Microbial Metabolism: Study Notes

Microbial Metabolism

Defining Metabolism

Metabolism encompasses all chemical reactions within a cell, including those that build up and break down molecules. These reactions are essential for cellular energy management and biosynthesis.

• Metabolism: The sum of all chemical reactions in a cell, including both energy-releasing and energy-consuming processes.

• Anabolism: Biosynthetic reactions that build complex molecules from simpler ones, requiring energy input.

• Catabolism: Degradative reactions that break down complex molecules into simpler ones, releasing energy.

• Interdependence: Catabolic reactions provide the energy and precursor molecules needed for anabolic reactions. Anabolism cannot proceed without the energy and building blocks supplied by catabolism.

• Example: The breakdown of glucose (catabolism) provides ATP and intermediates for the synthesis of amino acids (anabolism).

Adenosine Triphosphate (ATP) and Energy Coupling

ATP is the primary energy carrier in cells, efficiently storing and releasing energy through the ADP/ATP cycle.

• ATP Structure: Composed of adenine, ribose, and three phosphate groups.

• Energy Storage: High-energy bonds between phosphate groups store potential energy.

• ATP Hydrolysis: Breaking the terminal phosphate bond releases energy, converting ATP to ADP.

• ATP Regeneration: ADP is recharged to ATP via phosphorylation using energy from catabolic reactions.

Equation:

$\mathrm{ATP} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{ADP} + \mathrm{P_i} + \text{energy}$

ADP/ATP Cycle: The continuous conversion between ATP and ADP allows cells to store and release energy as needed.

Enzymes and Catalysis

Enzymes are biological catalysts that accelerate metabolic reactions, enabling life processes to occur efficiently under physiological conditions.

• Structure: Most enzymes are proteins with a specific three-dimensional structure and an active site for substrate binding.

• Function: Enzymes lower the activation energy of reactions, increasing reaction rates without being consumed.

• Specificity: The active site is highly specific for its substrate, forming an enzyme–substrate complex.

• Necessity: Without enzymes, most cellular reactions would proceed too slowly to sustain life.

Coenzymes and Ribozymes

• Coenzymes: Non-protein organic molecules that assist enzymes, often by carrying electrons or functional groups.

• Examples:

  • NAD+: Electron carrier; accepts electrons to become NADH.

  • FAD: Electron carrier; accepts electrons to become FADH2.

• Ribozymes: Catalytic RNA molecules that facilitate specific biochemical reactions, differing from protein enzymes in their nucleic acid composition.

Factors Affecting Enzyme Activity and Inhibition

• Factors: Substrate concentration, enzyme concentration, temperature, pH, presence of inhibitors or cofactors.

• Optimal Conditions: Each enzyme has an optimal temperature and pH for maximum activity.

• Enzyme Inhibition:

  • Competitive Inhibition: Inhibitor resembles substrate and competes for the active site; can be overcome by increasing substrate concentration.

  • Noncompetitive Inhibition: Inhibitor binds to a site other than the active site, altering enzyme shape and reducing activity regardless of substrate concentration.

  • Feedback Inhibition: End product of a pathway inhibits an earlier enzyme, regulating pathway activity.

Oxidation–Reduction (Redox) Reactions

Redox reactions involve the transfer of electrons between molecules and are fundamental to energy production in cells.

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• Coupled Reactions: Oxidation and reduction always occur together; when one molecule is oxidized, another is reduced.

Equation:

$\text{AH}_2 + \text{B} \rightarrow \text{A} + \text{BH}_2$

(A is oxidized, B is reduced)

Mechanisms of ATP Synthesis

Cells generate ATP through three main mechanisms, each involving different energy sources and processes.

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

• Oxidative Phosphorylation: Uses energy from electron transport chains to create a proton gradient, driving ATP synthesis via chemiosmosis.

• Photophosphorylation: Utilizes light energy to generate a proton gradient for ATP production (in photosynthetic organisms).

Cellular Respiration

Cellular respiration is a multi-step process that converts nutrients into ATP, involving glycolysis, the Krebs cycle, and the electron transport chain (ETC).

• Glycolysis: Occurs in the cytoplasm of both prokaryotes and eukaryotes; breaks down glucose into pyruvate.

• Pyruvate Oxidation & Krebs Cycle: Occur in the mitochondrial matrix (eukaryotes) or cytoplasm (prokaryotes); acetyl-CoA is oxidized, generating electron carriers and CO2.

• Electron Transport Chain (ETC): Located in the inner mitochondrial membrane (eukaryotes) or plasma membrane (prokaryotes); electrons from NADH and FADH2 are transferred through carriers, creating a proton gradient.

