Microbial Metabolism

Overview of Metabolism

Microbial metabolism encompasses all the chemical reactions that occur within microorganisms, enabling them to grow, reproduce, and maintain cellular structures. These reactions are broadly categorized into catabolism (breaking down molecules to release energy) and anabolism (building complex molecules from simpler ones).

• Metabolism: The sum of all biochemical reactions in a cell, including both catabolic and anabolic pathways.

• Catabolism: The breakdown of complex molecules into simpler ones, releasing energy (often in the form of ATP).

• Anabolism: The synthesis of complex molecules from simpler precursors, requiring energy input.

• Reduction-Oxidation (Redox) Reactions: Chemical reactions involving the transfer of electrons. These are central to energy production in cells.

• ATP (Adenosine Triphosphate): The primary energy currency of the cell, produced during catabolic reactions and consumed during anabolic reactions.

Example: During glycolysis, glucose is catabolized to pyruvate, generating ATP and NADH through redox reactions.

Enzymes in Metabolism

Enzymes are biological catalysts that accelerate metabolic reactions by lowering activation energy. They are highly specific for their substrates and can be regulated to control metabolic pathways.

• Role of Enzymes: Facilitate and speed up metabolic reactions without being consumed.

• Enzyme Regulation: Enzymes can be regulated by inhibitors, activators, or through covalent modification (e.g., phosphorylation).

• Allosteric Regulation: Enzyme activity is modulated by molecules binding to sites other than the active site, causing conformational changes.

Example: Phosphofructokinase, a key enzyme in glycolysis, is allosterically inhibited by ATP.

Glycolysis and Aerobic Glucose Catabolism

Glucose catabolism in aerobic microorganisms occurs in three main stages: glycolysis, pyruvate oxidation, and the citric acid cycle, followed by the electron transport chain (ETC).

• Cellular Respiration:

  • 1 Synthesis of Acetyl-CoA

  • 2 The Citric Acid Cycle

  • 3 A final series of redox reactions

• 1. Glycolysis

  Where it happens: Cytoplasm Big idea: Take one big sugar and split it so the cell can start getting energy.

  • Starts with 1 glucose molecule (6 carbons)

  • Ends with 2 pyruvate molecules (3 carbons each)

  • This process does NOT need oxygen, so it can happen anytime

  What the cell gets:

  • 2 ATP (net gain)

    • The cell actually uses 2 ATP at the beginning, but makes 4 later → net = 2

  • 2 NADH (these are energy carriers that will be used later)

  Why it matters:

  • It’s the first step in breaking down food for energy

  • Even if oxygen runs out, glycolysis can still keep the cell alive for a bit

  👉 Think of it like chopping a big log into two smaller logs so they fit into the fireplace.

  2. Pyruvate Oxidation (Link Reaction)

  Where it happens: Mitochondrial matrix Big idea: Prep pyruvate so it can enter the main energy cycle.

  • Each pyruvate (3C) is converted into acetyl-CoA (2C)

  • The extra carbon is released as CO₂ (waste)

  • This step connects glycolysis to the Krebs cycle

  What the cell gets (per glucose):

  • 2 NADH

  • 2 CO₂ released

  Why it matters:

  • Acetyl-CoA is the only molecule that can enter the Krebs cycle

  • This step is like checking your ticket before you’re allowed into the ride

  3. Citric Acid Cycle (Krebs Cycle)

  Where it happens: Mitochondrial matrix Big idea: Strip electrons from acetyl-CoA to store energy for later.

  • Each acetyl-CoA goes through the cycle once

  • Since 1 glucose makes 2 acetyl-CoA, the cycle runs twice per glucose

  What the cell gets (per glucose):

  • 6 NADH

  • 2 FADH₂

  • 2 ATP (or GTP)

  • 4 CO₂ released

  Why it matters:

  • This is where most electron carriers are made

  • Those carriers fuel the Electron Transport Chain

  👉 Think of Krebs like a phone charger that doesn’t power the phone yet—it just stores energy in the battery packs (NADH & FADH₂).

  4. Electron Transport Chain (ETC)

  Where it happens: Inner mitochondrial membrane Big idea: Use electrons to make a LOT of ATP.

  • NADH and FADH₂ donate electrons

  • Electrons move through membrane proteins

  • Energy released pumps H⁺ ions across the membrane

  • This creates a proton gradient

  Oxygen’s role:

  • Oxygen is the final electron acceptor

  • It combines with electrons and hydrogen to form water (H₂O)

  • Without oxygen → the chain stops

  ATP production:

  • Protons flow back through ATP synthase

  • This drives the production of ~26–28 ATP

  👉 This is like water building up behind a dam—when it flows through turbines, electricity (ATP) is generated.

  Big Picture Summary (per glucose)

  • Glycolysis: 2 ATP

  • Krebs Cycle: 2 ATP

  • ETC: ~26–28 ATP ➡️ Total: ~30–32 ATP

Key Equations:

Example: In Escherichia coli, aerobic respiration yields up to 38 ATP per glucose molecule.

Types of Respiration and Fermentation

Microorganisms utilize different metabolic pathways depending on environmental conditions and the availability of electron acceptors.

• Aerobic Respiration: Uses oxygen as the terminal electron acceptor in the ETC; produces the most ATP.

• Anaerobic Respiration: Uses alternative electron acceptors (e.g., nitrate, sulfate) when oxygen is absent; yields less ATP than aerobic respiration.

• Fermentation: Occurs in the absence of a functional ETC; organic molecules serve as both electron donors and acceptors; yields the least ATP.

Environmental Conditions:

• Aerobic respiration is favored when oxygen is available.

• Anaerobic respiration occurs in oxygen-limited environments with alternative electron acceptors.

• Fermentation is used when neither oxygen nor alternative electron acceptors are available.

Example: Lactobacillus species ferment glucose to lactic acid under anaerobic conditions.

Catabolism of Lipids, Carbohydrates, and Proteins

Microorganisms can catabolize a variety of substrates for energy and carbon, using specific enzymes for each type of macromolecule.

• Lipid Catabolism: Lipases hydrolyze triglycerides into glycerol and fatty acids. Fatty acids undergo β-oxidation to generate acetyl-CoA.

• Complex Carbohydrate Catabolism: Polysaccharides (e.g., starch, cellulose) are broken down by specific hydrolases (e.g., amylase, cellulase) into monosaccharides, which enter glycolysis.

• Protein Catabolism: Proteases degrade proteins into amino acids, which are deaminated and converted into intermediates of central metabolic pathways.

Example: Bacillus subtilis secretes amylase to degrade starch into glucose.

Oxygenic vs. Anoxygenic Photosynthesis

Photosynthetic microorganisms can be classified based on whether they produce oxygen during photosynthesis.

• Oxygenic Photosynthesis: Utilizes water as an electron donor, producing O2 as a byproduct. Carried out by cyanobacteria, algae, and plants.

• Anoxygenic Photosynthesis: Uses alternative electron donors (e.g., H2S, Fe2+), does not produce O2. Performed by purple and green sulfur bacteria.

Comparison Table:

Example: Synechocystis (a cyanobacterium) performs oxygenic photosynthesis, while Chlorobium (a green sulfur bacterium) performs anoxygenic photosynthesis.