Metabolism: The Sum of All Chemical Reactions in Microorganisms

Overview of Metabolism

Metabolism encompasses all chemical reactions occurring within a living organism. These reactions are divided into two main categories: catabolism (breakdown of complex molecules to release energy) and anabolism (synthesis of complex molecules from simpler ones, requiring energy). The interplay between these processes is fundamental to microbial life.

• Catabolism: Degradative, exergonic reactions that release energy by breaking down complex molecules (e.g., glucose breakdown).

• Anabolism: Biosynthetic, endergonic reactions that consume energy to build complex molecules (e.g., protein synthesis).

• ATP: The energy released from catabolic reactions is stored in adenosine triphosphate (ATP), which is then used to drive anabolic reactions.

Energy and Thermodynamics in Microbial Systems

Types of Energy

• Kinetic Energy: Energy of motion.

• Potential Energy: Stored energy, such as that found in chemical bonds.

• Chemical Energy: A form of potential energy available in chemical reactions, used for cellular work.

Thermodynamic Laws

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

• Second Law (Entropy): Every energy transfer increases the entropy (disorder) of the universe. Energy conversions are never 100% efficient; some energy is always lost as heat.

Chemical Reactions and Enzymes

Collision Theory and Activation Energy

Chemical reactions occur when atoms, ions, or molecules collide with sufficient energy to break or form bonds. The minimum energy required to initiate a reaction is called activation energy.

• Reaction Rate: The frequency of effective collisions. Increased by higher temperature, pressure, or the presence of enzymes.

• Catalysts: Substances (often enzymes) that speed up reactions by lowering activation energy without being consumed.

Enzymes: Biological Catalysts

• Nature: Enzymes are mostly globular proteins with specific three-dimensional shapes.

• Function: They catalyze biochemical reactions by lowering activation energy, increasing reaction rates under mild cellular conditions.

• Specificity: Each enzyme acts on a specific substrate due to the unique structure of its active site.

• Efficiency: Enzymes are highly efficient and regulated by the cell.

Naming and Classification

• Enzyme names typically end in -ase (e.g., lactase, DNA polymerase).

• Six major classes based on reaction type (e.g., oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases).

Enzyme Structure

• Holoenzyme: The complete, active enzyme, consisting of:

  • Apoenzyme: The protein portion.

  • Cofactor: The non-protein component, which may be:

    • Metal ion (e.g., Fe2+, Mg2+, Zn2+).

    • Coenzyme: An organic molecule (e.g., NAD+, FAD, coenzyme A).

Mechanism of Enzyme Action

• Enzyme binds substrate at the active site, forming an enzyme-substrate complex.

• Substrate is transformed into product(s), and the enzyme is released unchanged.

Factors Affecting Enzyme Activity

• Temperature: High temperatures can denature enzymes; low temperatures slow reaction rates.

• pH: Each enzyme has an optimum pH for maximal activity.

• Substrate Concentration: Activity increases with substrate concentration until enzymes are saturated.

• Inhibitors:

  • Competitive Inhibitors: Compete with substrate for the active site.

  • Noncompetitive Inhibitors: Bind elsewhere, altering enzyme function.

Energy Production: Oxidation-Reduction and ATP Generation

Redox Reactions

• Oxidation: Loss of electrons (often as H+ or addition of O2).

• Reduction: Gain of electrons (often as H+ or removal of O2).

• Redox reactions are coupled: when one molecule is oxidized, another is reduced.

• Example: NAD+ is reduced to NADH during glycolysis and the Krebs cycle.

ATP Synthesis Mechanisms

• Substrate-Level Phosphorylation: Direct transfer of a phosphate group to ADP from a phosphorylated intermediate.

• Oxidative Phosphorylation: Energy from redox reactions in the electron transport chain is used to generate ATP via chemiosmosis.

• Photophosphorylation: Light energy is used to generate ATP in photosynthetic organisms.

Metabolic Pathways of Energy Production

Carbohydrate Catabolism

Most cellular energy is derived from the oxidation of carbohydrates, primarily glucose. Two main pathways are used:

• Respiration: Complete breakdown of glucose to CO2 and H2O (aerobic) or to other inorganic molecules (anaerobic).

• Fermentation: Partial breakdown of glucose to organic end-products in the absence of O2.

Glycolysis

• Converts one glucose (C6) to two pyruvic acid (C3) molecules.

• Net yield: 2 ATP and 2 NADH per glucose.

• Occurs in the cytoplasm of both prokaryotes and eukaryotes.

Preparatory Step

• Pyruvic acid (C3) is decarboxylated to acetyl-CoA (C2), releasing CO2.

Krebs Cycle (Citric Acid Cycle)

• Acetyl-CoA (C2) combines with oxaloacetic acid (C4) to form citric acid (C6).

• Cycle regenerates oxaloacetic acid and produces:

  • 6 NADH, 2 FADH2, 2 ATP, and 6 CO2 per glucose.

Electron Transport Chain (ETC)

• NADH and FADH2 donate electrons to the ETC, a series of carriers (flavoproteins, cytochromes, ubiquinones).

• Electrons are passed to a final electron acceptor (O2 in aerobic respiration).

• ATP is generated by oxidative phosphorylation.

Summary of Aerobic Respiration

• In prokaryotes: up to 38 ATP per glucose.

• In eukaryotes: up to 36 ATP per glucose (due to mitochondrial transport costs).

Chemiosmotic Mechanism

• Protons are pumped across a membrane, creating a proton motive force.

• ATP synthase uses this gradient to synthesize ATP from ADP and phosphate.

• Location: inner mitochondrial membrane (eukaryotes), plasma membrane (prokaryotes).

Alternative Pathways to Glycolysis

• Pentose Phosphate Pathway: Metabolizes five-carbon sugars; yields 1 ATP and 12 NADPH per glucose.

• Entner-Doudoroff Pathway: Yields 1 ATP and 2 NADPH per glucose.

Types of Respiration and Fermentation

• Aerobic Respiration: O2 is the final electron acceptor; complete oxidation of glucose.

• Anaerobic Respiration: Inorganic molecules other than O2 (e.g., nitrate, sulfate) serve as final electron acceptors.

• Fermentation: Organic molecules serve as final electron acceptors; yields less ATP than respiration.

Definitions Table

Summary Table: Energy Production in Aerobic Respiration (Prokaryotes)

Key Equations

• ATP Synthesis (General):

• Overall Aerobic Respiration:

Examples and Applications

• Example: Escherichia coli can perform both aerobic and anaerobic respiration, depending on oxygen availability.

• Application: Understanding microbial metabolism is essential for biotechnology (e.g., fermentation in food production, antibiotic synthesis).

Additional info: Some details, such as the exact ATP yield in eukaryotes (36 ATP), are inferred from standard microbiology textbooks.