Microbial Metabolism and Bioenergetics: Study Guide

Microbial Metabolism and Bioenergetics

Introduction to Metabolism

Metabolism refers to the sum of all chemical reactions that occur within a living organism. These reactions are essential for maintaining life, providing energy, and synthesizing necessary cellular components.

• Metabolism: The total of all biochemical reactions in a cell, divided into two main types: anabolism and catabolism.

• Anabolism: The biosynthetic phase; building complex molecules from simpler ones, usually requiring energy.

• Catabolism: The degradative phase; breaking down complex molecules into simpler ones, releasing energy.

• Key Difference: Anabolism consumes energy (endergonic), while catabolism releases energy (exergonic).

• Example: Synthesis of proteins from amino acids (anabolism); breakdown of glucose during glycolysis (catabolism).

ATP: The Energy Currency

Adenosine triphosphate (ATP) is the primary energy carrier in cells, linking catabolic and anabolic reactions.

• ATP as an Intermediate: ATP stores energy from catabolic reactions and provides it for anabolic reactions.

• ATP Structure: Composed of adenine, ribose, and three phosphate groups.

• Hydrolysis of ATP: Releases energy by converting ATP to ADP and inorganic phosphate ($ATP \rightarrow ADP + P_i$).

• Example: Muscle contraction, active transport, and biosynthesis all require ATP.

Enzymes and Enzyme Activity

Enzymes are biological catalysts that speed up chemical reactions without being consumed. They are essential for metabolic processes.

• Enzyme: A protein that accelerates a specific chemical reaction.

• Cofactor: A non-protein component (metal ion or organic molecule) required for enzyme activity.

• Coenzyme: An organic cofactor, often derived from vitamins (e.g., NAD+, FAD).

• Mechanism: Enzymes lower the activation energy of reactions.

• Specificity: Each enzyme acts on a specific substrate due to its unique active site.

• Factors Affecting Activity: Temperature, pH, substrate concentration, and presence of inhibitors.

• Optimal Conditions: Each enzyme has an optimal temperature and pH for maximum activity.

• Inhibition: Competitive inhibition occurs when a molecule competes with the substrate for the active site. Noncompetitive inhibition occurs when an inhibitor binds elsewhere, altering enzyme function.

• Feedback Inhibition: The end product of a pathway inhibits an earlier step, regulating metabolic flow.

Ribozymes

Ribozymes are RNA molecules with catalytic activity, capable of catalyzing specific biochemical reactions, such as RNA splicing.

• Example: Self-splicing introns in some RNA molecules.

Redox Reactions and Energy Production

Redox (reduction-oxidation) reactions are central to energy production in cells, involving the transfer of electrons from one molecule to another.

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• Importance: Glucose oxidation is a major source of energy for most organisms.

Phosphorylation and ATP Generation

Cells generate ATP through three main types of phosphorylation:

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

• Oxidative phosphorylation: ATP generated via the electron transport chain and chemiosmosis, using oxygen as the final electron acceptor.

• Photophosphorylation: ATP generated using light energy in photosynthetic organisms.

Metabolic Pathways

Metabolic pathways are series of enzyme-catalyzed reactions that transform substrates into final products.

• Catabolic Pathways: Glycolysis, Krebs cycle, and electron transport chain.

• Anabolic Pathways: Synthesis of amino acids, nucleotides, and lipids.

• Amphibolic Pathways: Pathways that function in both anabolism and catabolism (e.g., Krebs cycle).

Glycolysis

Glycolysis is the breakdown of glucose to pyruvate, producing ATP and NADH. It consists of preparatory and energy-conserving stages.

• Preparatory Stage: Glucose is phosphorylated and split into two 3-carbon molecules.

• Energy-Conserving Stage: ATP and NADH are produced.

• Net Yield: 2 ATP, 2 NADH, and 2 pyruvate per glucose molecule.

Pentose Phosphate and Entner-Doudoroff Pathways

Alternative pathways for glucose catabolism, providing precursors for biosynthesis and varying ATP yields.

• Pentose Phosphate Pathway: Generates NADPH and pentoses for nucleotide synthesis; produces only one ATP per glucose.

• Entner-Doudoroff Pathway: Found in some bacteria; produces NADPH and ATP, but less efficient than glycolysis.

Krebs Cycle (Citric Acid Cycle)

The Krebs cycle oxidizes acetyl-CoA to CO2, generating NADH, FADH2, and ATP (or GTP).

• Main Products: 3 NADH, 1 FADH2, 1 ATP (per acetyl-CoA).

