Microbial Metabolism: Catabolism, Anabolism, and Energy Pathways

Microbial Metabolism

Overview of Metabolism

Metabolism encompasses all chemical reactions occurring within a living organism. These reactions are essential for maintaining life, providing energy, and synthesizing necessary cellular components.

• Catabolism: Breakdown of complex molecules into simpler ones, releasing energy.

• Anabolism: Synthesis of complex molecules from simpler ones, requiring energy input.

• ATP (Adenosine Triphosphate): The primary energy carrier in cells, linking catabolic and anabolic reactions.

Enzymes and Their Function

Nature and Mechanism of Enzymes

Enzymes are biological catalysts, primarily proteins, that accelerate chemical reactions by lowering the activation energy required.

• Activation Energy: The minimum energy needed for a reaction to occur.

• Enzyme Structure: Most are globular proteins with specific three-dimensional shapes, determining their specificity.

• Enzyme-Substrate Complex: The substrate binds to the enzyme's active site, is transformed, and the enzyme is recovered unchanged.

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

Naming and Classification of Enzymes

• Enzyme names typically end with -ase (e.g., sucrase, lipase).

• Six major classes based on reaction type: oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases.

Enzyme Components

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

  • Apoenzyme: The protein portion.

  • Cofactor: Nonprotein component, which may be a metal ion (e.g., Fe2+, Mg2+) or a coenzyme (organic molecule such as NAD+, FAD, coenzyme A).

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.

Regulation of Enzyme Activity

• Feedback Inhibition: The end-product of a metabolic pathway inhibits an early enzyme in the pathway, preventing overproduction.

• Ribozymes: RNA molecules with catalytic activity, especially in protein synthesis.

Energy Production in Microorganisms

Oxidation-Reduction (Redox) Reactions

Energy production in cells involves the transfer of electrons through oxidation and reduction reactions.

• Oxidation: Loss of electrons (often with protons, H+).

• Reduction: Gain of electrons.

• Redox reactions are coupled; one molecule is oxidized while another is reduced.

• NAD+ / NADH: Key electron carriers in metabolism.

ATP Generation Mechanisms

• Phosphorylation: Addition of inorganic phosphate (Pi) to ADP to form ATP.

• Substrate-Level Phosphorylation: Direct transfer of phosphate from a substrate to ADP.

• Oxidative Phosphorylation: Electrons pass through the electron transport chain to O2 (or another acceptor), generating ATP via chemiosmosis.

• Photophosphorylation: Light energy drives electron transport, producing ATP (in photosynthetic organisms).

Metabolic Pathways of Energy Production

Carbohydrate Catabolism

• Most cellular energy is derived from carbohydrate oxidation.

• Two main types:

  • Respiration: Complete breakdown of sugars.

  • Fermentation: Partial breakdown of sugars.

Glycolysis

• Main pathway for glucose oxidation; end-product is pyruvic acid.

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

Alternative Pathways

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

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

Cellular Respiration

• Organic molecules are oxidized; energy is generated via the electron transport chain (ETC).

• Aerobic Respiration: O2 is the final electron acceptor.

• Anaerobic Respiration: Final electron acceptor is an inorganic molecule other than O2 (e.g., NO3-, SO42-, CO32-).

• Krebs Cycle: Acetyl groups are oxidized; NAD+ and FAD are reduced.

• Per glucose: 6 NADH, 2 FADH2, 2 ATP, 6 CO2 produced.

• Electron Transport Chain: Series of carriers (flavoproteins, cytochromes, ubiquinones) transfer electrons, pumping protons to generate a proton motive force.

• ATP Synthase: Uses proton gradient to synthesize ATP from ADP and Pi.

• ATP yield: 38 ATP (prokaryotes), 36 ATP (eukaryotes) per glucose in aerobic respiration; less in anaerobic respiration.

Fermentation

• Energy released from sugars without O2; only 2 ATP per glucose (substrate-level phosphorylation).

• Final electron acceptor is an organic molecule from within the cell.

• Lactic Acid Fermentation: Pyruvic acid reduced to lactic acid.

• Alcohol Fermentation: Acetaldehyde reduced to ethanol.

• Heterolactic Fermentation: Produces lactic acid and ethanol (via pentose phosphate pathway).

Lipid and Protein Catabolism

• Lipases: Hydrolyze lipids to glycerol and fatty acids.

• Beta-Oxidation: Fatty acids broken down to acetyl CoA.

