Microbial Metabolism: Energy Production and Pathways

Metabolism: The Sum of All Chemical Reactions in a Cell

Overview of Metabolism

Metabolism encompasses all chemical reactions occurring within a living organism, divided into two main processes: catabolism and anabolism. These reactions are essential for cellular function, growth, and maintenance.

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

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

• The energy released from catabolic (exergonic) reactions is used to drive anabolic (endergonic) reactions.

• ATP (adenosine triphosphate) serves as the main energy currency in cells.

Energy and Thermodynamics in Biological Systems

Forms of Energy

• Kinetic Energy: Energy of motion.

• Potential Energy: Stored energy, including chemical energy available in molecules for cellular work.

Thermodynamic Laws

• First Law (Law of Conservation of Energy): Energy cannot be created or destroyed, only transformed or transferred.

• Second Law (Entropy): Every energy transfer increases the entropy (disorder) of the universe.

Catabolic and Anabolic Pathways

Metabolic Pathways

Metabolic pathways are sequences of enzymatically catalyzed chemical reactions in cells. Catabolic pathways release energy, while anabolic pathways consume energy.

• Exergonic reactions: Release energy (catabolism).

• Endergonic reactions: Require energy input (anabolism).

Chemical Reactions and Enzymes

Collision Theory and Activation Energy

• Chemical reactions occur when atoms, ions, or molecules collide with sufficient energy.

• Activation energy is the minimum energy required to initiate a reaction by disrupting electronic configurations.

• Reaction rate is the frequency of effective collisions; it can be increased by enzymes, temperature, or pressure.

Enzymes: Biological Catalysts

• Enzymes are globular proteins that catalyze chemical reactions by lowering activation energy.

• They are highly efficient, function at low temperatures, and are regulated by the cell.

• Enzyme names typically end in -ase and are classified by the reactions they catalyze.

Enzyme Structure

• Holoenzyme: The complete, active enzyme, consisting of:

  • Apoenzyme: Protein portion.

  • Cofactor: Non-protein component, which may be a metal ion (e.g., Fe, Mg, Zn) or a coenzyme (organic molecule such as NAD+, FAD, or coenzyme A).

Mechanism of Enzyme Action

• Enzymes bind substrates at their active sites, forming an enzyme-substrate complex.

• The substrate is transformed into products, and the enzyme is released unchanged.

• Enzymes exhibit specificity for their substrates.

Factors Affecting Enzyme Activity

• Temperature: High temperatures can denature enzymes; low temperatures slow reaction rates.

• pH: Each enzyme has an optimum pH for maximal activity.

• Substrate concentration: Activity increases with substrate concentration until enzymes are saturated.

• Inhibitors:

  • Competitive inhibitors: Compete with substrate for the active site.

  • Noncompetitive inhibitors: Bind elsewhere, altering enzyme function.

Energy Production: Oxidation-Reduction (Redox) Reactions

Redox Reactions

• Oxidation: Loss of electrons (or H+); addition of O2.

• Reduction: Gain of electrons (or H+); removal of O2.

• Redox reactions are coupled; when one molecule is oxidized, another is reduced.

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

ATP Generation Mechanisms

Phosphorylation Methods

• Substrate-level phosphorylation: Direct transfer of a phosphate group to ADP from an intermediate substrate.

• Oxidative phosphorylation: Electrons are transferred through an electron transport chain (ETC) to a final electron acceptor, generating ATP via chemiosmosis.

• Photophosphorylation: Light energy is used to generate ATP in photosynthetic organisms.

Metabolic Pathways of Energy Production

Carbohydrate Catabolism

Cells primarily obtain energy by oxidizing carbohydrates, especially glucose. Two main pathways are respiration and fermentation.

• Respiration: Complete breakdown of glucose to CO2 and H2O (aerobic) or to other inorganic molecules (anaerobic).

• Fermentation: Partial breakdown of glucose to organic end-products in the absence of O2.

