Microbial Metabolism

Overview of Metabolism

Metabolism encompasses all the controlled biochemical reactions occurring within a microorganism. These reactions are organized into pathways that either break down molecules to release energy (catabolism) or build complex molecules using energy (anabolism).

• Catabolism: The breakdown of nutrient molecules in a series of exergonic steps, releasing energy stored in ATP.

• Anabolism: The synthesis of macromolecules, requiring energy input from ATP (endergonic).

• Metabolic Pathways: Sequences of enzymatically catalyzed chemical reactions in a cell.

Basic Chemical Reactions in Metabolism

Oxidation and Reduction (Redox) Reactions

Redox reactions are fundamental to energy transfer in cells, involving the transfer of electrons between molecules.

• Oxidation: Loss of electrons from a molecule (electron donor).

• Reduction: Gain of electrons by a molecule (electron acceptor).

• Electron Carriers: Key molecules such as NAD+, NADP+, and FAD shuttle electrons in metabolic pathways.

ATP Production and Energy Storage

ATP (adenosine triphosphate) is the primary energy currency in cells, produced by phosphorylation processes.

• Phosphorylation: Addition of inorganic phosphate (PO43-) to a molecule.

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

• Oxidative phosphorylation: Phosphorylation of ADP using energy from electron transport (respiration).

• Photophosphorylation: Phosphorylation of ADP using light energy (photosynthesis).

The Roles of Enzymes in Metabolism

Enzyme Structure and Function

Enzymes are biological catalysts that speed up metabolic reactions by lowering activation energy.

• Apoenzyme: The protein portion of an enzyme.

• Cofactor: Nonprotein component (inorganic ions or organic coenzymes) required for enzyme activity.

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

• Ribozyme: RNA molecules with catalytic activity.

Enzyme Classification

Enzymes are classified based on their mode of action:

• Hydrolases: Catalyze hydrolysis reactions.

• Isomerases: Rearrange atoms within a molecule.

• Ligases/Polymerases: Join molecules together.

• Lyases: Split molecules without water.

• Oxidoreductases: Catalyze oxidation-reduction reactions.

• Transferases: Transfer functional groups between molecules.

Enzyme Activity and Regulation

• Activation Energy: The minimum energy required to initiate a chemical reaction. Enzymes lower this threshold.

• Active Site: The region on the enzyme where the substrate binds, exhibiting specificity (induced-fit model).

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

• Denaturation: Loss of enzyme structure and function due to extreme conditions.

• Inhibition:

  • Competitive Inhibitors: Bind to the active site, blocking substrate access.

  • Noncompetitive Inhibitors: Bind to allosteric sites, altering enzyme shape and function.

  • Allosteric Activation: Binding of a cofactor to an allosteric site activates the enzyme.

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

Carbohydrate Catabolism

Glycolysis

Glycolysis is the central pathway for glucose catabolism, occurring in the cytoplasm and yielding pyruvic acid, ATP, and NADH.

• Stages:

  • Energy-investment (steps 1-3)

  • Lysis (steps 4-5)

  • Energy-conserving (steps 6-10)

• Net Yield: 2 ATP (substrate-level phosphorylation), 2 NADH, 2 pyruvic acid per glucose.

Cellular Respiration

Cellular respiration is a three-stage process that completely oxidizes glucose to CO2 and H2O, generating ATP.

1. Synthesis of Acetyl-CoA: Pyruvic acid is decarboxylated and combined with coenzyme A, producing acetyl-CoA, CO2, and NADH.

2. Krebs Cycle (Citric Acid Cycle): Acetyl-CoA enters a cycle of eight enzymatic steps, producing ATP, NADH, and FADH2.

3. Electron Transport Chain (ETC): Electrons from NADH and FADH2 are transferred through membrane-bound carriers, generating a proton gradient for ATP synthesis.

Electron Transport Chain Carriers: Flavoproteins, ubiquinones, metal-containing proteins, cytochromes.

Final Electron Acceptors:

• Oxygen (aerobic respiration)

• Other inorganic molecules (anaerobic respiration)

Chemiosmosis

Chemiosmosis is the process by which ATP is synthesized as protons flow down their electrochemical gradient through ATP synthase.