• Role: Supplies electrons for the electron transport chain.

Electron Transport Chain and Chemiosmosis

The electron transport chain (ETC) transfers electrons from NADH and FADH2 to oxygen (aerobic) or other acceptors (anaerobic), creating a proton gradient used to synthesize ATP.

• Chemiosmotic Model: Proposed by Peter Mitchell; proton motive force drives ATP synthesis via ATP synthase.

• Equation: $\text{NADH} + H^+ + 1/2 O_2 \rightarrow \text{NAD}^+ + H_2O$

Aerobic vs. Anaerobic Respiration

Respiration can be classified based on the final electron acceptor.

• Aerobic Respiration: Uses oxygen as the final electron acceptor; yields more ATP.

• Anaerobic Respiration: Uses inorganic molecules (e.g., nitrate, sulfate) as final electron acceptors; yields less ATP.

Fermentation

Fermentation is an anaerobic process that regenerates NAD+ by transferring electrons from NADH to organic molecules, producing various end products.

• Common End Products: Lactic acid, ethanol, CO2, propionic acid, etc.

• Example: Lactic acid fermentation in muscle cells; alcoholic fermentation in yeast.

Lipid and Protein Catabolism

Lipids and proteins can also be catabolized for energy.

• Lipid Catabolism: Fatty acids undergo beta-oxidation to form acetyl-CoA, entering the Krebs cycle.

• Protein Catabolism: Proteins are broken down into amino acids, which are deaminated and converted into intermediates of glycolysis or the Krebs cycle.

Biochemical Tests in Microbiology

Biochemical tests are used to identify bacteria based on their metabolic capabilities.

• Examples: Differentiation of Pseudomonas and Escherichia based on their ability to oxidize or ferment glucose.

Photosynthesis and Photophosphorylation

Photosynthesis converts light energy into chemical energy. Photophosphorylation is the process of ATP generation using light energy.

• Cyclic Photophosphorylation: Electrons return to the original chlorophyll; only ATP is produced.

• Noncyclic Photophosphorylation: Electrons are transferred to NADP+; both ATP and NADPH are produced.

• Importance: Provides energy and reducing power for carbon fixation.

Oxidative vs. Photophosphorylation

Feature	Oxidative Phosphorylation	Photophosphorylation
Energy Source	Chemical (NADH, FADH2)	Light
Electron Donor	Organic molecules	Water or other donors
Final Electron Acceptor	Oxygen (aerobic) or other (anaerobic)	NADP+ (noncyclic) or chlorophyll (cyclic)
Location	Cell membrane (prokaryotes), mitochondria (eukaryotes)	Thylakoid membrane (chloroplasts, cyanobacteria)

Energy Production in Cells

Cells obtain energy by oxidizing organic or inorganic molecules, transferring electrons to generate ATP.

• Sources: Glucose, sulfur, ammonia, hydrogen, etc.

• Mechanism: Electron transport chains and chemiosmosis.

Classification of Microorganisms by Nutritional Patterns

Microorganisms are classified based on their carbon and energy sources.

Group	Energy Source	Carbon Source	Example
Photoautotroph	Light	CO2	Cyanobacteria
Photoheterotroph	Light	Organic compounds	Rhodobacter
Chemoautotroph	Chemicals	CO2	Nitrosomonas
Chemoheterotroph	Chemicals	Organic compounds	Escherichia coli

Anabolism and Catabolism Interactions

Anabolic and catabolic pathways are interconnected. Amino acids for protein synthesis can be derived from intermediates of catabolic pathways.

• Amphibolic Pathways: Serve both anabolic and catabolic functions.

• Integration: Metabolic intermediates are shared between biosynthetic and degradative processes.

Peptidoglycan Synthesis

Peptidoglycan synthesis is an example of an anabolic pathway, integrating precursors from catabolic processes for cell wall construction in bacteria.

Summary Table: Key Metabolic Pathways

Pathway	Main Function	Key Products
Glycolysis	Glucose breakdown	ATP, NADH, pyruvate
Krebs Cycle	Oxidation of acetyl-CoA	CO2, NADH, FADH2, ATP
Electron Transport Chain	ATP synthesis via chemiosmosis	ATP, H2O
Fermentation	Regenerate NAD+	Lactic acid, ethanol, CO2
Pentose Phosphate	Biosynthesis precursors	NADPH, ribose-5-phosphate

Additional info: These notes are based on a set of study questions covering microbial metabolism, energy production, and related biochemical pathways. The content has been expanded to provide definitions, examples, and context suitable for exam preparation in a college-level microbiology course.