Energy and Metabolism

Metabolic Pathways

Metabolic pathways are series of chemical reactions occurring within a cell, where the product of one reaction serves as the substrate for the next. These pathways are essential for maintaining life and are classified as either catabolic or anabolic.

• Catabolic pathway: Breaks down complex molecules into simpler ones, releasing energy (e.g., cellular respiration).

• Anabolic pathway: Builds complex molecules from simpler ones, requiring energy input (e.g., protein synthesis).

Example: Glycolysis is a catabolic pathway that breaks down glucose to produce ATP.

Types of Energy

Energy is the capacity to do work or cause change. In biological systems, energy exists in several forms:

• Kinetic energy: Energy of motion (e.g., movement of molecules).

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

• Chemical energy: Potential energy stored in chemical bonds of molecules.

Example: ATP stores chemical energy used by cells.

Thermodynamics in Biology

Biological systems obey the laws of thermodynamics:

• First law of thermodynamics: Energy cannot be created or destroyed, only transformed.

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

Entropy is a measure of disorder; spontaneous processes increase entropy.

Free Energy and Chemical Reactions

Free Energy (G) and Reaction Types

Free energy (G) determines whether a reaction is spontaneous. The change in free energy (ΔG) is calculated as:

• Exergonic reaction: Releases energy, ΔG < 0, spontaneous.

• Endergonic reaction: Requires energy input, ΔG > 0, non-spontaneous.

Example: Hydrolysis of ATP is exergonic; synthesis of glucose is endergonic.

Energy Coupling

Cells couple endergonic reactions to exergonic ones to drive non-spontaneous processes. ATP hydrolysis is commonly used to power endergonic reactions.

• Energy coupling: The use of energy released from exergonic reactions to drive endergonic reactions.

Example: ATP hydrolysis provides energy for muscle contraction.

Enzymes and Catalysis

Enzyme Function and Activation Energy

Enzymes are biological catalysts that speed up chemical reactions by lowering the activation energy required.

• Activation energy: The energy barrier that must be overcome for a reaction to proceed.

• Catalyst: Substance that increases reaction rate without being consumed.

Example: Amylase catalyzes the breakdown of starch.

Enzyme-Substrate Relationship

Enzymes bind to substrates at the active site, forming an enzyme-substrate complex. The substrate is converted to product, and the enzyme is released unchanged.

• Enzyme: Protein that catalyzes reactions.

• Substrate: Reactant molecule acted upon by the enzyme.

• Product: Resulting molecule after the reaction.

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

• Induced fit: The active site changes shape to better fit the substrate.

Example: Sucrase binds sucrose at its active site and breaks it into glucose and fructose.

Factors Affecting Enzyme Activity

The rate of enzymatic reactions depends on several factors:

• Concentration of enzyme: Higher enzyme concentration increases reaction rate (up to a limit).

• Concentration of substrate: Higher substrate concentration increases rate until saturation.

• Concentration of product: Accumulation of product can slow reaction due to feedback inhibition.

• Temperature: Optimal temperature increases rate; extreme temperatures denature enzymes.

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

Example: Pepsin works best at pH 2 in the stomach.

Cofactors and Coenzymes

Many enzymes require non-protein helpers:

• Cofactors: Inorganic molecules (e.g., metal ions) that assist enzyme activity.

• Coenzymes: Organic molecules (often vitamins) that assist enzymes.

Comparison Table:

Enzyme Inhibition

Enzyme activity can be regulated by inhibitors:

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

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

• Reversible inhibition: Inhibitor can dissociate from enzyme.

• Irreversible inhibition: Inhibitor binds permanently, inactivating enzyme.

Example: Cyanide irreversibly inhibits cytochrome c oxidase.

Allosteric Regulation and Feedback Inhibition

Enzymes can be regulated by molecules binding to sites other than the active site (allosteric regulation). Feedback inhibition occurs when the end product of a pathway inhibits an earlier enzyme, preventing overproduction.

• Allosteric regulation: Regulation of enzyme activity by binding at a site other than the active site.

• Feedback inhibition: End product inhibits an enzyme in the pathway.

Example: ATP inhibits phosphofructokinase in glycolysis.

Unit 4 Vocabulary

• Metabolic pathway

• Catabolic pathway

• Anabolic pathway

• Energy

• Kinetic energy

• Potential energy

• Chemical energy

• First law of thermodynamics

• Entropy

• Second law of thermodynamics

• Spontaneous process

• Free energy

• Exergonic reaction

• Endergonic reaction

• Energy coupling

• ATP

• Activation energy

• Catalyst

• Substrate

• Active site

• Induced fit

• Cofactors

• Coenzyme

• Competitive inhibitors

• Noncompetitive inhibitors

• Allosteric regulation

• Feedback inhibition