Microbial Metabolism

Metabolism: An Overview

Metabolism encompasses all chemical reactions within a living organism, serving as an energy-balancing act. It is divided into two fundamental processes:

• Catabolism: Reactions that break down complex molecules, releasing energy and providing building blocks for anabolism. These are often hydrolytic and exergonic (produce more energy than they consume). Example: Breakdown of sugars into carbon dioxide and water.

• Anabolism: Reactions that use energy and building blocks to synthesize complex molecules. Also known as biosynthetic reactions, they are typically dehydration synthesis and endergonic (consume more energy than they produce). Example: Formation of proteins from amino acids, nucleic acids from nucleotides, and polysaccharides from simple sugars.

ATP: The Energy Connection

ATP (Adenosine Triphosphate) acts as the energy intermediary between catabolism and anabolism.

• Catabolic reactions release energy, which is used to synthesize ATP.

• ATP stores usable energy and releases it to power anabolic reactions.

• Metabolic pathways are sequences of chemical reactions, each step catalyzed by specific enzymes determined by the cell’s genetic makeup.

Enzymes: Structure and Function

Enzymes are biological catalysts that accelerate chemical reactions by lowering activation energy.

• Some enzymes are purely proteins.

• Apoenzyme: Protein portion, inactive alone.

• Cofactor: Nonprotein component (e.g., iron, zinc, magnesium, calcium).

• Coenzyme: Organic cofactor.

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

Collision Theory: Chemical reactions occur as atoms, ions, and molecules collide.

• Activation Energy: Minimum collision energy required for a reaction.

• Reaction Rate: Frequency of effective collisions; increased by enzymes, temperature, pressure, or concentration.

Mechanism of Enzymatic Action

Enzymes facilitate reactions through a specific sequence:

1. Substrate contacts the enzyme's active site.

2. Substrate binds to the active site, forming an enzyme-substrate complex.

3. Substrate is transformed into products.

4. Products are released as they no longer fit the active site.

5. Enzyme is recovered unchanged and can catalyze further reactions.

Factors Influencing Enzymatic Activity

Several factors affect enzyme function:

• Temperature: Activity increases with temperature up to an optimum; excessive heat denatures enzymes.

• pH: Each enzyme has an optimal pH; extreme pH can denature proteins.

• Substrate Concentration: Increased concentration raises reaction rate until enzymes are saturated.

• Inhibitors: Decrease or interfere with enzyme activity.

Enzyme Inhibition

• Competitive Inhibition: Inhibitor competes for the active site, preventing substrate binding.

• Noncompetitive Inhibition: Inhibitor binds to an allosteric site, altering enzyme shape and rendering the active site nonfunctional. Can be reversible or irreversible.

• Feedback Inhibition: Final product inhibits an earlier enzyme in the pathway, a form of noncompetitive inhibition.

Ribozymes

Ribozymes are RNA molecules functioning as catalysts. They bind and act on substrates, are not consumed in reactions, and play roles in RNA splicing and protein synthesis in ribosomes.

Oxidation-Reduction (Redox) Reactions

• Oxidation: Removal of electrons from a molecule, often producing energy.

• Reduction: Gain of electrons.

• Redox reactions involve one substance being oxidized and another reduced. Mnemonic: OIL RIG (Oxidation Is Loss, Reduction Is Gain).

• Dehydrogenation: Biological oxidations often involve loss of hydrogen atoms.

Phosphorylation Reactions Generating ATP

Energy from redox reactions is trapped by ATP formation.

• Phosphorylation: Addition of a phosphate group (🅟) to a compound, generating ATP from ADP.

• Substrate-Level Phosphorylation: Direct transfer of a high-energy phosphate to ADP. Example: Glycolysis and Krebs cycle.

• Oxidative Phosphorylation: ATP synthesis via electron transport chain. Example: Aerobic respiration.

• Photophosphorylation: ATP production using light energy, electrons from chlorophyll initiate reactions. Example: Photosynthesis.

Metabolic Pathways: Function and Purpose

Metabolic pathways are series of enzyme-catalyzed reactions.

• Break down nutrients.

• Extract energy from organic molecules.

• Store energy in ATP.

• Produce molecules needed by the cell.

Glycolysis: Reactants, Products, and By-Products

• Reactant: Glucose

• Products: 2 Pyruvic acid, 2 ATP (net), 2 NADH

• Stages:

  • Preparatory: 2 ATP used, glucose phosphorylated and split into two 3-carbon molecules.

  • Energy Conserving: 4 ATP produced, 2 NADH produced, 2 pyruvic acid formed.

• Net gain: 2 ATP per glucose oxidized.

Krebs Cycle: Products

• Before entering, pyruvic acid is converted to acetyl-CoA.

• For one acetyl-CoA: 3 NADH, 1 FADH₂, 1 ATP, 2 CO₂

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

Chemiosmotic Model for ATP Generation

• ATP synthesis via electron transport chain (oxidative phosphorylation).

• Uses proton (H⁺) gradient across membrane.

• Steps:

  1. Electrons move through electron transport chain.

  2. Energy pumps H⁺ across membrane, creating proton motive force.

  3. H⁺ returns via ATP synthase, providing energy for ATP formation:

• In prokaryotes: ETC in plasma membrane; in eukaryotes: inner mitochondrial membrane.

Aerobic vs. Anaerobic Respiration

Fermentation: Reactions and Products

• Regenerates NAD⁺ for glycolysis in absence of oxygen.

• Does not require oxygen, Krebs cycle, or electron transport chain.

• Uses organic molecule (e.g., pyruvate) as final electron acceptor.

• Produces small amount of ATP.

• Lactic Acid Fermentation: Glucose → Pyruvic acid → Lactic acid

• Alcohol Fermentation: Glucose → Pyruvic acid → Acetaldehyde → Ethanol

• Products: Lactic acid, ethanol, acetic acid, propionic acid, butanol

• Example: Saccharomyces cerevisiae (yeast) produces ethanol for beer and wine.

Biochemical Tests for Bacterial Identification

• Fermentation Test: Determines if bacterium can ferment carbohydrate; acid production changes pH indicator color, gas production detected. Example: Escherichia

• Oxidase Test: Detects cytochrome c oxidase. Example: Pseudomonas

Photosynthesis: Purpose and Reactions

• Converts light energy to chemical energy, producing organic compounds (e.g., sugars).

• Provides organic material for metabolic processes.

Light-Dependent vs. Light-Independent Reactions

Nutritional Patterns Among Organisms

Additional info: Expanded explanations and tables were added for completeness and clarity.