Microbial Metabolism: Enzymes, Pathways, and Energy Production

Microbial Metabolism

Overview of Metabolism

Metabolism encompasses all the chemical reactions that occur within a cell, divided into two main categories: catabolism and anabolism. Catabolism involves the breakdown of nutrients to release energy, while anabolism uses energy to build macromolecules necessary for cellular structure and function.

• Catabolism: Degradative, energy-yielding reactions (e.g., breakdown of glucose).

• Anabolism: Biosynthetic, energy-consuming reactions (e.g., synthesis of proteins, nucleic acids).

• ATP: The main energy currency, linking catabolic and anabolic processes.

Enzymes and Their Function

Enzyme Structure and Mechanism

Enzymes are biological catalysts that accelerate chemical reactions by lowering the activation energy required. They are typically proteins, sometimes requiring non-protein cofactors for activity.

• Apoenzyme: The protein portion of an enzyme.

• Cofactor: Non-protein component required for enzyme activity (can be inorganic or organic).

• Holoenzyme: The complete, active enzyme with its cofactor.

Types of Cofactors

Cofactors are essential for the catalytic activity of many enzymes. They can be inorganic ions or organic molecules (coenzymes).

Cofactor	Example of Use	Substance Transferred	Vitamin Source
Magnesium (inorganic)	Forms bond with ADP during phosphorylation	Phosphate	None
Nicotinamide adenine dinucleotide (NAD+)	Carrier of reducing power	Two electrons and one hydrogen ion	Niacin (B3)
Flavin adenine dinucleotide (FAD)	Carrier of reducing power	Two electrons and two hydrogen ions	Riboflavin (B2)
Coenzyme A	Transfers acyl groups	Two-carbon molecule	Pantothenic acid (B5)
Pyridoxal phosphate	Transfers amino groups	Amino group	Pyridoxine (B6)
Thiamine pyrophosphate	Decarboxylation and aldehyde transfer	Aldehyde group (CHO)	Thiamine (B1)

Enzyme-Substrate Interaction

The lock and key hypothesis describes how the substrate fits precisely into the enzyme's active site, forming an enzyme-substrate complex that facilitates the reaction.

• Active Site: Region on the enzyme where the substrate binds.

• Enzyme-Substrate Complex: Temporary association during the reaction.

Enzyme Kinetics and Regulation

Enzyme activity is influenced by several factors, including temperature, pH, and substrate concentration. Extreme conditions can lead to denaturation, rendering the enzyme inactive.

• Temperature: Each enzyme has an optimal temperature for activity.

• pH: Each enzyme has an optimal pH range.

• Substrate Concentration: Activity increases with substrate concentration until saturation is reached.

Enzyme Inhibition

Enzyme activity can be regulated by inhibitors, which are molecules that decrease or halt enzyme function. There are two main types:

• Competitive Inhibitors: Compete with the substrate for the active site; can be overcome by increasing substrate concentration.

• Noncompetitive Inhibitors: Bind to an allosteric site, changing the enzyme's shape and function; cannot be overcome by substrate increase.

• Feedback Inhibition: End-product of a pathway inhibits an earlier step, regulating pathway activity.

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Example: Sulfanilamide as a Competitive Inhibitor

Sulfanilamide, a sulfa drug, acts as a competitive inhibitor by mimicking para-aminobenzoic acid (PABA), a substrate in bacterial folic acid synthesis.

Redox Reactions and Electron Carriers

Redox Reactions

Redox (reduction-oxidation) reactions are essential for energy transfer in cells. One molecule loses electrons (oxidation), while another gains them (reduction).

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

Electron Carriers

Electron carriers such as NAD+ and FAD collect electrons during catabolic reactions and deliver them to the electron transport chain (ETC).

• NAD+ (Nicotinamide Adenine Dinucleotide): Accepts two electrons and one proton to become NADH.

• FAD (Flavin Adenine Dinucleotide): Accepts two electrons and two protons to become FADH2.

ATP Synthesis and Phosphorylation

ATP Production

The energy released from catabolic reactions is stored in ATP through phosphorylation. There are three main types of phosphorylation:

Type	Source of Phosphate	Source of Energy	Location in Eukaryotes	Location in Prokaryotes
Substrate-level	Organic molecule	High-energy phosphate bond	Cytosol and mitochondrial matrix	Cytoplasm
Oxidative	Inorganic phosphate	Proton motive force	Inner mitochondrial membrane	Cytoplasmic membrane
Photophosphorylation	Inorganic phosphate	Proton motive force (light-driven)	Thylakoid of chloroplast	Thylakoid of cyanobacteria

Catabolic Pathways

Glycolysis (Embden-Meyerhof Pathway)

Glycolysis is the central pathway for glucose catabolism, occurring in the cytoplasm of both prokaryotes and eukaryotes. It converts one glucose molecule into two pyruvate molecules, generating ATP and NADH.