• Catabolic products enter glycolysis and Krebs cycle.

• Protein Catabolism: Amino acids are deaminated, decarboxylated, or desulfurized before entering the Krebs cycle.

Biochemical Tests and Bacterial Identification

• Microbes can be identified by their enzyme activities.

• Fermentation tests detect acid and gas production from carbohydrate fermentation.

Photosynthesis

Light-Dependent Reactions (Photophosphorylation)

• Photosynthesis: Conversion of light energy to chemical energy for carbon fixation.

• Chlorophyll a: Main pigment in green plants, algae, cyanobacteria.

• Electrons from chlorophyll pass through an ETC, generating ATP by chemiosmosis.

• Photosystems: Complexes of pigments in thylakoid membranes.

• Cyclic Photophosphorylation: Electrons return to chlorophyll.

• Noncyclic Photophosphorylation: Electrons reduce NADP+; replaced by electrons from H2O (producing O2) or H2S (producing S0 granules).

Light-Independent Reactions (Calvin-Benson Cycle)

• CO2 is fixed into sugars via the Calvin-Benson cycle.

Summary of Energy Production Mechanisms

• Phototrophs convert sunlight to chemical energy via oxidation reactions.

• Chemotrophs obtain energy from organic or inorganic compounds.

• Energy is derived from electron transfer in redox reactions.

• Cells require an electron donor, electron carriers, and a final electron acceptor for energy production.

Metabolic Diversity Among Organisms

Type	Energy Source	Carbon Source	Examples
Photoautotrophs	Light (photophosphorylation)	CO2 (Calvin-Benson cycle)	Cyanobacteria, plants
Photoheterotrophs	Light	Organic compounds	Green and purple non-sulfur bacteria
Chemoautotrophs	Inorganic compounds	CO2	Nitrifying bacteria
Chemoheterotrophs	Organic compounds	Organic compounds	Most bacteria, animals, fungi

Metabolic Pathways of Energy Use

Polysaccharide Biosynthesis

• Glycogen is synthesized from ADPG (adenosine diphosphoglucose) in bacteria.

• UDPNAc (uridine diphosphate N-acetylglucosamine) is the precursor for peptidoglycan biosynthesis.

Lipid Biosynthesis

• Lipids are formed from fatty acids and glycerol.

• Glycerol is derived from dihydroxyacetone phosphate (glycolysis intermediate); fatty acids are synthesized from acetyl CoA.

Amino Acid and Protein Biosynthesis

• Amino acids are synthesized from intermediates of carbohydrate metabolism, especially the Krebs cycle.

Purine and Pyrimidine Biosynthesis

• Nucleotide sugars are derived from the pentose phosphate or Entner-Doudoroff pathways.

• Backbones of purines and pyrimidines are formed from carbon and nitrogen atoms of certain amino acids.

Integration of Metabolism

• Anabolic and catabolic pathways are interconnected via common intermediates, forming amphibolic pathways.

Key Equations

• ATP Formation:

  • Substrate-level phosphorylation: $\mathrm{ADP} + \mathrm{P_i} \rightarrow \mathrm{ATP}$

  • Oxidative phosphorylation: $\mathrm{NADH} + \mathrm{H^+} + 1/2\,\mathrm{O_2} \rightarrow \mathrm{NAD^+} + \mathrm{H_2O}$

  • Photophosphorylation: $\mathrm{ADP} + \mathrm{P_i} \xrightarrow{\text{light}} \mathrm{ATP}$

• Glycolysis (net reaction): $\mathrm{Glucose} + 2\,\mathrm{NAD^+} + 2\,\mathrm{ADP} + 2\,\mathrm{P_i} \rightarrow 2\,\mathrm{Pyruvate} + 2\,\mathrm{NADH} + 2\,\mathrm{ATP} + 2\,\mathrm{H_2O}$

• Aerobic Respiration (overall): $\mathrm{C_6H_{12}O_6} + 6\,\mathrm{O_2} + 38\,\mathrm{ADP} + 38\,\mathrm{P_i} \rightarrow 6\,\mathrm{CO_2} + 6\,\mathrm{H_2O} + 38\,\mathrm{ATP}$

Example Application

• Biochemical Tests: Used in clinical microbiology to identify bacteria based on their metabolic capabilities (e.g., fermentation of lactose, production of hydrogen sulfide).

Additional info: Amphibolic pathways allow cells to efficiently switch between energy production and biosynthesis, depending on cellular needs.