Microbial Metabolism

Overview of Metabolism

Metabolism encompasses all controlled biochemical reactions within a microbe, serving the ultimate function of reproducing the organism. It is divided into two major classes: catabolism and anabolism.

• Catabolism: Breakdown of larger molecules into smaller products; releases energy (exergonic).

• Anabolism: Synthesis of larger molecules from smaller products; requires energy (endergonic).

Eight Elementary Statements Guiding Metabolic Processes

• Every cell acquires nutrients.

• Metabolism requires energy from light or catabolism of nutrients.

• Energy is stored in adenosine triphosphate (ATP).

• Cells catabolize nutrients to form precursor metabolites.

• Precursor metabolites, energy from ATP, and enzymes are used in anabolic reactions.

• Enzymes plus ATP form macromolecules.

• Cells grow by assembling macromolecules.

• Cells reproduce once they have doubled in size.

Catabolism vs. Anabolism

• Catabolic pathways: Break larger molecules into smaller products; use NAD+ as electron collector for ATP production.

• Anabolic pathways: Synthesize large molecules; use NADP+ as electron donor for biosynthesis.

Oxidation and Reduction Reactions

Oxidation-reduction (redox) reactions involve the transfer of electrons from an electron donor to an electron acceptor. These reactions always occur simultaneously and are essential for energy transfer in cells.

• Cells use electron carriers: NAD+, NADP+, FAD.

ATP Production and Energy Storage

Organisms release energy from nutrients and store it in high-energy phosphate bonds of ATP. ATP is produced by phosphorylation of ADP in three ways:

• Substrate-level phosphorylation

• Oxidative phosphorylation

• Photophosphorylation

The Roles of Enzymes in Metabolism

Enzyme Structure and Function

Enzymes are organic catalysts that increase the likelihood of a reaction. They are classified based on their mode of action:

• Hydrolases

• Isomerases

• Ligases or polymerases

• Lyases

• Oxidoreductases

• Transferases

Many protein enzymes are complete in themselves, but some require nonprotein cofactors (inorganic ions or coenzymes) to become active. The combination of an apoenzyme and its cofactor forms a holoenzyme. Some enzymes are RNA molecules called ribozymes.

Enzyme Activity and Reaction Rates

Enzymes lower the activation energy required for chemical reactions, thus speeding up metabolic processes.

Enzyme-Substrate Interaction

Enzymes bind substrates at their active sites, often inducing a fit that enhances catalytic activity.

Factors Influencing Enzyme Activity

Several factors affect the rate of enzymatic reactions:

• Temperature

• pH

• Enzyme and substrate concentrations

• Presence of inhibitors

Control of Enzymatic Activity

• Activators: Some enzymes are activated when a cofactor binds to a site other than the active site (allosteric activation).

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• Inhibitors: Substances that block enzyme activity, including competitive and noncompetitive inhibitors. Feedback inhibition controls the action of some enzymes.

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Carbohydrate Catabolism

Glucose Catabolism: Cellular Respiration and Fermentation

Many organisms oxidize carbohydrates, primarily glucose, as their main energy source. Glucose is catabolized by two processes:

• Cellular respiration: Complete oxidation of pyruvic acid to produce ATP via redox reactions.

• Fermentation: Partial oxidation of sugar to release energy using an organic molecule as the final electron acceptor.

Glycolysis

Glycolysis occurs in the cytoplasm and involves splitting a six-carbon glucose into two three-carbon pyruvic acid molecules. It consists of three stages:

• Energy-investment stage

• Lysis stage

• Energy-conserving stage

Net gain: 2 ATP, 2 NADH, and 2 pyruvic acid.

Cellular Respiration

Cellular respiration consists of three stages:

1. Synthesis of acetyl-CoA

2. Krebs cycle

3. Electron transport chain (ETC)

Electron Transport Chain and Chemiosmosis

The ETC is a series of carrier molecules that pass electrons to a final electron acceptor, using the energy to pump protons and establish a gradient. Chemiosmosis uses this gradient to generate ATP via ATP synthase.

Metabolic Diversity: Alternative Pathways

• Entner-Doudoroff pathway: Found only in prokaryotes; produces 1 ATP, 1 NADH, and 1 NADPH.

• Pentose phosphate pathway: Alternative to glycolysis; produces precursor metabolites and NADPH for biosynthesis.

Fermentation

Fermentation regenerates NAD+ by using an organic molecule as the final electron acceptor. End products vary by organism (e.g., lactic acid, ethanol).

Other Catabolic Pathways

Lipid Catabolism

Lipids are broken down by lipases. Glycerol enters glycolysis, while fatty acids undergo beta-oxidation to form acetyl-CoA, which enters the citric acid cycle.

Protein Catabolism

Proteins are broken down into amino acids, which are deaminated and converted into metabolites that enter the citric acid cycle. Waste products include ammonia and urea.

Other Anabolic Pathways

Gluconeogenesis and Biosynthesis

Anabolic reactions synthesize complex molecules using energy from ATP and precursor metabolites. Many anabolic pathways are the reverse of catabolic pathways and are termed amphibolic.

Integration and Regulation of Metabolic Function

Regulation of Metabolism

Cells regulate metabolism by controlling gene expression and metabolic expression. Feedback inhibition, allosteric regulation, and compartmentalization in eukaryotes are key mechanisms.

Key Equations and Concepts

• ATP Formation:

• Glycolysis Net Reaction:

• Citric Acid Cycle Net Reaction:

• Electron Transport Chain:

Summary Table: Catabolic Pathways

Summary Table: Enzyme Regulation

Conclusion

Microbial metabolism is a complex network of catabolic and anabolic reactions, regulated by enzymes and cellular mechanisms. Understanding these pathways is essential for comprehending microbial growth, energy production, biosynthesis, and adaptation.