Microbial Metabolism: Anabolism, Catabolism, and Enzyme Function

Metabolism: An Overview

Definition and Importance

Metabolism encompasses all the chemical reactions occurring within a living cell, enabling the cell to extract energy from nutrients and synthesize the molecules necessary for life. These reactions are organized into metabolic pathways, which are determined by enzymes encoded by genes.

• Catabolism: The breakdown of complex molecules into simpler ones, releasing energy.

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

• Metabolic Pathways: Sequences of enzymatically catalyzed reactions, often interconnected.

Example: Catabolic pathways break down glucose to release energy, while anabolic pathways use that energy to build proteins from amino acids.

Catabolism vs. Anabolism

Key Differences

• Catabolism:

  • Hydrolytic reactions (breaking bonds with water)

  • Exergonic (energy-releasing)

  • Example: Glucose breakdown to CO2 and H2O

• Anabolism:

  • Dehydration synthesis (forming bonds by removing water)

  • Endergonic (energy-consuming)

  • Example: Protein synthesis from amino acids

Energy Transfer: Energy released from catabolic reactions is stored in ATP and used to drive anabolic reactions.

ATP: The Energy Currency

Role in Metabolism

ATP (adenosine triphosphate) acts as an intermediate, capturing energy from catabolic reactions and providing it for anabolic processes.

• ATP Structure: Adenosine molecule with three phosphate groups

• Energy Release: Hydrolysis of ATP to ADP + Pi releases energy

Equation:

$\text{ATP} \rightarrow \text{ADP} + \text{P}_i + \text{Energy}$

Enzymes: Biological Catalysts

Mechanism and Components

Enzymes are proteins that accelerate chemical reactions by lowering the activation energy required. They are highly specific for their substrates.

• Active Site: Region where substrate binds

• Enzyme-Substrate Complex: Temporary association during reaction

• Turnover Number: Number of substrate molecules converted per second

Example: The enzyme sucrase catalyzes the hydrolysis of sucrose into glucose and fructose.

Enzyme Components

• Apoenzyme: Protein portion (inactive alone)

• Cofactor: Non-protein component (can be inorganic or organic)

• Coenzyme: Organic cofactor (often derived from vitamins)

• Holoenzyme: Apoenzyme plus cofactor (active form)

Example: NAD+, NADP+, FAD, and Coenzyme A are important coenzymes in metabolism.

Factors Influencing Enzyme Activity

Environmental and Chemical Factors

• Temperature: Enzyme activity increases with temperature up to an optimum, then decreases due to denaturation.

• pH: Each enzyme has an optimal pH; extreme pH can denature enzymes.

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

• Inhibitors: Chemicals that decrease enzyme activity (competitive or noncompetitive).

Enzyme Inhibition

Types of Inhibition

• Competitive Inhibition: Inhibitor competes with substrate for active site; can be overcome by increasing substrate concentration.

• Noncompetitive Inhibition: Inhibitor binds to allosteric site, changing enzyme shape and function; cannot be overcome by substrate increase.

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

Example: Feedback inhibition is common in amino acid biosynthesis pathways.

Oxidation-Reduction (Redox) Reactions

Definitions and Biological Importance

Redox reactions involve the transfer of electrons between molecules, fundamental to energy production in cells.

• Oxidation: Loss of electrons

• Reduction: Gain of electrons

• Redox Pair: One molecule is oxidized, another is reduced

Example: NAD+ is reduced to NADH during glycolysis and the Krebs cycle.

ATP Generation: Phosphorylation Mechanisms

Three Types of Phosphorylation

• Substrate-Level Phosphorylation: Direct transfer of phosphate group to ADP from a substrate.

• Oxidative Phosphorylation: ATP generated via electron transport chain and chemiosmosis.

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

Equation:

$\text{ADP} + \text{P}_i + \text{Energy} \rightarrow \text{ATP}$

Metabolic Pathways of Energy Production

Carbohydrate Catabolism

Microorganisms primarily use carbohydrates, especially glucose, as energy sources. The two main processes are cellular respiration and fermentation.

• Cellular Respiration: Includes glycolysis, Krebs cycle, and electron transport chain.

• Fermentation: Partial breakdown of glucose without oxygen.

Glycolysis

Glycolysis is the oxidation of glucose to pyruvic acid, producing ATP and NADH. It consists of a preparatory stage (ATP investment) and an energy-conserving stage (ATP and NADH production).

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

• Energy-Conserving Stage: Glyceraldehyde 3-phosphate is oxidized to pyruvic acid, generating ATP and NADH.

Equation:

$\text{Glucose} + 2\text{ATP} + 2\text{ADP} + 2\text{P}_i + 2\text{NAD}^+ \rightarrow 2\text{Pyruvic acid} + 4\text{ATP} + 2\text{NADH} + 2\text{H}^+$

Alternative Pathways

• Pentose Phosphate Pathway: Breaks down pentose sugars, produces NADPH, and provides intermediates for biosynthesis.

• Entner-Doudoroff Pathway: Produces NADPH, NADH, and ATP; occurs in some bacteria.

Cellular Respiration

Aerobic Respiration

Aerobic respiration uses oxygen as the final electron acceptor and includes glycolysis, the Krebs cycle, and the electron transport chain.

• Transition Step: Pyruvic acid is converted to acetyl-CoA.

• Krebs Cycle: Acetyl-CoA is oxidized, producing NADH, FADH2, ATP, and CO2.

• Electron Transport Chain: Electrons are transferred through carrier molecules, generating ATP via chemiosmosis.

Overall Reaction:

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

Anaerobic Respiration

Anaerobic respiration uses a molecule other than oxygen as the final electron acceptor, yielding less ATP.

Electron Acceptor	Products
NO3-	NO2-, N2, H2O
SO42-	H2S, H2O
CO32-	CH4, H2O

Fermentation

Process and Products

Fermentation is the partial oxidation of organic molecules without oxygen, producing small amounts of ATP and various end-products.

• Lactic Acid Fermentation: Produces lactic acid (homolactic or heterolactic).

• Alcohol Fermentation: Produces ethanol and CO2.

Example: Yeast (Saccharomyces cerevisiae) ferments sugars to produce ethanol in brewing.

Lipid and Protein Catabolism

Pathways and End-Products

Lipids and proteins are catabolized by microorganisms to extract energy and generate intermediates for the Krebs cycle.

• Lipid Catabolism: Lipids are broken down into glycerol and fatty acids; glycerol enters glycolysis, fatty acids undergo beta-oxidation to form acetyl-CoA.

• Protein Catabolism: Proteins are degraded to amino acids, which are deaminated and converted to intermediates that enter the Krebs cycle.

Example: Bacteria can use proteins and lipids as energy sources when carbohydrates are unavailable.

Summary Table: ATP Yield During Prokaryotic Aerobic Respiration

Stage	Electron Carrier	ATP Yield (Method)
Glycolysis	2 NADH	2 ATP (substrate-level)
Transition Step	2 NADH	-
Krebs Cycle	6 NADH, 2 FADH2	2 ATP (substrate-level)
Electron Transport Chain	All NADH, FADH2	34 ATP (oxidative phosphorylation)
Total	-	38 ATP

Additional info: These notes cover the essential concepts of microbial metabolism, including the roles of catabolism, anabolism, enzymes, ATP, and the major metabolic pathways. The included images directly reinforce the explanations and are selected for their clear relevance to the adjacent content.