Introduction to Metabolism: Energy, Enzymes, and Regulation

Guida di studio - Note intelligenti

Appunti personalizzati basati sui tuoi materiali, ampliati con definizioni chiave, esempi e contesto.

Introduction to Metabolism

Overview of Metabolism

Metabolism encompasses all chemical reactions occurring within a living organism. These reactions are organized into metabolic pathways, where a starting molecule is converted through a series of steps, each catalyzed by a specific enzyme, to yield a final product.

Catabolic pathways break down complex molecules into simpler ones, releasing energy (e.g., cellular respiration).
Anabolic pathways build complex molecules from simpler ones, consuming energy (e.g., protein synthesis).
Bioenergetics is the study of how energy flows through living organisms.

Metabolic pathway with enzymes Anabolic vs Catabolic Reactions

Energy and Thermodynamics in Biological Systems

Forms and Laws of Energy

Energy is the capacity to cause change and is essential for cellular work. Cells transform energy from one form to another to perform life processes.

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.

Bear illustrating first and second law of thermodynamics

ATP: The Energy Currency of the Cell

Adenosine triphosphate (ATP) is the primary energy carrier in cells. It consists of adenine, ribose, and three phosphate groups. Energy is released when ATP is hydrolyzed to ADP and inorganic phosphate (Pi), which is used to drive cellular work.

Structure of ATP

ATP is regenerated from ADP and Pi through cellular respiration (catabolism of glucose).
Cells use ATP for chemical work (driving endergonic reactions), transport work (pumping substances across membranes), and mechanical work (movement of cilia, muscle contraction).

ATP in transport and mechanical work ATP cycle: coupling catabolism and anabolism

Free Energy and Metabolic Reactions

Gibbs Free Energy (ΔG)

Free energy is the portion of a system's energy that can perform work. The change in free energy (ΔG) during a reaction determines whether the process is spontaneous.

ΔG < 0: Spontaneous (exergonic) reaction; energy is released.
ΔG > 0: Nonspontaneous (endergonic) reaction; energy is required.
ΔG = 0: System is at equilibrium.

Exergonic and endergonic reactions Exergonic reaction graph Endergonic reaction graph

Enzymes and Activation Energy

Role of Enzymes

Enzymes are biological catalysts, usually proteins, that speed up chemical reactions by lowering the activation energy (EA) required for the reaction to proceed. They do not alter the ΔG of the reaction.

Activation energy (EA): The initial energy input needed to start a reaction.
Enzymes bind specific substrates at their active site, forming an enzyme-substrate complex.
Enzymes are highly specific, typically catalyzing only one reaction or type of reaction.

Activation energy and transition state Enzyme catalyzed hydrolysis of sucrose Catalyzed vs uncatalyzed reaction energy profile Enzyme-substrate complex formation Enzyme-substrate complex molecular model Enzyme lowers activation energy

Enzyme Kinetics and Substrate Concentration

The rate of an enzyme-catalyzed reaction increases with substrate concentration until the enzyme becomes saturated. At saturation, the reaction rate can only be increased by adding more enzyme.

Reaction rate vs substrate concentration

Regulation of Enzyme Activity

Environmental Effects

Enzyme activity is influenced by environmental factors such as temperature and pH. Each enzyme has an optimal temperature and pH at which its activity is maximal. Deviations can lead to reduced activity or denaturation (loss of structure and function).

Optimal temperature for enzymes Protein denaturation Optimal pH for enzymes Protein charge and pH

Cofactors and Coenzymes

Cofactors are nonprotein molecules that assist enzymes. They may be inorganic (e.g., metal ions) or organic (coenzymes, often derived from vitamins). Cofactors are essential for the catalytic activity of many enzymes.

Cofactor binding activates protein

Enzyme Inhibition

Enzyme inhibitors are molecules that decrease or block enzyme activity. Inhibition can be reversible or irreversible.

Competitive inhibitors resemble the substrate and bind to the active site, blocking substrate access. Their effect can be overcome by increasing substrate concentration.
Noncompetitive inhibitors bind to a site other than the active site, causing a conformational change that reduces enzyme activity. Their effect cannot be overcome by increasing substrate concentration.

Competitive and noncompetitive inhibition Normal binding of substrate to enzyme Effect of inhibitors on reaction rate

Feedback Inhibition

Cells regulate metabolic pathways through feedback inhibition, where the end product of a pathway inhibits an enzyme involved earlier in the pathway. This prevents the overproduction of products and conserves resources.

Summary Table: Types of Enzyme Regulation

Type	Mechanism	Effect
Competitive Inhibition	Inhibitor binds active site	Can be overcome by increasing substrate
Noncompetitive Inhibition	Inhibitor binds allosteric site	Cannot be overcome by increasing substrate
Feedback Inhibition	End product inhibits pathway	Prevents overproduction

Additional info: Enzyme regulation is essential for maintaining homeostasis and efficient cellular function. Cells can also regulate enzymes by controlling gene expression or by post-translational modifications.