Metabolism Concepts and Chemical Reactions

Concepts in Metabolism

• Metabolism involves various chemical reactions: substitution, cleavage, condensation, addition, and transfer.
• Chemical equilibrium indicates a balance between forward and reverse reactions, symbolized by Keq.
• Steady-state describes conditions in cells where concentrations of metabolites fluctuate slightly despite continuous input and output.

Chemical Reactions

• Isomerization: Rearrangement of molecular structure without changing the molecular formula.
• Substitution: One atom or group is replaced by another, exemplified by hydrogen replacing a methyl group.
• Redox Reactions: Oxidation and reduction occur simultaneously; coenzymes often involved.
• Cleavage: A reaction type that results in splitting a molecule into parts.
• Condensation: Two molecules combine, releasing water; integral in forming macromolecules (e.g., peptide and glycosidic bonds).
• Addition: Joining of two molecules without water elimination, often across double bonds.
• Transfer: Involves movement of functional groups or phosphate groups between molecules.

Equilibrium and Steady-State

• Biochemical reactions generally run in reversible directions under cellular conditions.
• Cells operate as open systems, maintaining relatively constant concentrations of reactants and products.
• Reactions perpetually ‘pulled’ towards completion due to substrate and product interchanges.
• True equilibria would stop substrate flow, hence cells maintain a net flow of materials.

Bioenergetics

• Bioenergetics studies energy changes in metabolic processes, essential for predicting reaction significance within pathways.
• Key energy concepts include enthalpy (H), entropy (S), and free energy (G).
  • Enthalpy: Overall energy change during a reaction.
  • Entropy: Energy representing disorder; not usable biologically.
  • Free Energy (G): Usable energy for biological work (moving molecules, muscle contraction).

Free Energy Parameters

• ΔG’ represents actual free energy change under physiological conditions (e.g., 37°C, pH 7).
• Standard free energy change (ΔG°) assessed under consistent conditions, critical for comparing different reactions.
• ΔG°’ refers to reactions specifically measured at pH 7; serves as a benchmark for metabolic reactions.

Reaction Coupling

• Physiologically irreversible reactions with positive ΔG require energy to continue, usually supplied by other cellular reactions.
• Energy from sunlight or food fuels endergonic processes via reduced coenzymes (e.g., NADH, FADH2) and high-energy phosphate compounds (e.g., ATP).

ATP: Energy Shuttle

• ATP acts as an energy carrier formed from the energy of glucose breakdown.
• ATP rapidly transfers energy to various cellular processes, illustrating its transient nature.

Special Types of Anhydrides

• Phosphoric Acid Anhydrides (e.g., ATP, ADP): Exhibit large negative free energy changes on hydrolysis due to factors like electrostatic repulsion and resonance stabilization.
• Phosphoric-Carboxylic Anhydrides: Energy-rich compounds (e.g., acetyl-phosphate, 1,3-BPG) with significant free energy changes due to bond strains.
• Enol Phosphates: Phosphoenolpyruvate (PEP) has the highest hydrolysis free energy, pivotal in metabolic reactions.

Enzymes and Kinetics

• Enzymes may function independently or as multienzyme complexes.
• Reaction rate influences include substrate concentration, enzyme concentration, presence of inhibitors or activators, coenzyme levels, pH, and temperature.
• Enzyme kinetics explores how these factors affect the rates of reactions within metabolic pathways.

Intermediary Metabolism

• Encompasses reactions that store and generate metabolic energy essential for biosynthesis of compounds.
• Structural configurations of enzymes critically determine their specific reaction properties.
• Metabolic pathways consist of interlinked chemical reactions, emphasizing the importance of substrate and product transformations.