

Enzyme-Substrate Interaction: Induced Fit Model

When a substrate binds to the enzyme's active site, the enzyme changes shape slightly to create a tighter fit, known as induced fit.

Steps of Enzyme Action

1. Substrate binds to the active site.

2. Induced fit occurs.

3. Bonds in the substrate are stressed.

4. Bonds are broken.

5. New bonds are formed.

6. Product is released.

Factors Affecting Enzyme Activity

• Substrate concentration: Reaction rate increases with substrate concentration up to a point.

• pH: Each enzyme has an optimal pH; too high or too low can denature the enzyme.

• Temperature: Reaction rate increases with temperature up to an optimum; higher temperatures can denature enzymes.

Cofactors and Coenzymes

• Cofactors: Non-protein helpers required for enzyme activity (e.g., metal ions like Zn2+, Fe2+).

• Coenzymes: Organic cofactors (e.g., vitamins such as NAD+, FAD).

Example: Pepsin is a human enzyme that functions well at pH 2 and is found in the stomach.

Enzyme Inhibition

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

• Noncompetitive inhibitors: Bind to another part of the enzyme, causing a shape change that reduces activity.

Summary Table: Catabolic vs. Anabolic Pathways

Key Equations

• ATP Hydrolysis:

• Free Energy Change:

Additional info:

• Enzyme specificity is determined by the unique three-dimensional structure of the enzyme, particularly the shape of the active site.

• Enzyme-catalyzed reactions are essential for regulating metabolism and maintaining homeostasis in living organisms.