Glycolysis: Reactants and Products

• Reactants: 1 glucose, 2 ATP, 2 NAD+, 2 ADP, 2 Pi

• Products: 2 pyruvate, 2 NADH, net gain of 2 ATP

Equation:

$\mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 + 2\ \mathrm{NAD}^+ + 2\ \mathrm{ADP} + 2\ \mathrm{P_i} \rightarrow 2\ \mathrm{C}_3\mathrm{H}_4\mathrm{O}_3 + 2\ \mathrm{NADH} + 2\ \mathrm{ATP}$

Krebs Cycle: Reactants and Products

• Reactants (per acetyl-CoA): Acetyl-CoA, 3 NAD+, FAD, ADP, Pi

• Products (per acetyl-CoA): 2 CO2, 3 NADH, 1 FADH2, 1 ATP (or GTP), CoA

• Per glucose (2 acetyl-CoA): 4 CO2, 6 NADH, 2 FADH2, 2 ATP, 2 CoA

Equation (per glucose):

$2\ \mathrm{Acetyl{-}CoA} + 6\ \mathrm{NAD}^+ + 2\ \mathrm{FAD} + 2\ \mathrm{ADP} + 2\ \mathrm{P_i} \rightarrow 4\ \mathrm{CO}_2 + 6\ \mathrm{NADH} + 2\ \mathrm{FADH}_2 + 2\ \mathrm{ATP} + 2\ \mathrm{CoA}$

Electron Transport Chain and Chemiosmosis

• Electrons from NADH and FADH2 pass through a series of membrane-bound carriers.

• Energy released is used to pump H+ ions across the membrane, creating a proton motive force.

• H+ flows back through ATP synthase, driving the phosphorylation of ADP to ATP (chemiosmosis).

Aerobic vs. Anaerobic Respiration

Feature	Aerobic Respiration	Anaerobic Respiration
Final Electron Acceptor	Oxygen (O2)	Inorganic molecule other than O2 (e.g., nitrate, sulfate)
ATP Yield	High	Lower than aerobic
Pathways Used	Glycolysis, Krebs cycle, ETC	Glycolysis, Krebs cycle, ETC

Similarity: Both use glycolysis, Krebs cycle, and ETC. Difference: The final electron acceptor and ATP yield.

Fermentation and Alternative Catabolic Pathways

Fermentation enables ATP production in the absence of oxygen or other terminal electron acceptors, relying solely on substrate-level phosphorylation during glycolysis.

• Process: Glycolysis produces ATP and NADH; pyruvate or its derivatives serve as the final electron acceptor, regenerating NAD+.

• Types of Fermentation:

  • Lactic Acid Fermentation: Pyruvate is reduced to lactate; used by certain bacteria and muscle cells.

  • Alcoholic Fermentation: Pyruvate is converted to ethanol and CO2; common in yeasts.

  • Mixed Acid Fermentation: Produces a mixture of acids (lactic, acetic, formic) and gases; characteristic of some enteric bacteria.

Catabolism of Lipids and Proteins

Cells can break down lipids and proteins for energy, especially when carbohydrates are scarce.

• Lipid Catabolism: Lipases hydrolyze lipids into glycerol and fatty acids. Glycerol enters glycolysis; fatty acids undergo beta-oxidation to form acetyl-CoA for the Krebs cycle.

• Protein Catabolism: Proteases degrade proteins into amino acids, which are deaminated and their carbon skeletons enter glycolysis or the Krebs cycle.

Anabolic Reactions: Biosynthesis

Anabolism uses energy and precursor molecules to synthesize essential cellular components.

• Carbohydrates: Synthesized via gluconeogenesis from glycolytic and Krebs cycle intermediates.

• Lipids: Formed by linking glycerol (from glycolysis) with fatty acids (from acetyl-CoA).

• Proteins: Built from amino acids, which are synthesized from metabolic intermediates and assembled into polypeptides during translation.

Regulation of Metabolism

Cells coordinate anabolic and catabolic pathways to maintain energy balance and efficient resource use.

• Feedback Inhibition: End products inhibit key enzymes to prevent overproduction.

• Enzyme Regulation: Activity is adjusted based on substrate availability, energy status, and environmental conditions.

Metabolic Diversity Among Organisms

Microorganisms display diverse metabolic strategies, classified by their sources of carbon, energy, and electrons.

Category	Source	Example
Autotroph	CO2 (inorganic carbon)	Cyanobacteria
Heterotroph	Organic compounds	Escherichia coli
Phototroph	Light energy	Algae
Chemotroph	Chemical compounds	Nitrifying bacteria
Lithotroph	Inorganic electron donors	Thiobacillus
Organotroph	Organic electron donors	Most fungi

• ATP Production: Phototrophs use photophosphorylation; chemotrophs use aerobic/anaerobic respiration or fermentation.

• Combined Categories: Organisms can be classified by combining these terms (e.g., photoautotroph, chemoorganoheterotroph).

Additional info: The above notes expand on the provided learning outcomes with definitions, equations, and examples for clarity and completeness.