Microbial Metabolism: Core Concepts and Pathways

Microbial Metabolism

Overview of Metabolism

Metabolism is the sum of all chemical reactions occurring within a living organism. These reactions are essential for growth, reproduction, and maintenance of cellular structures. Metabolism is divided into two fundamental processes: catabolism and anabolism.

• Catabolism: Energy-releasing reactions that break down complex molecules into simpler ones. Catabolic pathways produce ATP, NADH, and FADH2.

• Anabolism: Energy-consuming reactions that build complex molecules from simpler ones. Anabolic pathways use ATP generated by catabolism.

• ATP: Acts as the energy currency, coupling catabolic and anabolic reactions. Energy released from catabolism is stored as ATP and used for anabolic processes.

Enzymes: Mechanism, Activity, and Classification

Enzymes are biological catalysts that accelerate chemical reactions by lowering the activation energy required. They are highly specific due to their unique active sites and are not consumed during the reaction.

• Mechanism of Enzymatic Action:

  1. Enzyme binds to substrate, forming an enzyme-substrate complex.

  2. Substrate is transformed into product(s).

  3. Enzyme is released unchanged and can catalyze further reactions.

• Factors Influencing Enzyme Activity:

  • Temperature: Increased temperature raises reaction rate up to an optimum; excessive heat causes denaturation.

  • pH: Each enzyme has an optimal pH; extreme pH values lead to denaturation.

  • Substrate Concentration: Higher substrate concentration increases activity until the enzyme is saturated.

  • Competitive Inhibition: Inhibitor competes with substrate for the active site.

  • Noncompetitive Inhibition: Inhibitor binds elsewhere, altering enzyme shape and function.

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

• Components of a Holoenzyme:

  • Apoenzyme: Protein portion of the enzyme.

  • Cofactor: Non-protein component; may be inorganic (e.g., metal ions) or organic (coenzymes such as NAD+, NADP+, FAD, Coenzyme A).

• Enzyme Classification:

  1. Oxidoreductases: Catalyze oxidation-reduction reactions.

  2. Transferases: Transfer functional groups between molecules.

  3. Hydrolases: Catalyze hydrolysis reactions.

  4. Lyases: Remove atoms without hydrolysis.

  5. Isomerases: Rearrangement of atoms within a molecule.

  6. Ligases: Join molecules using ATP.

Oxidation-Reduction (Redox) Reactions

Redox reactions are central to energy production in cells. Oxidation is the loss of electrons, while reduction is the gain of electrons. These processes always occur together.

• In biological systems, electrons are often transferred with hydrogen atoms (dehydrogenation).

• Redox reactions drive the flow of energy through metabolic pathways.

General Redox Equation:

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

Here, A is oxidized and B is reduced.

Major Metabolic Pathways

Glycolysis

• Occurs in the cytoplasm of both prokaryotes and eukaryotes.

• Oxidizes one molecule of glucose to two molecules of pyruvic acid.

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

• Does not require oxygen (anaerobic process).

Krebs Cycle (Citric Acid Cycle)

• Location: Mitochondrial matrix (eukaryotes), cytoplasm (prokaryotes).

• Acetyl-CoA is oxidized, releasing CO2 and transferring electrons to NAD+ and FAD.

• Per glucose molecule: 2 ATP, 6 NADH, 2 FADH2, 4 CO2.

Electron Transport Chain (ETC)

• Series of carrier molecules (cytochromes, flavoproteins) embedded in membranes.

• Electrons from NADH and FADH2 are passed through the chain, releasing energy.

• Energy is used to pump protons, generating a proton gradient (chemiosmosis).

• ATP is produced by oxidative phosphorylation.

• Oxygen is the final electron acceptor in aerobic respiration, forming water.

Key Metabolic Terms

• Aerobic: Processes that require oxygen as the final electron acceptor.

• Anaerobic: Processes that occur without oxygen; use inorganic molecules other than O2 as final electron acceptors.

• Fermentation: ATP generation using an organic molecule as the final electron acceptor; does not use the electron transport chain.

• Chemoautotroph: Organism that obtains energy from chemicals and carbon from CO2.

• Chemoheterotroph: Organism that obtains both energy and carbon from organic compounds.

• Photoautotroph: Uses light for energy and CO2 as a carbon source.

• Photoheterotroph: Uses light for energy and organic compounds for carbon.

• Ribozyme: RNA molecule with catalytic (enzymatic) activity.

Comparison of Aerobic and Anaerobic Respiration

Feature	Aerobic Respiration	Anaerobic Respiration
Final Electron Acceptor	O2	Inorganic molecule (NO3-, SO42-, CO32-)
ATP Yield	High	Lower
Krebs Cycle	Complete	Partial
Electron Transport Chain	Full	Limited

Products of Fermentation

• Alcohol Fermentation: Produces ethanol and CO2 (e.g., by yeast).

• Lactic Acid Fermentation: Produces lactic acid (e.g., by Lactobacillus, Streptococcus).

• Other Products: Various acids, gases, and alcohols depending on the organism and pathway.

Biochemical Tests for Bacterial Identification

• Fermentation Tests: Detect the ability of bacteria to ferment specific sugars (e.g., Staphylococcus aureus ferments mannitol).

• Enzyme Detection: Identifies specific enzymes produced by bacteria, aiding in differentiation and identification.

• These tests are essential tools in clinical and research microbiology laboratories.

Example: The mannitol fermentation test distinguishes Staphylococcus aureus (mannitol fermenter) from other staphylococci.

Additional info: The above notes expand on the original points with definitions, examples, and context to provide a comprehensive overview suitable for exam preparation.