Microbial Metabolism

Overview of Metabolism

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

• Catabolism: The breakdown of complex molecules into simpler ones, releasing energy.

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

• Example: Glycolysis is a catabolic pathway; protein synthesis is anabolic.

Redox Reactions in Metabolism

Oxidation-reduction (redox) reactions are central to energy transfer in cells.

• Oxidation: Loss of electrons from a molecule.

• Reduction: Gain of electrons by a molecule.

• Redox pairs: One molecule is oxidized while another is reduced.

• Example: NAD+ is reduced to NADH during glycolysis.

ATP Production Mechanisms

Cells generate ATP through three main mechanisms:

• Substrate-level phosphorylation: Direct transfer of a phosphate group to ADP.

• Oxidative phosphorylation: ATP synthesis using energy from the electron transport chain and chemiosmosis.

• Photophosphorylation: ATP production using light energy (in photosynthetic organisms).

• Equation:

Enzymes in Metabolism

Enzymes are biological catalysts that speed up metabolic reactions by lowering activation energy.

• Active site: Region where substrate binds.

• Cofactors: Non-protein helpers (e.g., metal ions, coenzymes like NAD+).

• Example: Hexokinase catalyzes the first step of glycolysis.

Glycolysis

Glycolysis is the catabolic pathway that breaks down glucose into pyruvate, generating ATP and NADH.

• Reactants: Glucose, NAD+, ADP, Pi

• Products: 2 Pyruvate, 2 ATP, 2 NADH

• Location: Cytoplasm

Pentose Phosphate Pathway

This pathway operates alongside glycolysis, generating NADPH and pentoses for biosynthesis.

• Key products: NADPH, ribose-5-phosphate

• Importance: Provides reducing power and precursors for nucleotide synthesis.

Entner-Doudoroff Pathway

An alternative to glycolysis found in some bacteria, producing ATP, NADPH, and pyruvate.

• Key difference: Yields less ATP than glycolysis.

Comparing Glycolytic Pathways

Transition to the Krebs Cycle

Pyruvate from glycolysis is converted to acetyl-CoA, which enters the Krebs cycle (citric acid cycle).

• Reactants: Acetyl-CoA, NAD+, FAD, ADP

• Products: CO2, NADH, FADH2, ATP

Krebs Cycle (Citric Acid Cycle)

A series of reactions that generate high-energy electron carriers and CO2 from acetyl-CoA.

• Main products per turn: 3 NADH, 1 FADH2, 1 ATP, 2 CO2

• Location: Cytoplasm (prokaryotes), mitochondria (eukaryotes)

Electron Transport Chain (ETC) and Chemiosmosis

The ETC uses electrons from NADH and FADH2 to pump protons across a membrane, creating a proton gradient used by ATP synthase to generate ATP.

• Final electron acceptor: Oxygen (aerobic), other molecules (anaerobic)

• Equation:

Aerobic vs. Anaerobic Respiration

Fermentation

Fermentation allows ATP production without oxygen by regenerating NAD+ from NADH.

• Products: Lactic acid, ethanol, CO2, other organic acids

• Importance: Enables energy production in anaerobic conditions.

Lipid and Protein Catabolism

Microbes can break down lipids and proteins for energy when carbohydrates are unavailable.

• Lipids: Broken into glycerol and fatty acids; fatty acids enter β-oxidation.

• Proteins: Deaminated to enter glycolysis or the Krebs cycle.

Photosynthesis in Microbes

Some microbes use light energy to synthesize organic molecules via photosynthesis.

• Cyclic photophosphorylation: Electrons return to the original chlorophyll.

• Noncyclic photophosphorylation: Electrons are transferred to NADP+, forming NADPH.

• Equation:

Summary Table: Key Metabolic Pathways

Additional info: For exam preparation, review the mechanisms of enzyme action, the regulation of metabolic pathways, and the integration of catabolic and anabolic processes in microbial cells.