• Proton Motive Force: The potential energy stored in the proton gradient across a membrane.

• ATP Yield: Up to 34 ATP per glucose via ETC; total theoretical yield from aerobic respiration is 38 ATP per glucose.

Pentose Phosphate Pathway

An alternative to glycolysis, this pathway generates precursor metabolites for biosynthesis and yields NADPH and ribose-5-phosphate, but less ATP.

• Functions: Synthesis of nucleotides, amino acids, and photosynthetic glucose.

Fermentation

Fermentation is the partial oxidation of sugar without an external electron acceptor, regenerating NAD+ for glycolysis.

• Common End Products: Lactic acid, ethanol, and other organic acids or alcohols.

• Importance: Used in food industry (e.g., yogurt, bread, alcohol) and for microbial identification.

Other Catabolic Pathways

Lipid Catabolism

Lipids are hydrolyzed by lipases into glycerol and fatty acids. Glycerol enters glycolysis, while fatty acids undergo beta-oxidation to form acetyl-CoA.

• Beta-oxidation: Sequential removal of two-carbon units from fatty acids, generating acetyl-CoA, NADH, and FADH2.

Protein Catabolism

Proteins are degraded by proteases into amino acids, which are deaminated and converted into intermediates for the Krebs cycle or used in anabolism.

• Deamination: Removal of the amino group from amino acids.

Photosynthesis

Light-Dependent and Light-Independent Reactions

Photosynthesis captures light energy to synthesize carbohydrates from CO2 and H2O. It occurs in two stages:

• Light-dependent reactions: Use chlorophylls in photosystems to convert light energy into chemical energy (ATP, NADPH).

• Light-independent reactions (Calvin-Benson cycle): Use ATP and NADPH to fix CO2 into glucose.

Photosystems: Networks of pigment molecules embedded in thylakoid membranes.

Other Anabolic Pathways

Amphibolic Reactions

Amphibolic reactions can function in both catabolic and anabolic directions, depending on cellular needs.

• Examples: Biosynthesis and breakdown of carbohydrates, lipids, amino acids, and nucleotides.

Carbohydrate Biosynthesis (Gluconeogenesis)

Gluconeogenesis is the synthesis of sugars from noncarbohydrate precursors, involving highly endergonic, amphibolic reactions.

• Precursors: Amino acids, glycerol, fatty acids.

Lipid Biosynthesis

Lipids are synthesized from glycerol and fatty acids (reverse of catabolism), and more complex lipids (e.g., steroids, waxes) are formed via specialized pathways.

Amino Acid Biosynthesis

Amino acids are synthesized by:

• Amination: Addition of an amine group from ammonia to a precursor metabolite.

• Transamination: Transfer of an amine group from one amino acid to another, catalyzed by enzymes with pyridoxal phosphate.

• Essential Amino Acids: Must be obtained from the environment if the organism cannot synthesize them.

Nucleotide Biosynthesis

Nucleotides are assembled from a 5-carbon sugar, phosphate group, and nitrogenous base, using precursors from glycolysis and the Krebs cycle.

Integration and Regulation of Metabolic Functions

Interaction of Catabolic and Anabolic Pathways

Catabolic and anabolic pathways are interconnected, with energy and precursor metabolites from catabolism fueling anabolism.

• Regulatory Mechanisms:

  • Synthesis/degradation of transport proteins

  • Enzyme production in response to substrate availability

  • Catabolism of metabolites for energy

  • Synthesis of metabolites as needed

  • Compartmentalization in eukaryotes

  • Allosteric regulation and feedback inhibition

  • Regulation of cofactor availability

• Levels of Regulation:

  • Control of gene expression (enzyme production)

  • Control of metabolic expression (activity of existing enzymes)

Summary Table: Major Metabolic Pathways

Key Equations

• ATP Hydrolysis:

• Glycolysis (overall):

• Aerobic Respiration (overall):

Example: Lactobacillus species ferment glucose to lactic acid, which is used in yogurt production.