Metabolism and Metabolic Pathways

Overview of Metabolism

Metabolism encompasses all chemical reactions within an organism, managing the material and energy resources of the cell. These reactions are organized into metabolic pathways, each consisting of a series of steps catalyzed by specific enzymes.

• Metabolic pathway: A sequence of chemical reactions in which each step is catalyzed by a specific enzyme, transforming a starting molecule into a product.

• Catabolic pathways: Pathways that release energy by breaking down complex molecules into simpler compounds. Example: Cellular respiration breaks down glucose to produce energy.

• Anabolic pathways: Pathways that consume energy to build complex molecules from simpler ones. Example: Synthesis of proteins from amino acids.

• Bioenergetics: The study of how energy flows through living organisms.

Forms of Energy

Definition and Types

Energy is the capacity to cause change or do work, such as moving matter against opposing forces like gravity or friction. Energy exists in various forms and can be transformed from one form to another to perform the work of life.

• Kinetic energy: Energy of motion. Includes radiant (electromagnetic), thermal (heat), motion, sound, and electrical energy.

• Potential energy: Stored energy due to an object's position or structure. Example: Water at the top of a hill, or electrons in chemical bonds.

• Chemical energy: A form of potential energy stored in molecules, such as glucose. This energy is released during chemical reactions.

• Energy conversion: Energy can be converted from one form to another, such as potential energy to kinetic energy.

Thermodynamics in Biological Systems

First Law of Thermodynamics

The first law, or the law of conservation of energy, states that energy can be transferred and transformed, but it cannot be created or destroyed.

• Organisms absorb energy (from light or food) and release metabolic wastes (such as CO2) to their surroundings.

Second Law of Thermodynamics

Every energy transfer or transformation increases the entropy (disorder) of the universe. Some energy is lost as heat and becomes unavailable to do work.

• Entropy: A measure of molecular disorder or randomness.

• Spontaneous processes increase entropy and occur without energy input.

• Nonspontaneous processes decrease entropy and require energy input.

• Living systems create ordered structures but increase the disorder of their surroundings through catabolic reactions.

Free Energy and Spontaneity of Reactions

Free Energy Change (ΔG)

Free energy (G) is the portion of a system's energy that can perform work when temperature and pressure are uniform. The change in free energy (ΔG) determines whether a reaction is spontaneous.

• = change in enthalpy (total energy)

• = change in entropy

• = temperature in Kelvin

• If , the reaction is spontaneous.

• If , the reaction is nonspontaneous.

Stability and Equilibrium

Systems tend to move toward lower free energy (greater stability). At equilibrium, forward and reverse reactions occur at the same rate, and no net change occurs.

Exergonic and Endergonic Reactions

Classification Based on Free Energy

Reactions are classified by their free-energy changes:

• Exergonic reaction: Releases free energy (); is spontaneous.

• Endergonic reaction: Absorbs free energy (); is nonspontaneous.

• In closed systems, reactions eventually reach equilibrium and can do no work.

• Open systems (like cells) prevent equilibrium, enabling continuous work.

ATP and Cellular Work

ATP Structure and Function

Adenosine triphosphate (ATP) is the cell's energy currency, used to power cellular work by coupling exergonic and endergonic reactions.

• Structure: ATP consists of adenine, ribose, and three phosphate groups.

• Hydrolysis of ATP: ATP + H2O → ADP + Pi (inorganic phosphate) + energy

• ATP hydrolysis is exergonic and powers endergonic reactions via phosphorylation (transfer of a phosphate group).

• The recipient molecule becomes a phosphorylated intermediate, which is more reactive.

Types of Cellular Work Powered by ATP

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

• Transport work: Pumping substances across membranes against spontaneous movement

• Mechanical work: Movement, such as beating cilia or muscle contraction

ATP Cycle

ATP is regenerated by the addition of a phosphate group to ADP. The energy for this process comes from catabolic (exergonic) reactions.

• The ATP cycle couples energy-yielding processes to energy-consuming ones.

Enzymes and Catalysis

Enzymes as Catalysts

Enzymes are biological catalysts, usually proteins, that speed up metabolic reactions by lowering the activation energy barrier.

• Activation energy (Ea): The initial energy needed to start a chemical reaction.

• Enzymes do not change ΔG; they only speed up reactions that would occur eventually.

• Enzymes are not consumed during the reaction and can be reused.

Mechanism of Enzyme Action

• Substrate binds to the enzyme's active site, forming an enzyme-substrate complex.

• Enzyme lowers Ea by orienting substrates, straining bonds, providing a favorable microenvironment, or participating directly in the reaction.

• Products are released, and the enzyme is free to catalyze another reaction.

Factors Affecting Enzyme Activity

• Temperature: Each enzyme has an optimal temperature for activity.

• pH: Each enzyme has an optimal pH.

• Cofactors: Non-protein helpers (e.g., metal ions, vitamins) required for enzyme function.

Enzyme Inhibition and Regulation

Types of Inhibitors

• Competitive inhibitors: Bind to the active site, blocking substrate binding. Can be overcome by increasing substrate concentration.

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

• Some inhibitors (e.g., poisons, pesticides) bind irreversibly and can be harmful.

Regulation of Enzyme Activity

• Allosteric regulation: Regulatory molecules bind to a site other than the active site, affecting enzyme function. Can inhibit or activate enzymes.

• Cooperativity: Substrate binding to one active site affects binding at other sites (in multi-subunit enzymes).

• Feedback inhibition: The end product of a metabolic pathway inhibits an earlier step, preventing overproduction.

• Enzymes may be organized within cellular structures to increase efficiency.

Summary Table: Types of Enzyme Inhibition

Summary Table: Metabolic Pathways

Additional info: The review and expansion of the provided slides has included definitions, examples, and academic context for key concepts in metabolism, energy, thermodynamics, ATP, enzymes, and regulation, suitable for General Biology students.