Microbial Metabolism: Enzymes, Energy, and Catabolic Pathways

Microbial Metabolism

Metabolism: Energy Conservation and Cellular Processes

Microbial metabolism encompasses all chemical reactions within a cell, enabling energy conservation and the synthesis or breakdown of molecules. These reactions are categorized as anabolism and catabolism.

• Anabolism: The assembly of smaller molecules into larger macromolecules. This process requires energy input, typically from ATP.

• Catabolism: The breakdown of macromolecules into smaller molecules, releasing energy that can be conserved as ATP or heat.

Enzymes: Catalysts of Life

Enzymes are biological catalysts that accelerate chemical reactions by lowering the energy of activation. They are essential for both anabolic and catabolic processes.

• Catalysts: Enzymes speed up reactions without being consumed.

• Energy of Activation: The minimum energy required for a reaction; enzymes lower this threshold by orienting substrates in their active sites.

Enzyme Structure

• Simple enzymes: Composed solely of polypeptide chains.

• Conjugated enzymes: Consist of a protein (apoenzyme) and a non-protein component (cofactor).

• Holoenzyme: The complete, active enzyme (apoenzyme + cofactor).

• Cofactors: Non-protein helpers; can be coenzymes (organic molecules, often vitamins) or inorganic elements (metal ions).

• Ribozymes: RNA molecules with catalytic activity, acting on other RNA substrates.

Enzyme-Substrate Interactions

• Active Site: The region on the enzyme where the substrate binds, often a crevice or groove.

• Binding: Temporary, weak, and reversible; facilitates the reaction and release of product.

Cofactors and Coenzymes

• Metallic cofactors: Trace elements (Fe, Zn, Cu, Mg, Mn, Co, Se) alter the active site and regulate enzyme activity.

• Coenzymes: Organic molecules (often vitamins) act as transient carriers of chemical groups, shuttling substrates for bond formation or cleavage.

Naming and Classification of Enzymes

• Enzyme names typically reflect their function, ending in -ase (e.g., lactase, DNA polymerase).

• Transfer reactions include oxidation-reduction (electron transfer), aminotransferase, phosphotransferase, and methyltransferase.

Enzyme Location and Regulation

• Exoenzymes: Function outside the cell.

• Endoenzymes: Function within the cell.

• Constitutive enzymes: Always present and active.

• Regulated enzymes: Produced or activated in response to environmental changes.

Microbial Enzymes as Virulence Factors

• Streptokinase: Produced by Streptococcus pyogenes; digests blood clots, aiding invasion.

• Elastase/Collagenase: Produced by Pseudomonas aeruginosa; increases severity of lung infections.

• Lecithinase C: Produced by Clostridium perfringens; damages cell membranes.

Enzyme Sensitivity to Environment

• Enzymes are optimized for their organism's habitat.

• Denaturation: Disruption of weak bonds leads to loss of enzyme activity.

Regulation of Enzymatic Activity and Metabolic Pathways

Cells regulate enzyme activity and metabolic pathways to maintain homeostasis and respond to environmental changes.

• Metabolic Pathways: Multistep, interconnected series of reactions.

• Direct Controls:

  • Competitive inhibition: A mimic competes with the substrate for the active site.

  • Noncompetitive inhibition: A regulator binds elsewhere, causing negative feedback.

• Enzyme Synthesis Controls:

  • Enzyme repression: e.g., trp operon in Escherichia coli.

  • Enzyme induction: e.g., lac operon in Escherichia coli.

The Pursuit and Utilization of Energy

Energy in Cells: Coupled Reactions

Cellular energy transformations involve coupled reactions, where energy released from one process is used to drive another.

• Exergonic reactions: Release energy, often stored as ATP.

• Endergonic reactions: Require energy input to form bonds.

Biological Oxidation and Reduction (Redox Reactions)

Redox reactions are fundamental to energy transfer in cells, involving electron donors and acceptors.

• Phosphorylation: ATP synthesis via electron transfer.

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

• Electron carriers: NAD/NADH act as molecular shuttles.

• Final electron acceptor: Oxygen in aerobic metabolism; other compounds in anaerobic metabolism.

Adenosine Triphosphate (ATP): Metabolic Money

• Structure: Three phosphates, ribose sugar, adenine base.

• Energy: Derived from the strain between phosphate groups.

• Metabolic roles:

  • Substrate-level phosphorylation

  • Oxidative phosphorylation

  • Photophosphorylation

Catabolism: Getting Materials and Energy

Overview of Catabolic Pathways

Catabolism provides energy and building blocks for cells by breaking down nutrients.

• Glycolysis: Breakdown of glucose to pyruvate.

• Aerobic respiration: Glycolysis, Krebs cycle, respiratory chain; oxygen is the final electron acceptor.

• Anaerobic respiration: Similar pathway, but final electron acceptor is not oxygen.

Aerobic Respiration

• Glucose: Primary substrate; other hexoses can enter pathways.

• Glycolysis:

  • Converts glucose to pyruvate in nine steps.

  • Two ATP consumed initially; four ATP generated; net yield = 2 ATP.