• Energy Investment: 2 ATP used.

• Lysis: 6-carbon glucose split into two 3-carbon molecules.

• Energy Payoff: 4 ATP produced (net gain: 2 ATP), 2 NADH, 2 pyruvate.

Alternative Pathways

• Entner-Doudoroff Pathway: Found in some prokaryotes; yields 1 ATP and 1 NADPH per glucose.

• Pentose Phosphate Pathway: Fo

• und in both prokaryotes and eukaryotes; yields 1 ATP and 1 NADPH, important for biosynthesis.

Krebs Cycle (Citric Acid Cycle)

Before entering the Krebs cycle, pyruvate is converted to acetyl-CoA. The cycle completes the oxidation of glucose derivatives, producing NADH, FADH2, ATP, and CO2.

• Location: Mitochondrial matrix (eukaryotes), cytoplasm (prokaryotes).

• Products per glucose: 6 NADH, 2 FADH2, 2 ATP, 4 CO2.

Electron Transport Chain (ETC) and Oxidative Phosphorylation

The ETC is a series of membrane-bound proteins and coenzymes that transfer electrons from NADH and FADH2 to a final electron acceptor, generating a proton gradient used to synthesize ATP.

• Location: Inner mitochondrial membrane (eukaryotes), plasma membrane (prokaryotes).

• Proton Motive Force: H+ ions are pumped across the membrane, creating an electrochemical gradient.

• ATP Synthase: Enzyme that uses the proton gradient to produce ATP.

• Final Electron Acceptor: Oxygen in aerobic respiration (forms water).

Summary of ATP Yield in Aerobic Respiration

Pathway	ATP Produced	ATP Used	NADH Produced	FADH2 Produced
Glycolysis	4	2	2	0
Synthesis of acetyl-CoA and Krebs cycle	2	0	8	2
Electron transport chain	34	0	0	0
Net total	38

Anaerobic Respiration

In anaerobic respiration, the final electron acceptor is an inorganic molecule other than oxygen (e.g., nitrate, sulfate, carbonate). The amount of ATP generated is less than in aerobic respiration.

• ATP Yield: Varies from 2 to 36 ATP per glucose.

• Example: Escherichia coli can use nitrate as a terminal electron acceptor.

Fermentation

Fermentation is an anaerobic process that allows cells to regenerate NAD+ from NADH, enabling glycolysis to continue. The final electron acceptor is an organic molecule, and the process yields various end-products.

• Lactic Acid Fermentation: Pyruvate is reduced to lactic acid (e.g., in bacteria, muscle cells).

• Ethanol Fermentation: Pyruvate is converted to ethanol and CO2 (e.g., in yeast).

• ATP Yield: Only 2 ATP per glucose.

Comparison of Catabolic Pathways

	Aerobic Respiration	Anaerobic Respiration	Fermentation
Oxygen required?	Yes	No	No
Type of phosphorylation	Substrate-level and oxidative	Substrate-level and oxidative	Substrate-level
Final electron acceptor	Oxygen	NO3-, SO4^2-, CO3^2-, or externally acquired organic molecules	Cellular organic molecules
ATP yield per glucose	38 (prokaryotes), 36 (eukaryotes)	2–36	2

Anabolism and Integrated Metabolic Pathways

Anabolism

Anabolism refers to the synthesis of macromolecules from smaller precursors, requiring energy input (usually from ATP or NADPH). Many anabolic pathways are the reverse of catabolic pathways (e.g., gluconeogenesis is the reverse of glycolysis).

• Gluconeogenesis: Synthesis of glucose from non-carbohydrate precursors.

• Macromolecule Synthesis: Formation of proteins, nucleic acids, lipids, and polysaccharides.

Integrated Metabolic Pathways

Metabolic pathways are interconnected, allowing cells to adapt to different nutrient sources and energy demands. Intermediates from catabolic pathways can be diverted into anabolic pathways for biosynthesis.

Classification of Microorganisms by Metabolism

Energy and Carbon Sources

Microorganisms are classified based on their energy and carbon sources:

• Phototrophs: Use light as an energy source.

• Chemotrophs: Use chemicals as an energy source.

• Autotrophs: Use CO2 as a carbon source.

• Heterotrophs: Use organic compounds as a carbon source.

Examples include photoautotrophs (plants, algae), chemoheterotrophs (animals, most bacteria), and others.