Glycolysis

• Converts one glucose (C6) to two pyruvic acid (C3) molecules.

• Produces 2 ATP and 2 NADH per glucose.

• End-product: pyruvic acid.

Preparatory Step

• Pyruvic acid (C3) is decarboxylated to acetyl-CoA (C2), releasing CO2.

Krebs Cycle (Citric Acid Cycle)

• Acetyl-CoA (C2) combines with oxaloacetic acid (C4) to form citric acid (C6).

• Produces 6 NADH, 2 FADH2, 2 ATP, and 6 CO2 per glucose.

• Electrons are transferred to NAD+ and FAD for the ETC.

Electron Transport Chain (ETC) and Oxidative Phosphorylation

• NADH and FADH2 donate electrons to the ETC.

• ETC consists of carriers such as flavoproteins, cytochromes, and ubiquinones.

• Electrons move through the chain, generating a proton gradient used by ATP synthase to produce ATP.

• In prokaryotes, ETC is in the plasma membrane; in eukaryotes, in the inner mitochondrial membrane.

Summary of Aerobic Respiration

• In prokaryotes: Up to 38 ATP per glucose.

• In eukaryotes: Up to 36 ATP per glucose.

Chemiosmotic Mechanism of ATP Generation

• Protons are pumped across a membrane, creating a proton motive force.

• ATP synthase uses this force to convert ADP and phosphate into ATP.

Alternative Pathways to Glycolysis

• Pentose Phosphate Pathway: Metabolizes five-carbon sugars; yields 1 ATP and 12 NADPH per glucose.

• Entner-Doudoroff Pathway: Yields 1 ATP and 2 NADPH per glucose.

Cellular Respiration Types

• Aerobic respiration: O2 is the final electron acceptor.

• Anaerobic respiration: Final electron acceptor is an inorganic molecule other than O2 (e.g., S or N).

Key Definitions and Processes

Term	Definition
Substrate-level phosphorylation	High-energy phosphate from an intermediate molecule is added to ADP to form ATP.
Photophosphorylation	Energy from light is trapped by chlorophyll to provide energy for ATP production.
Oxidative phosphorylation	Energy is released as electrons are passed to oxygen through a series of cytochromes in an ETC.
Fermentation	Release of energy from the breakdown of sugars to other organic end-products in the absence of O2.
Aerobic respiration	Oxidation of glucose to CO2 and water with O2 as the final electron acceptor.
Anaerobic respiration	Oxidation of organic molecules in an ETC with inorganic molecules other than O2 as the final electron acceptor (e.g., S or N).

Summary Table: Energy Production in Aerobic Respiration (Prokaryotes)

Pathway	ATP Produced (per glucose)	NADH Produced	FADH2 Produced	CO2 Produced
Glycolysis	2	2	0	0
Preparatory Step	0	2	0	2
Krebs Cycle	2	6	2	4
Electron Transport Chain	34	--	--	--
Total	38	--	--	6

Additional info: The above table summarizes the ATP yield and byproducts of aerobic respiration in prokaryotes. In eukaryotes, the total ATP yield is typically 36 due to differences in mitochondrial transport.

Key Equations

• Overall equation for aerobic respiration:

$\mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 + 6\,\mathrm{O}_2 \rightarrow 6\,\mathrm{CO}_2 + 6\,\mathrm{H}_2\mathrm{O} + \text{energy (ATP)}$

• ATP formation (substrate-level phosphorylation):

$\mathrm{ADP} + \mathrm{P}_i \rightarrow \mathrm{ATP}$

• Reduction of NAD+:

$\mathrm{NAD}^+ + 2\,\mathrm{H}^+ + 2\,e^- \rightarrow \mathrm{NADH} + \mathrm{H}^+$

Examples and Applications

• Example: During vigorous exercise, muscle cells switch from aerobic respiration to fermentation, producing lactic acid as an end-product.

• Application: Industrial fermentation is used to produce ethanol, lactic acid, and other chemicals by exploiting microbial metabolic pathways.