Energy, Enzymes, and Metabolism

Metabolism

Metabolism refers to all the chemical reactions that occur within a living organism to maintain life. These reactions are organized into metabolic pathways, which are sequences of chemical reactions, each catalyzed by a specific enzyme.

• Metabolic Pathways: Series of interconnected chemical reactions that transform a starting molecule into a final product through a series of steps. Each step is catalyzed by a different enzyme.

• Single Chemical Reaction: Involves only one transformation, whereas metabolic pathways involve multiple, regulated steps.

• Substrates/Reactants: The starting materials in a chemical reaction.

• Products: The molecules produced by the reaction.

Catabolic vs. Anabolic Pathways:

• Catabolic Pathways: Break down complex molecules into simpler ones, releasing energy (e.g., cellular respiration).

• Anabolic Pathways: Build complex molecules from simpler ones, requiring energy input (e.g., protein synthesis).

• Similarities: Both involve multiple steps and are regulated by enzymes.

Free Energy and the Laws of Thermodynamics

Energy is the capacity to do work or cause change. In biological systems, energy exists in various forms and is governed by the laws of thermodynamics.

• Forms of Energy:

  • Kinetic Energy: Energy of motion (e.g., movement of molecules, muscle contraction).

  • Potential Energy: Stored energy due to position or structure (e.g., chemical bonds, concentration gradients).

• First Law of Thermodynamics: Energy cannot be created or destroyed, only transformed from one form to another.

• Second Law of Thermodynamics: Every energy transfer increases the entropy (disorder) of the universe; some energy is lost as heat.

Free Energy (G): The portion of a system's energy that can perform work at constant temperature and pressure.

• Free Energy Change (ΔG): Determines whether a reaction is spontaneous.

  • If , the reaction is spontaneous (exergonic).

  • If , the reaction is nonspontaneous (endergonic).

  • If , the system is at equilibrium.

Key Terms and Relationships:

• System: The part of the universe being studied (e.g., a cell).

• Surroundings: Everything outside the system.

• Universe: System + surroundings.

• Entropy (S): Measure of disorder; increases in spontaneous processes.

• Spontaneous Reaction: Occurs without energy input; increases entropy; .

• Nonspontaneous Reaction: Requires energy input; .

• Exergonic Reaction: Releases energy; .

• Endergonic Reaction: Requires energy input; .

Interpreting Free Energy Graphs: Free energy diagrams plot the progress of a reaction versus free energy. The difference between reactants and products indicates and whether the reaction is exergonic or endergonic.

ATP (Adenosine Triphosphate)

ATP is the primary energy currency of the cell, used to power many cellular processes.

• Structure of ATP: Consists of an adenine base, a ribose sugar, and three phosphate groups. It is structurally similar to a nucleotide (the monomeric subunit of nucleic acids).

• ATP Hydrolysis: The reaction in which ATP is broken down into ADP (adenosine diphosphate) and inorganic phosphate (), releasing energy.

• Cellular Work Powered by ATP:

  • Mechanical Work: Movement of motor proteins (e.g., muscle contraction).

  • Transport Work: Pumping substances across membranes (e.g., sodium-potassium pump).

  • Chemical Work: Driving endergonic reactions (e.g., synthesis of macromolecules).

• ATP-ADP Cycle: ATP is continuously regenerated from ADP and by energy from exergonic reactions (e.g., cellular respiration). ATP hydrolysis (exergonic) powers endergonic cellular processes.

Enzymes

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

• Active Site: The region on the enzyme where the substrate binds and the reaction occurs. It has a unique shape and chemical environment specific to its substrate.

• Catalytic Cycle of Enzymes:

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

  2. The enzyme catalyzes the conversion of substrate to product.

  3. The product is released, and the enzyme is free to catalyze another reaction (enzymes are recycled).

• Activation Energy (Ea): The initial energy input required to start a reaction. Enzymes lower , making reactions proceed faster without changing .

• Cofactors: Non-protein helpers (e.g., metal ions, vitamins) required by some enzymes for activity. Without cofactors, these enzymes are inactive.

• Environmental Effects on Enzyme Activity:

  • Enzyme/Substrate Concentration: Increasing concentration increases reaction rate up to a point (saturation).

  • Temperature: Each enzyme has an optimal temperature; too high or too low reduces activity.

  • pH: Each enzyme has an optimal pH; deviations can denature the enzyme or reduce activity.

Regulation of Enzyme Activity

Cells regulate enzyme activity to control metabolic pathways and conserve resources.

• Competitive Inhibition: Inhibitor resembles the substrate and competes for binding at the active site.

• Noncompetitive Inhibition: Inhibitor binds to a site other than the active site, changing the enzyme's shape and reducing activity.

• Allosteric Enzymes: Enzymes with regulatory sites (allosteric sites) distinct from the active site. Binding of regulators can increase or decrease activity. Allosteric inhibition occurs when an inhibitor binds to the allosteric site, stabilizing the inactive form of the enzyme.

• Feedback Inhibition: The end product of a metabolic pathway inhibits an enzyme involved earlier in the pathway, preventing overproduction. Example: In amino acid biosynthesis, the final amino acid product inhibits the first enzyme in the pathway.

Example of Feedback Inhibition:

• Pathway: A → B → C → D (final product)

• Product D inhibits the enzyme that converts A to B, stopping the pathway when enough D is present.

Additional info: Enzyme regulation is essential for homeostasis and efficient resource use in cells. Many drugs and toxins act as enzyme inhibitors.