Microbial Metabolism: Key Concepts and Pathways

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Microbial Metabolism

Overview of Metabolism

Metabolism encompasses all controlled biochemical reactions occurring within a microbe, ultimately enabling reproduction and growth. It is guided by a series of elementary statements that describe nutrient acquisition, energy utilization, and macromolecule synthesis.

Metabolism: The sum of all chemical reactions in a cell.
Cells acquire nutrients and extract energy from light or catabolism.
Energy is stored in ATP (adenosine triphosphate).
Catabolism produces precursor metabolites, which are used in anabolic reactions.
Cells grow and reproduce by assembling macromolecules.

Diagram of metabolism showing the relationship between catabolism and anabolism Cellular diagram showing catabolism and anabolism

Catabolism and Anabolism

Metabolic reactions are divided into two major classes: catabolic and anabolic pathways. These processes are interconnected and essential for cellular function.

Catabolic pathways: Break down larger molecules into smaller products; exergonic (release energy).
Anabolic pathways: Synthesize large molecules from smaller products; endergonic (require energy).
ATP is central to energy transfer between these pathways.

Oxidation and Reduction Reactions

Redox reactions involve the transfer of electrons between molecules, playing a critical role in energy production and metabolic pathways.

Oxidation: Loss of electrons by a molecule.
Reduction: Gain of electrons by a molecule.
Electron carriers (NAD+, NADP+, FAD) shuttle electrons during metabolism.
Redox reactions always occur simultaneously.

Diagram of oxidation and reduction reactions Alternative diagram of oxidation and reduction reactions

ATP Production and Energy Storage

Cells release energy from nutrients and store it in ATP, which is generated through phosphorylation. There are three main mechanisms for ATP synthesis:

Substrate-level phosphorylation: Direct transfer of phosphate between substrates.
Oxidative phosphorylation: Uses energy from electron transport chain.
Photophosphorylation: Utilizes light energy (in photosynthetic organisms).

The Roles of Enzymes in Metabolism

Enzyme Structure and Function

Enzymes are organic catalysts that increase the likelihood of chemical reactions. They are classified based on their mode of action and can be composed of protein or RNA (ribozymes).

Six categories: Hydrolases, Isomerases, Ligases/polymerases, Lyases, Oxidoreductases, Transferases.
Apoenzyme: Inactive protein portion.
Cofactor: Non-protein component (inorganic or organic).
Holoenzyme: Active enzyme formed by apoenzyme and cofactor.

Structure of a holoenzyme

Enzyme-Substrate Interaction

Enzymes bind substrates at their active sites, forming enzyme-substrate complexes and facilitating reactions by lowering activation energy.

Enzymes are highly specific for their substrates.
Activation energy is reduced in the presence of enzymes.

Effect of enzymes on activation energy Enzyme fitted to substrate Process of enzymatic activity

Factors Affecting Enzyme Activity

Several factors influence the rate of enzymatic reactions, including temperature, pH, substrate concentration, and the presence of inhibitors.

Optimal conditions are required for maximum enzyme activity.
Inhibitors block enzyme activity without denaturing the enzyme.
Types of inhibition: Competitive, noncompetitive, and allosteric.

Graphs showing effects of temperature, pH, and substrate concentration on enzyme activity Functional and denatured protein structures Competitive inhibition of enzyme activity Allosteric control of enzyme activity

Carbohydrate Catabolism

Glucose Catabolism

Glucose is the primary energy source for many organisms and is catabolized through cellular respiration and fermentation.

Glycolysis: Splits glucose into two pyruvic acid molecules; net gain of 2 ATP and 2 NADH.
Three stages: Energy-investment, lysis, energy-conserving.

Cellular Respiration

Cellular respiration completely oxidizes pyruvic acid to produce ATP via three stages: synthesis of acetyl-CoA, Krebs cycle, and electron transport chain.

Synthesis of acetyl-CoA: Produces acetyl-CoA, CO2, and NADH.
Krebs cycle: Transfers energy to NAD+ and FAD; produces ATP, FADH2, NADH, and CO2.
Electron transport chain (ETC): Series of carrier molecules; establishes proton gradient for ATP synthesis.
Chemiosmosis: ATP generated as protons flow through ATP synthase.

Formation of acetyl-CoA Krebs cycle diagram

Fermentation

Fermentation provides an alternative pathway for NAD+ regeneration when cellular respiration is not possible, resulting in partial oxidation of sugars and production of organic end products.

Fermentation products include lactic acid, ethanol, and other organic acids.
Different organisms produce distinct fermentation products.

Fermentation pathway diagram Fermentation products and organisms

Other Catabolic Pathways

Lipids and proteins can also be catabolized to provide energy and precursor metabolites for cellular processes.

Lipid catabolism: Hydrolysis and beta-oxidation yield glycerol and fatty acids, which enter glycolysis and Krebs cycle.
Protein catabolism: Proteases break down polypeptides; deamination allows entry into Krebs cycle.

Catabolism of a fat molecule Protein catabolism diagram

Photosynthesis

Overview and Structures

Photosynthesis enables organisms to synthesize organic molecules from inorganic carbon dioxide using light energy. Chlorophylls and photosystems are key components.

Chlorophylls: Pigments with a hydrocarbon tail and magnesium-centered active site.
Photosystems: Light-harvesting matrices embedded in thylakoid membranes.
Thylakoids are arranged in stacks called grana; stroma is the surrounding space.

Photosynthetic structures in a prokaryote Reaction center of photosystem

Light-Dependent and Light-Independent Reactions

Photosynthesis consists of light-dependent reactions (requiring light) and light-independent reactions (Calvin-Benson cycle).

Light-dependent reactions: Use light energy to generate ATP and NADPH via photophosphorylation.
Light-independent reactions: Use ATP and NADPH to fix carbon dioxide into glucose.
Calvin-Benson cycle is the key pathway for carbon fixation.

Calvin-Benson cycle diagram

Other Anabolic Pathways

Amphibolic Pathways and Biosynthesis

Anabolic reactions synthesize macromolecules using energy and metabolites derived from catabolic pathways. Many pathways are amphibolic, proceeding in both directions.

Gluconeogenesis: Synthesis of glucose from non-carbohydrate precursors.
Biosynthesis of fats: Reverse of beta-oxidation.
Synthesis of amino acids: Amination and transamination reactions.
Biosynthesis of nucleotides: Involves pentose phosphate pathway and other intermediates.

Gluconeogenesis diagram Biosynthesis of fat Synthesis of amino acids by amination and transamination Biosynthesis of nucleotides

Integration and Regulation of Metabolic Function

Regulation Mechanisms

Cells regulate metabolism by controlling enzyme synthesis and activity, isolating pathways within organelles, and using feedback inhibition.

Enzyme synthesis is regulated by gene expression.
Metabolic activity is regulated by allosteric sites and feedback inhibition.
Cells prioritize energy-efficient substrates and cease synthesis of metabolites when available.
Amphibolic pathways are regulated by requiring different coenzymes for each direction.

Pathway	Key Features
Catabolism	Breakdown, energy release, precursor metabolites
Anabolism	Synthesis, energy consumption, macromolecule formation
Amphibolic	Bidirectional, shared intermediates

Example: Feedback inhibition in amino acid biosynthesis slows production when the end product accumulates.