Microbiology 145: Metabolism – Enzymes, Pathways, and Energy

Metabolism

Overview of Metabolism

Metabolism refers to the sum total of all biochemical processes occurring within a cell. These processes are essential for maintaining life, enabling growth, reproduction, and response to environmental changes.

• Anabolism: The synthesis of chemical compounds. This process consumes energy and is termed endergonic.

• Catabolism: The breakdown of chemical compounds. This process releases energy and is termed exergonic.

• Energy Flow: Energy is stored in molecules such as carbohydrates and lipids, and is released or used during cellular processes like cell growth and division.

Example: The conversion of glucose to ATP during cellular respiration is a catabolic process.

Selective Toxicity and Mitochondria

Antibiotics and Eukaryotic Cells

Some antibiotics exhibit selective toxicity against bacteria but can also affect mitochondria in eukaryotic cells. This is because mitochondria share evolutionary ancestry with bacteria and possess similar ribosomes and metabolic pathways.

• Selective Toxicity: The ability of a drug to target microbial cells without harming host cells.

• Mitochondrial Vulnerability: Antibiotics targeting bacterial ribosomes may inadvertently affect mitochondrial ribosomes, leading to side effects.

Example: Tetracycline antibiotics can inhibit mitochondrial protein synthesis.

Bacterial Morphology

Classification by Shape and Arrangement

Bacteria are classified based on their shape and arrangement. The image depicts a chain of spherical bacteria.

• Coccus: Spherical-shaped bacteria.

• Streptococcus: Chains of cocci.

Example: Streptococcus pyogenes forms chains of cocci.

Metabolic Pathways

Energy Transformation in Cells

Cells release or store energy via metabolic pathways, which are sequences of chemical reactions where the product of one reaction serves as the substrate for the next.

• Metabolic Pathway: A connected sequence of chemical reactions.

• ATP: The primary energy currency of the cell.

• Energy Storage: Carbohydrates and lipids serve as energy reserves.

Enzymes

Structure and Function

Enzymes are organic molecules, usually proteins, that catalyze biochemical reactions without being consumed. They lower the activation energy required for reactions, allowing them to occur rapidly.

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

• Enzyme-Substrate Complex: Temporary association between enzyme and substrate.

• Product Formation: Substrate is converted to product, which is then released.

• Enzyme Recycling: Enzymes are not consumed and can catalyze multiple reactions.

Example: The enzyme lysozyme breaks down peptidoglycan in bacterial cell walls.

Enzyme Composition and Nomenclature

Types of Enzyme Components

Enzymes may consist of only protein or may require additional non-protein components for activity.

Type	Component	Example
Protein only	Apoenzyme	Lysozyme
Protein + ion	Cofactor (e.g., Mg2+, Zn2+, Fe2+)	Taq polymerase
Protein + organic molecule	Coenzyme (e.g., NAD+, FAD)	NAD+, FAD
Complete enzyme	Holoenzyme (apoenzyme + cofactor/coenzyme)	Many metabolic enzymes

Enzyme Naming: Enzymes are typically named based on their substrate or reaction type (e.g., lactase, peptidase, lipase).

ATP: Structure and Function

Role of ATP in Cellular Metabolism

Adenosine triphosphate (ATP) is the chemical energy currency for most cellular processes. The hydrolysis of ATP releases energy that can be used for cellular work.

• High-Energy Bonds: Breaking the bond holding the last phosphate group yields approximately 7.3 kcal/mole ATP.

• Portable Battery: ATP is readily used but not ideal for long-term energy storage due to instability and bulkiness.

• Energy Storage: Cells store energy in stable compounds like carbohydrates and lipids until needed.

Equation:

$\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

Electron Carriers

Oxidation and Reduction in Metabolism

Electron carriers are molecules that transport electrons during cellular respiration and other metabolic processes.

Carrier (Oxidized)	Carrier (Reduced)
NAD+	NADH
NADP+	NADPH
FAD	FADH2

Function: These carriers shuttle electrons to the electron transport chain, facilitating ATP production.

Glucose Catabolism

Pathways for Energy Production

Glucose catabolism is the process by which cells break down glucose to produce energy. There are two main types: respiration and fermentation. Both begin with glycolysis.

• Glycolysis: The initial pathway in glucose catabolism, resulting in a net gain of 2 ATP and 2 NADH per glucose molecule. The end product is 2 molecules of pyruvic acid (pyruvate).

• Respiration: Complete oxidation of glucose, producing maximum ATP.

• Fermentation: Incomplete oxidation, yielding less ATP.

Equation for Aerobic Respiration:

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 + 38\text{ADP} + 38\text{P}_i \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + 38\text{ATP}$

Aerobic Respiration

Three Main Steps

Aerobic respiration consists of three major steps:

1. Synthesis of Acetyl-CoA: Pyruvic acid is decarboxylated to form acetyl-CoA, producing NADH.

2. TCA/Krebs Cycle: Acetyl-CoA enters the cycle, generating NADH, FADH2, ATP, and CO2.

3. Electron Transport Chain (ETC): NADH and FADH2 donate electrons, driving ATP synthesis.

Why generate so much NADH and FADH2? These molecules provide protons and electrons to the ETC, which is responsible for the majority of ATP production from glucose.

Summary Table: Steps of Aerobic Respiration

Step	Main Inputs	Main Outputs	Purpose
Synthesis of Acetyl-CoA	Pyruvic acid, NAD+	Acetyl-CoA, NADH, CO2	Prepares substrate for Krebs cycle
Krebs Cycle	Acetyl-CoA, NAD+, FAD	NADH, FADH2, ATP, CO2	Generates electron carriers and ATP
Electron Transport Chain	NADH, FADH2, O2	ATP, H2O	Produces majority of ATP

Additional info: The notes above expand on the brief points in the original slides, providing definitions, examples, and context for key microbiology concepts related to metabolism and energy production.