Microbial Metabolism and Enzyme Function: Study Notes for Microbiology

Microbial Metabolism

Definitions: Metabolism, Catabolism, and Anabolism

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

• Catabolism: The breakdown of complex molecules into simpler ones, releasing energy (often as ATP).

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

• Metabolism: The sum of catabolic and anabolic reactions.

Example: Glucose breakdown (catabolism) provides ATP for protein synthesis (anabolism).

Catabolic Processes and Energy Conversion

Catabolic reactions convert substrates into products, often releasing energy and producing ATP.

• ATP is generated from ADP and inorganic phosphate during catabolic reactions.

• Energy released is used for cellular work or stored for later use.

Oxidation-Reduction (Redox) Reactions

Redox reactions are central to energy transfer in cells, involving the movement of electrons.

• Oxidation: Loss of electrons from a molecule.

• Reduction: Gain of electrons by a molecule.

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

Equation:

$\text{A} + \text{B} \rightarrow \text{A}^+ + \text{B}^-$

Role of NAD+ and FAD in Metabolism

NAD+ (Nicotinamide adenine dinucleotide) and FAD (Flavin adenine dinucleotide) are electron carriers in metabolic pathways.

• They accept electrons during catabolic reactions, becoming NADH and FADH2.

• They transfer electrons to the electron transport chain (ETC), facilitating ATP production.

Equation:

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

Enzymes and Their Function

Structure of Enzymes

Enzymes are biological catalysts, mostly proteins, that speed up chemical reactions.

• Some enzymes require non-protein components called cofactors (inorganic ions) or coenzymes (organic molecules).

• Enzyme structure includes an active site where substrates bind.

Substrate, Cofactor, and Coenzyme

• Substrate: The molecule upon which an enzyme acts.

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

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

Enzyme Function and Catalysis

Enzymes lower the activation energy of reactions, increasing reaction rates without being consumed.

• They are highly specific for their substrates.

• Enzyme activity can be regulated by temperature, pH, and substrate concentration.

Factors Affecting Enzyme Activity

• Temperature: High temperatures can denature enzymes, reducing activity.

• pH: Extreme pH values can alter enzyme structure and function.

• Substrate concentration: Increasing substrate increases reaction rate up to a saturation point.

Example: Human enzymes typically function best at 37°C and neutral pH.

Enzyme Inhibition

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

• Noncompetitive inhibitors: Bind elsewhere on the enzyme, altering its shape and reducing activity.

Additional info: Feedback inhibition is a regulatory mechanism where the end product of a pathway inhibits an earlier step.

Glycolysis and Fermentation Pathways

Glycolysis Overview

Glycolysis is the metabolic pathway that converts glucose into pyruvate, generating ATP and NADH.

• Occurs in the cytoplasm of cells.

• Produces 2 ATP, 2 NADH, and 2 pyruvate per glucose molecule.

Pentose Phosphate Pathway vs. Entner-Doudoroff Pathway

Pathway	Starting Molecule	Products
Pentose Phosphate	Glucose	NADPH, Ribose-5-phosphate
Entner-Doudoroff	Glucose	1 NADPH, 1 ATP, 1 NADH, 2 pyruvate

Additional info: Glycolysis produces the highest ATP yield among these pathways.

Fermentation vs. Cellular Respiration

• Cellular respiration: Uses oxygen, produces more ATP, includes glycolysis, Krebs cycle, and ETC.

• Fermentation: Does not use oxygen, produces less ATP, includes glycolysis only.

• Fermentation produces organic acids, alcohols, and gases as end products.

Summary Table: Glycolysis and Its Products

Process	ATP Produced	NADH Produced	Pyruvate Produced
Glycolysis	2	2	2
Pentose Phosphate	Variable	0	Variable
Entner-Doudoroff	1	1	2

Cellular Respiration: Krebs Cycle and Electron Transport Chain

Role of Acetyl-CoA, Krebs Cycle, and ETC

Acetyl-CoA enters the Krebs cycle, producing NADH and FADH2, which donate electrons to the ETC for ATP synthesis.

• Krebs cycle produces CO2, NADH, FADH2, and ATP.

• ETC uses NADH and FADH2 to generate ATP via oxidative phosphorylation.

Equation:

$\text{Glucose} + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP}$

Aerobic vs. Anaerobic Respiration

• Aerobic respiration: Uses oxygen as the final electron acceptor.

• Anaerobic respiration: Uses other molecules (e.g., nitrate, sulfate, CO2) as final electron acceptors.

Example: Bacteria, archaea, and eukaryotes can perform anaerobic respiration.

Fermentation Pathways and Products

Fermentation End Products

• Common products: acetic acid, acetone, isopropanol, CO2, propionic acid, cheese, bread, beer, kimchi.

• Fermentation recycles NADH to NAD+ to allow glycolysis to continue in the absence of oxygen.

Integration of Catabolism and Anabolism

Connection Between Catabolism and Anabolism

Catabolism provides energy and building blocks for anabolism, while anabolism uses ATP and precursors to synthesize cellular components.

• ATP produced by catabolism is consumed during anabolic reactions.

• ADP generated by anabolism is recycled by catabolic pathways.

Additional info: This integration ensures efficient energy use and cellular growth.