Metabolism: Transforming Matter and Energy

Overview of Metabolism

Metabolism encompasses all chemical reactions within a cell, including those that generate or consume energy. It is an emergent property resulting from the orderly interactions between molecules.

• Catabolic pathways break down complex molecules into simpler ones, releasing energy.

• Anabolic pathways build complex molecules from simpler ones, consuming energy.

• ATP acts as the energy currency, linking catabolic and anabolic processes.

Metabolic Pathways

Metabolic pathways consist of a series of enzyme-catalyzed steps, transforming a starting molecule into a product.

• Each step is catalyzed by a specific enzyme.

• Pathways can be linear or branched, depending on the cell's needs.

Forms of Energy and Thermodynamics

Types of Energy

Energy is the capacity to cause change and is essential for cellular work.

• Kinetic energy: Energy of motion.

• Potential energy: Energy stored due to position or structure.

• Chemical energy: Potential energy available for release in chemical reactions.

The Laws of Thermodynamics

Thermodynamics governs energy transformations in biological systems.

• First Law: Energy can be transferred and transformed, but not created or destroyed.

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

Free Energy and Spontaneity

Gibbs Free Energy (ΔG)

Free energy is the portion of a system's energy available to do work at constant temperature and pressure.

• ΔG = ΔH - TΔS

• ΔH: Change in enthalpy (total energy)

• ΔS: Change in entropy

• T: Temperature in Kelvin

Spontaneous vs. Nonspontaneous Reactions

• Spontaneous reactions have negative ΔG and increase stability.

• Nonspontaneous reactions require energy input and decrease entropy.

Exergonic and Endergonic Reactions

Classification of Reactions

• Exergonic reactions: Release free energy (ΔG < 0), occur spontaneously.

• Endergonic reactions: Absorb free energy (ΔG > 0), are nonspontaneous.

ATP: The Energy Currency of the Cell

Structure of ATP

ATP (adenosine triphosphate) consists of ribose, adenine, and three phosphate groups. It belongs to the nucleic acid macromolecule class.

ATP Hydrolysis

Hydrolysis of ATP releases energy by breaking the terminal phosphate bond, converting ATP to ADP and inorganic phosphate.

• Energy is released due to the repulsion between phosphate groups.

• ATP hydrolysis is highly exergonic.

ATP Coupling and Cellular Work

ATP powers cellular work by coupling exergonic reactions (ATP hydrolysis) to endergonic reactions.

• Phosphorylation: Transfer of a phosphate group to another molecule, driving endergonic reactions.

• ATP is used for chemical, transport, and mechanical work.

ATP Cycle

ATP is regenerated from ADP and inorganic phosphate using energy from catabolic reactions.

• This cycle couples energy-yielding and energy-consuming processes.

Enzymes: Biological Catalysts

Function of Enzymes

Enzymes are proteins that act as catalysts, speeding up reactions by lowering activation energy barriers.

• Enzymes are not consumed in the reaction.

• They do not change ΔG, only the rate of reaction.

Activation Energy

Activation energy is the initial energy required to start a reaction.

• Heat often supplies activation energy.

• Enzymes lower the activation energy, allowing reactions to proceed at moderate temperatures.

Enzyme Specificity and Induced Fit

• Enzymes bind to specific substrates at the active site.

• Induced fit: Enzyme changes shape to better fit the substrate, enhancing catalysis.

Factors Affecting Enzyme Activity

• Substrate concentration: Higher concentration increases reaction rate until saturation.

• Temperature: Each enzyme has an optimal temperature; too high leads to denaturation.

• pH: Each enzyme has an optimal pH.

• Cofactors: Nonprotein helpers (metal ions or coenzymes).

• Inhibitors: Chemicals that reduce enzyme activity (competitive or noncompetitive).

Regulation of Enzyme Activity

Allosteric Regulation

Allosteric regulation involves binding of regulatory molecules at sites other than the active site, affecting enzyme activity.

• Can either inhibit or activate enzymes.

• Enzymes oscillate between active and inactive forms.

Cooperativity

Binding of a substrate or regulatory molecule to one subunit of a multi-subunit enzyme can increase the activity of other subunits (e.g., oxygen binding to hemoglobin).

Feedback Inhibition

Feedback inhibition occurs when the end product of a pathway inhibits an earlier step, preventing overproduction and conserving resources.

Summary Table: Exergonic vs. Endergonic Reactions

Key Equations

• Gibbs Free Energy:

Example: ATP Hydrolysis Coupled to Glutamine Synthesis

• ATP hydrolysis provides energy for the synthesis of glutamine from glutamic acid and ammonia.

• Coupling exergonic and endergonic reactions is essential for cellular metabolism.

Conclusion

Metabolism is a complex, regulated network of chemical reactions that transform matter and energy within cells. Understanding the principles of energy, thermodynamics, and enzyme function is fundamental to biology. Review these concepts for mastery of cellular metabolism.