Microbial Metabolism and Enzyme Function: Study Notes

Microbial Metabolism and Enzyme Function

Enzymes: Mechanism of Action

Enzymes are biological catalysts that accelerate chemical reactions in living organisms. They function by lowering the activation energy required for reactions, allowing processes to occur rapidly and efficiently under physiological conditions.

• Substrate: The molecule upon which an enzyme acts.

• Active Site: The specific region of the enzyme where the substrate binds.

• Substrate-Enzyme Complex: The temporary association formed when a substrate binds to the enzyme's active site.

• Transition State: A high-energy state during the reaction where old bonds are breaking and new bonds are forming.

• Cofactors and Coenzymes: Non-protein molecules that assist enzymes in catalysis. Cofactors are often metal ions, while coenzymes are organic molecules (e.g., NAD+).

Example: The enzyme hexokinase catalyzes the phosphorylation of glucose in glycolysis.

Illustration: Enzyme + Substrate → Enzyme-Substrate Complex → Enzyme + Product

Thermodynamics in Biology

Thermodynamics governs the energy changes in biological systems. The first law states that energy cannot be created or destroyed, only transformed.

• First Law of Thermodynamics: $\Delta E = q + w$ (where $\Delta E$ is the change in internal energy, $q$ is heat, and $w$ is work).

• Application: In cells, chemical energy from nutrients is converted into ATP and heat.

Metabolic Reactions: Anabolism and Catabolism

Metabolism consists of all chemical reactions in a cell, divided into two main types:

• Anabolism: Biosynthetic reactions that build complex molecules from simpler ones; require energy (endergonic).

• Catabolism: Degradative reactions that break down molecules to release energy (exergonic).

• Endergonic Reaction: Requires energy input ($\Delta G > 0$).

• Exergonic Reaction: Releases energy ($\Delta G < 0$).

Example: Protein synthesis (anabolic) vs. glucose breakdown (catabolic).

ATP Cycle and Energy Transfer

ATP (adenosine triphosphate) is the primary energy currency in cells. Its hydrolysis releases energy for cellular processes.

• ATP Hydrolysis: $\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{Energy}$

• ATP Regeneration: Energy from catabolic reactions is used to re-phosphorylate ADP to ATP.

Redox Reactions in Metabolism

Redox (reduction-oxidation) reactions involve the transfer of electrons between molecules, crucial for energy production.

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• Example: NAD+ + 2e- + 2H+ → NADH + H+

Types of Microbial Nutrition: Chemotrophs

Microorganisms are classified based on their energy and carbon sources.

• Aerobic Chemotrophs: Use oxygen as the terminal electron acceptor.

• Anaerobic Chemotrophs: Use other molecules (e.g., nitrate, sulfate) as electron acceptors.

• Example: Escherichia coli can grow aerobically or anaerobically.

Aerobic Respiration: Stages and Substrates

Aerobic respiration is a multi-step process that converts glucose to ATP using oxygen.

• Glycolysis: Glucose → Pyruvate

• Krebs Cycle (Citric Acid Cycle): Pyruvate → CO2 + NADH + FADH2

• Electron Transport Chain: NADH/FADH2 → ATP (via oxidative phosphorylation)

Substrates: Glucose, oxygen

Products: CO2, H2O, ATP

Oxidative Phosphorylation and Chemiosmosis

Oxidative phosphorylation uses the electron transport chain to create a proton gradient, driving ATP synthesis via chemiosmosis.

• Electron Transport Chain: Transfers electrons, pumps protons across membrane.

• ATP Synthase: Uses proton gradient to synthesize ATP from ADP and Pi.

• Equation: $\text{ADP} + \text{P}_i \rightarrow \text{ATP}$

Location: Plasma membrane (prokaryotes), mitochondrial inner membrane (eukaryotes)

Fermentation vs. Anaerobic Respiration

Both processes occur in the absence of oxygen but differ in their mechanisms and ATP yield.

Process	Electron Acceptor	ATP Yield
Fermentation	Organic molecules (e.g., pyruvate)	Low (2 ATP per glucose)
Anaerobic Respiration	Inorganic molecules (e.g., nitrate, sulfate)	Moderate (more than fermentation, less than aerobic)
Aerobic Respiration	Oxygen	High (up to 38 ATP per glucose)

Example: Lactic acid fermentation in Lactobacillus; nitrate respiration in Pseudomonas.

Examples of Fermentation

Fermentation is used by many microbes to generate energy under anaerobic conditions.

• Lactic Acid Fermentation: Pyruvate → Lactic acid (e.g., Lactobacillus)

• Alcoholic Fermentation: Pyruvate → Ethanol + CO2 (e.g., Saccharomyces cerevisiae)

• Mixed Acid Fermentation: Produces a variety of acids and gases (e.g., Escherichia coli)

Exoenzymes

Exoenzymes are enzymes secreted by cells to break down large molecules outside the cell.

• Function: Degrade polymers (e.g., proteins, starch) into smaller units for uptake.

• Example: Amylase breaks down starch into maltose.

Additional info: These notes expand on the brief questions by providing definitions, examples, and context for each concept relevant to microbial metabolism and enzyme function.