Microbial Metabolism and Growth: Enzymes, Energy Pathways, and Cell Division

Enzymes: Structure, Function, and Regulation

Structure and Function of Enzymes

Enzymes are biological catalysts, typically proteins, that accelerate chemical reactions in microbial cells by lowering the activation energy required. They are essential for metabolic processes.

• Active Site: The region on the enzyme where substrate molecules bind and undergo a chemical reaction.

• Specificity: Enzymes are highly specific for their substrates due to the unique shape of their active sites.

• Induced Fit Model: The enzyme changes shape slightly to accommodate the substrate, enhancing catalysis.

Types of Enzyme Regulation

• Allosteric Regulation: Regulatory molecules bind to sites other than the active site (allosteric sites), causing conformational changes that increase or decrease enzyme activity.

• Competitive Inhibition: Inhibitors resemble the substrate and compete for binding at the active site, blocking substrate access.

• Noncompetitive Inhibition: Inhibitors bind to a different part of the enzyme, altering its function regardless of substrate concentration.

• Feedback Inhibition: The end product of a metabolic pathway inhibits an upstream enzyme, regulating pathway activity.

Example: The enzyme hexokinase is inhibited by its product, glucose-6-phosphate, via feedback inhibition.

Oxidation and Reduction in Microbial Metabolism

Definitions and Examples

• Oxidation: The loss of electrons from a molecule, often associated with the gain of oxygen or loss of hydrogen.

• Reduction: The gain of electrons by a molecule, often associated with the gain of hydrogen or loss of oxygen.

Example of Oxidation: In cellular respiration, glucose is oxidized to carbon dioxide.

Example of Reduction: Oxygen is reduced to water in the electron transport chain.

General Equation:

$\text{Oxidation:} \quad \text{A} \rightarrow \text{A}^+ + e^-$

$\text{Reduction:} \quad \text{B} + e^- \rightarrow \text{B}^-$

Binary Fission: Microbial Cell Division

Definition and Steps

Binary fission is the primary method of asexual reproduction in prokaryotes, resulting in two genetically identical daughter cells.

1. DNA Replication: The circular chromosome is duplicated.

2. Cell Elongation: The cell grows, and the chromosomes move to opposite poles.

3. Septum Formation: A septum (dividing wall) begins to form at the cell's midpoint.

4. Cell Separation: The septum is completed, and the cell splits into two daughter cells.

Example: Escherichia coli divides every 20 minutes under optimal conditions by binary fission.

Products of Major Metabolic Pathways

Glycolysis

• Products (per glucose): 2 ATP (net), 2 NADH, 2 pyruvate

• Location: Cytoplasm (both prokaryotes and eukaryotes)

Pyruvate Oxidation

• Products (per glucose): 2 NADH, 2 CO2, 2 acetyl-CoA

• Location: Cytoplasm (prokaryotes), mitochondrial matrix (eukaryotes)

Citric Acid Cycle (Krebs Cycle)

• Products (per glucose): 2 ATP (or GTP), 6 NADH, 2 FADH2, 4 CO2

• Location: Cytoplasm (prokaryotes), mitochondrial matrix (eukaryotes)

Electron Transport Chain (ETC) and Chemiosmosis

• Products (per glucose): ~34 ATP (via oxidative phosphorylation), water (if oxygen is terminal electron acceptor)

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

Pathway	Main Products	Location (Prokaryote)	Location (Eukaryote)
Glycolysis	2 ATP, 2 NADH, 2 pyruvate	Cytoplasm	Cytoplasm
Pyruvate Oxidation	2 NADH, 2 CO2, 2 acetyl-CoA	Cytoplasm	Mitochondrial matrix
Citric Acid Cycle	2 ATP, 6 NADH, 2 FADH2, 4 CO2	Cytoplasm	Mitochondrial matrix
ETC/Chemiosmosis	~34 ATP, H2O	Plasma membrane	Inner mitochondrial membrane

Role of Fermentation

Fermentation is an anaerobic process that allows cells to regenerate NAD+ from NADH, enabling glycolysis to continue in the absence of oxygen. It results in the partial oxidation of organic substrates and the production of various end products.

• Purpose: To recycle NAD+ for glycolysis when the electron transport chain is unavailable.

• Common Products: Lactic acid, ethanol, CO2, and other organic acids or alcohols.

• Applications: Food production (e.g., yogurt, bread, beer), survival in anaerobic environments.

Example: Lactobacillus species ferment glucose to lactic acid in yogurt production.

Oxygenic vs. Anoxygenic Photosynthesis

Comparison and Contrast

Feature	Oxygenic Photosynthesis	Anoxygenic Photosynthesis
Electron Donor	Water (H2O)	Substances other than water (e.g., H2S, Fe2+)
Oxygen Production	Produces O2	Does not produce O2
Organisms	Cyanobacteria, algae, plants	Green and purple sulfur bacteria, some non-sulfur bacteria
Photosystems	Two (PSI and PSII)	One

• Oxygenic Photosynthesis: Utilizes water as an electron donor, releasing oxygen as a byproduct. Found in cyanobacteria, algae, and plants.

• Anoxygenic Photosynthesis: Uses alternative electron donors (e.g., hydrogen sulfide), does not produce oxygen. Found in certain bacteria.

Example: Chlorobium (green sulfur bacteria) perform anoxygenic photosynthesis using H2S as an electron donor.