  • Six-carbon glucose becomes two three-carbon pyruvate molecules.

Pyruvic Acid: Central Metabolite

• Enters Krebs cycle in strict aerobes and some anaerobes.

• Converted to fermentation products in facultative anaerobes.

Transition Step (Preparatory Step)

• Occurs in cytoplasm (bacteria) or mitochondria (eukaryotes).

• Converts pyruvate to acetyl CoA, releasing NADH and CO2 via decarboxylation.

• COO- becomes CO2; hydrogen reduces NAD to NADH.

Krebs Cycle (Citric Acid Cycle)

• Occurs in cytoplasm (microbes) or mitochondrial matrix (eukaryotes).

• NADH and FADH2 generated for ATP production via oxidative phosphorylation.

• CO2 released; ATP generated by substrate-level phosphorylation.

Respiratory Chain: Electron Transport and Oxidative Phosphorylation

• Redox enzymes transfer electrons from NADH/FADH2 to oxygen, forming water.

• ATP synthase: Protein complex synthesizes ATP at the end of the electron transport chain.

• Oxidative phosphorylation: ATP synthesis coupled to electron transport.

• Chemiosmosis: ETS pumps H+ to create proton motive force; ATPase rotates to produce ATP.

• ATP yield:

  • NADH: 3 ATP

  • FADH2: 2 ATP

Summary of Aerobic Respiration

• Oxygen is the terminal electron acceptor.

• Total possible yield: 38 ATP per glucose.

• Terminal step: Catalyzed by cytochrome oxidase; forms water.

• Oxidase presence: Neisseria, Pseudomonas, Bacillus.

• Lack oxidase: Klebsiella, Enterobacter.

• Streptococcus: Contains peroxidase.

• Staphylococcus and other aerobes: Produce superoxide dismutase and catalase to detoxify oxygen products.

Anaerobic Respiration

• Terminal electron acceptor is not oxygen; often inorganic compounds (sulfate, carbonate, nitrate).

• Nitrate/nitrite reduction: e.g., E. coli.

• Denitrification: e.g., Pseudomonas, Bacillus.

• ATP yield is lower than aerobic respiration.

Fermentation

• Incomplete oxidation of glucose in absence of oxygen; uses organic compounds (pyruvate, acetaldehyde) as electron acceptors.

• Allows colonization of anaerobic environments.

• Glycolysis rate increases to meet ATP needs.

• Fermentation products:

  • Alcoholic fermentation: Pyruvate to ethanol.

  • Acidic fermentation:

    • Homolactic: Pyruvate to lactic acid.

    • Heterolactic: Pyruvate to lactic acid, acetic acid, and CO2.

  • Mixed acid fermentation: Pyruvate to multiple acids.

• Applications: Food and beverage production (e.g., yogurt, beer, sauerkraut).

Catabolism of Non-Carbohydrate Compounds

• Lipid catabolism:

  • Lipases: Break down lipids to fatty acids and glycerol.

  • Beta oxidation: Fatty acids are degraded to acetyl CoA.

• Protein catabolism:

  • Proteases: Digest proteins to amino acids.

  • Deamination: Removal of amine group (-NH2), producing precursors for lipids or carbohydrates.

Key Equations and Concepts

• ATP Hydrolysis:

• $\mathrm{ATP} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{ADP} + \mathrm{P}_i + \text{energy}$

• Glycolysis Net Reaction:

• $\mathrm{Glucose} + 2\mathrm{NAD}^+ + 2\mathrm{ADP} + 2\mathrm{P}_i \rightarrow 2\mathrm{Pyruvate} + 2\mathrm{NADH} + 2\mathrm{ATP} + 2\mathrm{H}_2\mathrm{O}$

• Krebs Cycle Net Reaction:

• $2\mathrm{Acetyl~CoA} + 2\mathrm{ADP} + 2\mathrm{P}_i + 6\mathrm{NAD}^+ + 2\mathrm{FAD} + 4\mathrm{H}_2\mathrm{O} \rightarrow 4\mathrm{CO}_2 + 2\mathrm{ATP} + 6\mathrm{NADH} + 2\mathrm{FADH}_2 + 2\mathrm{CoA}$

• Electron Transport Chain:

• $\mathrm{NADH} + \mathrm{H}^+ + 1/2\mathrm{O}_2 \rightarrow \mathrm{NAD}^+ + \mathrm{H}_2\mathrm{O}$

Summary Table: Types of Microbial Catabolism

Pathway	Final Electron Acceptor	ATP Yield (per glucose)	Key Products
Aerobic Respiration	Oxygen	~38	CO2, H2O
Anaerobic Respiration	Inorganic compounds (e.g., nitrate, sulfate)	Less than 38	CO2, reduced inorganic compounds
Fermentation	Organic compounds (e.g., pyruvate, acetaldehyde)	2	Alcohols, acids, gases

Example: Escherichia coli can perform aerobic respiration, anaerobic respiration (using nitrate as electron acceptor), and fermentation (producing mixed acids).

Additional info: The notes expand on basic points to provide context for enzyme regulation, metabolic pathways, and catabolic diversity, ensuring completeness for exam preparation.