Introduction to Metabolism and Enzyme Function: General Biology Study Notes (chapter 6)

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Introduction to Metabolism

Overview of Metabolism

Metabolism encompasses all chemical reactions occurring within a living organism, enabling it to maintain life, grow, and reproduce. These reactions are organized into metabolic pathways, which can be classified as either catabolic or anabolic.

Metabolism: The sum of all chemical reactions in a cell or organism.
Catabolic pathway: Releases energy by breaking down complex molecules into simpler compounds. Example: Cellular respiration breaks down glucose into carbon dioxide and water, releasing energy.
Anabolic pathway: Consumes energy to build complex molecules from simpler ones. Example: Synthesis of proteins from amino acids.

Metabolic Pathways

Metabolic pathways consist of a series of enzyme-catalyzed reactions, where the product of one reaction serves as the substrate for the next.

Pathways are often multistep, allowing for regulation and efficient energy use.
Each step is catalyzed by a specific enzyme.

Energy and Thermodynamics in Biology

Forms of Energy

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

Kinetic energy: Energy of motion; associated with moving objects.
Thermal energy: Random movement of molecules; often measured as heat.
Potential energy: Energy due to location or structure; stored in chemical bonds.
Chemical energy: A form of potential energy stored in molecules.

Thermodynamics

Thermodynamics is the study of energy transformations. Two fundamental laws govern energy in biological systems:

First Law of Thermodynamics

Energy can be transferred and transformed, but it cannot be created or destroyed. This is also known as the law of conservation of energy.

Example: Chemical energy in food is converted to kinetic energy and heat in animals.

Second Law of Thermodynamics

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

Entropy: A measure of disorder or randomness.
Living systems require a constant input of energy to maintain order.

Free Energy and Chemical Reactions

Free Energy (G)

Free energy is the portion of a system's energy that can perform work when temperature and pressure are uniform.

Change in free energy (): Determines whether a reaction is spontaneous.
If , the reaction is spontaneous (exergonic).
If , the reaction is nonspontaneous (endergonic).

Types of Chemical Reactions

Exergonic reaction: Releases energy; spontaneous; .
Endergonic reaction: Requires energy input; nonspontaneous; .
Equilibrium: The forward and reverse reactions occur at the same rate; no net change in free energy.

Adenosine Triphosphate (ATP)

Structure and Function of ATP

Adenosine triphosphate (ATP) is the primary energy carrier in cells.

Composed of adenine (a nitrogenous base), ribose (a sugar), and three phosphate groups.
The bonds between phosphate groups are unstable due to negative charges, making ATP an effective energy source.

ATP Hydrolysis and Energy Coupling

ATP hydrolysis releases energy by breaking the terminal phosphate bond:

This energy is used to drive endergonic reactions in the cell.
ATP is regenerated from ADP and inorganic phosphate through catabolic pathways.

Enzymes and Catalysis

Role of Enzymes

Enzymes are biological catalysts that speed up chemical reactions without being consumed.

Catalyst: An agent that increases the rate of a reaction.
Activation energy (Ea): The energy required to start a reaction.
Enzymes lower the activation energy, allowing reactions to proceed more rapidly.

Enzyme Specificity and Mechanism

Substrate: The reactant an enzyme acts upon.
Active site: The region of the enzyme where the substrate binds.
Enzyme-substrate complex: The temporary association between enzyme and substrate.
Induced fit model: The enzyme changes shape to fit the substrate more tightly, facilitating catalysis.

Factors Affecting Enzyme Activity

Temperature: Each enzyme has an optimal temperature; too high or too low can denature the enzyme.
pH: Each enzyme has an optimal pH; deviations can disrupt enzyme structure and function.

Enzyme Regulation

Enzyme Inhibition

Competitive inhibition: Inhibitor competes with substrate for the active site; can be overcome by increasing substrate concentration.
Noncompetitive inhibition: Inhibitor binds elsewhere on the enzyme, altering its shape and reducing activity.
Irreversible inhibition: Inhibitor binds permanently, often through covalent bonds.

Allosteric Regulation and Cooperativity

Allosteric inhibition: Regulatory molecule binds to a site other than the active site, stabilizing the inactive form.
Allosteric activation: Regulatory molecule stabilizes the active form of the enzyme.
Cooperativity: Binding of one substrate molecule primes the enzyme to accept additional substrate molecules.

Feedback Inhibition

Feedback inhibition occurs when the end product of a metabolic pathway inhibits an enzyme involved earlier in the pathway, preventing overproduction of the product.

Common mode of metabolic regulation in cells.

Summary Table: Types of Enzyme Inhibition

Type	Binding Site	Effect on Enzyme	Can be Overcome by Substrate?
Competitive	Active site	Blocks substrate binding	Yes
Noncompetitive	Allosteric site	Alters enzyme shape	No
Irreversible	Active or allosteric site	Permanently inactivates enzyme	No

Key Equations

Change in Free Energy:
ATP Hydrolysis:

Example: Cellular respiration is a catabolic pathway that releases energy by breaking down glucose, which is then used to regenerate ATP for cellular work.

Additional info: Some details, such as the structure of ATP and the mechanism of feedback inhibition, were expanded for clarity and completeness.