Microbial Metabolism and Enzyme Function: Study Notes

Microbial Metabolism

Definitions: Metabolism, Catabolism, and Anabolism

Metabolism encompasses all chemical reactions occurring within a cell, divided into two main categories: catabolism and anabolism.

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

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

• Metabolism: The sum of all catabolic and anabolic processes in a cell.

Example: Glycolysis is a catabolic pathway that breaks down glucose to pyruvate, releasing energy.

Catabolic and Anabolic Processes

• Catabolic processes: Glycolysis, Krebs cycle, fermentation, beta-oxidation of fatty acids.

• Anabolic processes: Protein synthesis, DNA replication, cell wall biosynthesis.

ATP Production and Energy Transfer

• Catabolic reactions generate ATP, which is then used to drive anabolic reactions.

• ATP is produced by substrate-level phosphorylation, oxidative phosphorylation, and photophosphorylation.

Redox Reactions in Metabolism

Oxidation-Reduction (Redox) Reactions

Redox reactions are essential for energy transfer in cells.

• Oxidation: Loss of electrons from a molecule.

• Reduction: Gain of electrons by a molecule.

• Redox reactions often involve electron carriers such as NAD+ and FAD.

Role of NAD+ and FAD in Metabolism

• NAD+ (Nicotinamide adenine dinucleotide) and FAD (Flavin adenine dinucleotide) are coenzymes that act as electron carriers.

• They accept electrons during catabolic reactions and transfer them to the electron transport chain, facilitating ATP production.

• Example: In glycolysis and the Krebs cycle, NAD+ is reduced to NADH, which then donates electrons to the electron transport chain.

Enzymes and Their Function

Structure and Function of Enzymes

Enzymes are biological catalysts that speed up chemical reactions without being consumed.

• Most enzymes are proteins, some require cofactors (inorganic ions) or coenzymes (organic molecules such as vitamins).

• Enzymes have an active site where substrates bind and reactions occur.

• Substrate: The specific molecule upon which an enzyme acts.

• Cofactor: Non-protein component required for enzyme activity (e.g., metal ions).

• Coenzyme: Organic molecule that assists enzyme function (e.g., NAD+, FAD).

Enzyme Activity and Regulation

• Enzyme activity is influenced by temperature, pH, and substrate concentration.

• High temperatures or extreme pH can denature enzymes, reducing activity.

• Enzyme activity increases with substrate concentration up to a saturation point.

Enzyme Inhibition

• Competitive inhibitors: Bind to the active site, blocking substrate binding.

• Noncompetitive inhibitors: Bind to another part of the enzyme, altering its shape and reducing activity.

Feedback Inhibition

• End-product of a metabolic pathway inhibits an earlier enzyme, preventing overproduction.

Glycolysis, Pentose Phosphate Pathway, and Entner-Doudoroff Pathway

Overview of Glycolysis

Glycolysis is the central pathway for glucose catabolism, producing ATP, NADH, and pyruvate.

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

Pentose Phosphate Pathway (PPP)

• Starts with glucose and produces NADPH and ribose-5-phosphate.

• Important for biosynthesis and antioxidant defense.

Entner-Doudoroff Pathway

• Alternative to glycolysis, found in some bacteria.

• Produces 1 ATP, 1 NADH, and 1 NADPH per glucose.

Pathway	Main Products	ATP Yield	Unique Features
Glycolysis	2 Pyruvate, 2 NADH	2 ATP	Universal in most organisms
Pentose Phosphate	NADPH, Ribose-5-phosphate	1 ATP (variable)	Biosynthesis, antioxidant defense
Entner-Doudoroff	2 Pyruvate, 1 NADH, 1 NADPH	1 ATP	Found in some bacteria

Cellular Respiration and Fermentation

Cellular Respiration

• Includes glycolysis, Krebs cycle, and electron transport chain (ETC).

• Requires oxygen (aerobic) or other final electron acceptors (anaerobic).

• Produces more ATP than fermentation.

Fermentation

• Occurs when oxygen is absent.

• Regenerates NAD+ from NADH, allowing glycolysis to continue.

• Produces less ATP and various end products (e.g., lactic acid, ethanol).

Comparison Table: Cellular Respiration vs. Fermentation

Process	Oxygen Required?	ATP Yield	End Products
Cellular Respiration	Yes (aerobic) or No (anaerobic)	~38 (aerobic)	CO2, H2O (aerobic); varied (anaerobic)
Fermentation	No	2	Lactic acid, ethanol, CO2, etc.

Electron Transport Chain (ETC) and ATP Synthesis

• ETC uses electrons from NADH and FADH2 to pump protons across a membrane, creating a proton gradient.

• ATP synthase uses this gradient to produce ATP from ADP and inorganic phosphate.

• Final electron acceptor is O2 in aerobic respiration; nitrate, sulfate, or CO2 in anaerobic respiration.

Fermentation Pathways and Products

• Common products: acetic acid, acetone, isopropanol, propionic acid, cheese, bread, beer, kimchi.

Integration of Catabolism and Anabolism

Connection Between Catabolism and Anabolism

• Catabolism provides ATP and precursor metabolites for anabolic pathways.

• Anabolism uses ATP and building blocks to synthesize macromolecules.

• ADP produced by anabolism is recycled by catabolic reactions.

Summary Table: Key Metabolic Pathways

Pathway	Main Function	Key Products
Glycolysis	Glucose breakdown	ATP, NADH, Pyruvate
Pentose Phosphate	Biosynthesis, NADPH production	NADPH, Ribose-5-phosphate
Entner-Doudoroff	Alternative glucose catabolism	ATP, NADH, NADPH, Pyruvate
Krebs Cycle	Oxidation of acetyl-CoA	CO2, NADH, FADH2, ATP
Electron Transport Chain	ATP synthesis	ATP, H2O (aerobic)
Fermentation	Regenerate NAD+	Organic acids, alcohols, gases

Additional info:

• Some details, such as the exact ATP yield in glycolysis and the role of NADPH in biosynthesis, were expanded for clarity.

• Tables were inferred and constructed to aid comparison and understanding.