Microbial Metabolism

Overview of Metabolic Pathways

Microbial metabolism encompasses all chemical reactions that occur within a microorganism, enabling it to grow, reproduce, and maintain cellular functions. These reactions are organized into metabolic pathways, which can be classified as catabolic, anabolic, or amphibolic.

• Catabolic Pathways: Break down complex molecules into simpler ones, releasing energy. Example: Breakdown of glucose during glycolysis.

• Anabolic Pathways: Build complex molecules from simpler ones, requiring energy input. Example: Synthesis of proteins from amino acids.

• Amphibolic Pathways: Participate in both catabolic and anabolic processes. Example: Krebs (TCA) cycle.

ATP (Adenosine Triphosphate) acts as the cell’s primary energy currency, linking energy-releasing (catabolic) and energy-consuming (anabolic) reactions. ATP is regenerated from ADP and inorganic phosphate (Pi) using energy from catabolic reactions.

ATP–ADP Cycling and Phosphorylation

• ATP Structure: Contains three phosphate groups; ADP has two, AMP has one.

• ATP Hydrolysis: Releases usable energy for cellular work.

• Phosphorylation: Adds a phosphate group to ADP, storing energy.

Enzymes: Biological Catalysts

Enzyme Structure and Function

Enzymes are reusable biological catalysts that accelerate chemical reactions by lowering activation energy without altering the overall free energy change (ΔG) of the reaction.

• Substrate: The molecule upon which an enzyme acts.

• Active Site: The region of the enzyme where the substrate binds.

• Cofactor: Nonprotein helper (e.g., Mg2+, Zn2+, Fe2+).

• Coenzyme: Organic cofactor, often derived from vitamins (e.g., NAD+, FAD, NADP+).

• Apoenzyme: Enzyme lacking its required cofactor.

• Holoenzyme: Complete, active enzyme with its cofactor.

Enzyme Activity Influences:

• Temperature: Low temperatures slow activity; high temperatures can denature enzymes.

• pH: Extreme pH can denature enzymes.

• Substrate Concentration: Reaction rate increases with substrate until enzyme saturation.

• Cofactors: Required for optimal enzyme activity.

• Phosphorylation State: Can activate or inhibit enzymes.

• Inhibitors: Competitive and noncompetitive inhibitors regulate enzyme activity.

Enzyme Regulation Mechanisms

• Competitive Inhibition: Inhibitor competes with substrate for the active site (e.g., sulfa drugs vs. PABA in folate synthesis).

• Noncompetitive (Allosteric) Inhibition: Inhibitor binds to a site other than the active site, altering enzyme function; excess substrate does not overcome inhibition.

• Phosphorylation/Dephosphorylation: Kinases add phosphates; phosphatases remove them, modulating enzyme activity.

Redox Reactions and Electron Carriers

Oxidation and Reduction

Redox reactions involve the transfer of electrons between molecules. Oxidation is the loss of electrons, while reduction is the gain of electrons. These processes always occur together.

• OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).

• When an atom is oxidized, its oxidation number increases; when reduced, it decreases.

Major Electron Carriers

Reduced carriers (NADH, FADH2, NADPH) act as electron shuttles, transferring high-energy electrons to the electron transport chain (ETC).

Cellular Respiration: Pathway of Glucose

Stages of Cellular Respiration

Note: In prokaryotes, all steps except the ETC occur in the cytoplasm; the ETC is in the plasma membrane. In eukaryotes, glycolysis occurs in the cytosol, the Krebs cycle in the mitochondrial matrix, and the ETC in the inner mitochondrial membrane.

ATP Generation Mechanisms

• Substrate-Level Phosphorylation: Direct transfer of a phosphate group to ADP by an enzyme (occurs in glycolysis and Krebs cycle).

• Oxidative Phosphorylation: Electron flow through the ETC creates a proton (H+) gradient, which drives ATP synthesis via ATP synthase.

• Chemiosmosis: Movement of H+ down its electrochemical gradient through ATP synthase, generating ATP.

Sequence of Events:

1. NADH/FADH2 deliver electrons to the ETC.

2. ETC transfers electrons and pumps H+ across the membrane.

3. H+ gradient forms.

4. H+ flows through ATP synthase.

5. ATP is generated.

Respiration Versus Fermentation

• Fermentation Pathways: - Homolactic fermentation: produces lactate - Alcohol fermentation: produces ethanol and CO2 - Other types: heterolactic, mixed-acid, butanediol fermentation

• Fermentation yields much less ATP than aerobic respiration.

Self-Check and Application Questions

• Classify metabolic pathways: - Breaking down glucose: Catabolic - Building a protein: Anabolic - Krebs cycle: Amphibolic

• Enzyme energy diagrams: Catalyzed reactions have lower activation energy but the same ΔG as uncatalyzed reactions.

• Temperature effects: Cooling slows enzyme activity; overheating can denature (irreversibly damage) enzymes.

• Enzyme regulation: - Competitive inhibition: Inhibitor at active site - Allosteric inhibition: Inhibitor at allosteric site - Phosphorylation: Modifies enzyme activity

• Electron carriers: - NAD+/NADH and FAD/FADH2 shuttle electrons via redox reactions.

• Order of respiration steps: Glycolysis → Intermediate step → Krebs cycle → ETC; know cellular locations for each.

• ATP synthesis: ETC creates proton gradient → chemiosmosis through ATP synthase → ATP production.

• Fermentation: Regenerates NAD+ so glycolysis can continue in the absence of an ETC.

• ATP yield comparison: Aerobic respiration (up to 38 ATP/glucose) vs. homolactic fermentation (2 ATP/glucose).

Additional info: The above notes expand on the study guide by providing definitions, examples, and context for each process, as well as summarizing key tables and comparisons for exam preparation.