Metabolism, Bioenergetics, and Enzymes

Learning Objectives

By the end of this section, you should be able to:

• Describe the differences between catabolic and anabolic reactions in metabolism.

• Explain the differences between kinetic and potential energy.

• Discuss how the laws of thermodynamics affect energy transformations in living organisms.

• Explain why catabolic metabolism uses exergonic reactions and why anabolic metabolism uses endergonic reactions.

• Give examples of enzymes and explain their role as biological catalysts.

• Describe how enzymes affect reaction rates and how substrates bind to enzymes.

• Explain how enzyme function is affected by temperature, pH, cofactors, and inhibitors.

• Describe forms of enzyme regulation: allosteric regulation, cooperativity, and feedback inhibition.

What is Metabolism?

Definition and Overview

Metabolism is the sum of all chemical reactions that occur within a living organism. These reactions are essential for maintaining life and are catalyzed by enzymes.

• Metabolism includes both the synthesis (anabolism) and breakdown (catabolism) of complex molecules.

• Metabolic reactions occur continuously at both the cellular and whole-organism level.

• Examples of metabolic activities: muscle contraction, blood circulation, cell repair, and hormone regulation.

Anabolic and Catabolic Pathways

Metabolic pathways can be divided into two main types:

• Anabolic pathways: Consume energy to build complicated molecules from simpler ones (e.g., synthesis of proteins from amino acids).

• Catabolic pathways: Release energy by breaking down complex molecules into simpler ones (e.g., cellular respiration, breakdown of macromolecules).

Both types of pathways are essential for managing the material and energy resources of the cell.

Forms of Energy

Kinetic vs. Potential Energy

• Kinetic energy: The energy of motion (e.g., movement, thermal, sound, electrical energy).

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

In biological systems, chemical energy stored in molecular bonds is a key form of potential energy.

Laws of Thermodynamics in Biology

First Law of Thermodynamics

• Energy cannot be created or destroyed, only transformed from one form to another.

• In living organisms, energy is transformed during metabolic reactions but the total energy remains constant.

Second Law of Thermodynamics

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

• Some energy is always lost as heat during energy transformations, making it less available for work.

• Biological systems require a constant input of energy to maintain order and counteract entropy.

Energy and Chemical Reactions

Gibbs Free Energy and Reaction Types

• Gibbs free energy (G): The amount of energy in a system that can do work.

• The change in free energy () during a reaction determines whether the reaction is spontaneous.

Equation:

Exergonic vs. Endergonic Reactions

• Exergonic reactions: Release energy (), are spontaneous (e.g., cellular respiration).

• Endergonic reactions: Require an input of energy (), are nonspontaneous (e.g., synthesis of macromolecules).

Enzymes and Catalysis

What are Enzymes?

• Enzymes are biological catalysts, usually proteins, that speed up chemical reactions without being consumed.

• Some enzymes are RNA molecules (ribozymes).

• Enzymes are highly specific for their substrates and reactions.

Examples of enzymes:

• Sucrase: Breaks down sucrose into glucose and fructose.

• Lactase: Breaks down lactose into glucose and galactose.

• Proteases: Digest proteins.

• Lipases: Break down lipids.

How Enzymes Work

• Enzymes lower the activation energy required for a reaction to proceed.

• They do not change the overall of the reaction.

• Enzymes increase the rate of specific reactions, unlike heat which is non-specific.

Enzyme-Substrate Interaction

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

• Lock-and-key model: Substrate fits exactly into the active site.

• Induced fit model: The enzyme changes shape slightly to fit the substrate more snugly, stabilizing the transition state and lowering activation energy.

Enzyme Activity and Regulation

• Enzyme activity can be affected by environmental factors such as temperature, pH, and salinity.

• Each enzyme has an optimal temperature and pH at which it functions best.

• High temperatures or extreme pH can denature enzymes, altering their structure and function.

Cofactors and Coenzymes

• Cofactors: Inorganic molecules (e.g., zinc, iron) required for enzyme activity.

• Coenzymes: Organic molecules (e.g., vitamins) that assist enzymes.

Enzyme Inhibitors

• Competitive inhibitors: Molecules that resemble the substrate and compete for binding at the active site.

• Noncompetitive inhibitors: Bind to a different site (allosteric site) on the enzyme, changing its shape and reducing activity.

Enzyme Regulation Mechanisms

• Allosteric regulation: Enzyme activity is regulated by molecules binding to sites other than the active site, causing conformational changes.

• Cooperativity: Binding of a substrate to one active site affects the activity at other active sites (common in multimeric enzymes).

• Feedback inhibition: The end product of a metabolic pathway inhibits an enzyme involved earlier in the pathway, preventing overproduction.

Summary Table: Anabolic vs. Catabolic Pathways

Key Terms to Know

• Metabolism

• Anabolism

• Catabolism

• Kinetic energy

• Potential energy

• Thermodynamics

• Exergonic

• Endergonic

• Activation energy

• Enzyme

• Substrate

• Active site

• Cofactor

• Coenzyme

• Competitive inhibitor

• Noncompetitive inhibitor

• Allosteric regulation

• Feedback inhibition