Microbial Metabolism

Introduction to Metabolism

Microbial metabolism encompasses all controlled biochemical reactions that occur within cells, enabling them to grow, reproduce, and respond to their environment. These reactions are broadly classified into catabolic (energy-releasing) and anabolic (energy-consuming) processes.

• Metabolism: The sum of all chemical reactions in a cell.

• Catabolism: Breakdown of molecules to release energy (exergonic).

• Anabolism: Synthesis of complex molecules from simpler ones (endergonic).

• Energy Storage: Energy is primarily stored in the bonds of adenosine triphosphate (ATP).

Example: The breakdown of glucose during cellular respiration is a catabolic process, while the synthesis of proteins from amino acids is anabolic.

Oxidation-Reduction (Redox) Reactions

Redox Reactions in Metabolism

Redox reactions involve the transfer of electrons from one molecule (the donor) to another (the acceptor). These reactions are central to energy production in cells.

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• Electron Carriers: Molecules that transport electrons during metabolic reactions.

• Major electron carriers:

  • NAD+ / NADH (Nicotinamide adenine dinucleotide)

  • NADP+ / NADPH (Nicotinamide adenine dinucleotide phosphate)

  • FAD / FADH2 (Flavine adenine dinucleotide)

Example: In glycolysis, NAD+ is reduced to NADH as glucose is oxidized.

ATP Production and Phosphorylation

Mechanisms of ATP Generation

ATP is generated by adding a phosphate group to ADP, a process called phosphorylation. There are three main types of phosphorylation in metabolism:

• Substrate-level phosphorylation: Direct transfer of phosphate from a substrate to ADP (occurs in glycolysis and Krebs cycle).

• Oxidative phosphorylation: Uses energy from electron transport chain and chemiosmosis to add phosphate to ADP.

• Photophosphorylation: Uses light energy (in photosynthetic organisms).

Equation:

Example: Substrate-level phosphorylation produces ATP during glycolysis.

Enzymes and Their Role in Metabolism

Structure and Function of Enzymes

Enzymes are biological catalysts, mostly proteins, that speed up chemical reactions by lowering activation energy. They are essential for metabolic processes.

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

• Induced-fit model: Enzyme changes shape slightly to fit the substrate.

• Cofactors: Non-protein components (e.g., metal ions, vitamins) required by some enzymes.

• Ribozyme: RNA molecules with catalytic activity.

Factors Affecting Enzyme Activity:

• Temperature

• pH

• Ionic concentration

• Enzyme and substrate concentrations

• Presence of inhibitors

Denaturation: Extreme conditions can permanently inactivate enzymes by altering their structure.

Enzyme Inhibition

• Competitive Inhibitor: Resembles the substrate and binds to the active site, blocking substrate access.

• Noncompetitive Inhibitor: Binds to an allosteric site, changing the enzyme's shape and reducing activity.

Example: Sulfanilamide is a competitive inhibitor of an enzyme in folic acid synthesis.

Carbohydrate Catabolism

Overview of Glucose Catabolism

Most organisms use carbohydrates, especially glucose, as their primary energy source. Glucose catabolism occurs via two main pathways: cellular respiration and fermentation.

• Cellular Respiration: Complete oxidation of glucose to CO2 and H2O (includes glycolysis, Krebs cycle, and electron transport chain).

• Fermentation: Partial oxidation of glucose; organic molecules serve as electron acceptors.

Glycolysis

• Occurs in the cytoplasm of all cells.

• Glucose (6C) is split into two pyruvate (3C each).

• Net gain: 2 ATP (substrate-level phosphorylation), 2 NADH, 2 pyruvate.

Equation:

Cellular Respiration

• Three stages: Conversion of pyruvate to acetyl-CoA, Krebs cycle, Electron Transport Chain (ETC).

• Occurs in cytosol (prokaryotes) or mitochondria (eukaryotes).

Conversion of Pyruvate to Acetyl-CoA

• Pyruvate is converted to acetyl-CoA by pyruvate dehydrogenase.

• Produces acetyl-CoA, CO2, and NADH.

Krebs Cycle (Citric Acid Cycle)

• Acetyl-CoA enters the cycle; energy is transferred to NAD+ and FAD.

• Produces ATP, NADH, FADH2, and CO2.

Electron Transport Chain (ETC)

• NADH and FADH2 donate electrons to the ETC.

• Electrons are passed through a series of carriers, releasing energy used to pump H+ ions across the membrane.

• ATP synthase uses the proton gradient to generate ATP (oxidative phosphorylation).

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

Electron Acceptors

• Aerobic Respiration: O2 is the final electron acceptor.

• Anaerobic Respiration: Other molecules (e.g., SO42-, NO3-, CO32-) serve as final electron acceptors.

ATP Yield from Aerobic Respiration

Additional info: In eukaryotes, the total ATP yield is slightly lower (~36) due to the cost of transporting NADH into mitochondria.

Fermentation

• Used by cells lacking an ETC or when O2 is absent.

• Regenerates NAD+ from NADH, allowing glycolysis to continue.

• Yields less ATP than respiration.

• Common products: lactic acid, ethanol, acetate.

Example: Lactobacillus species ferment glucose to lactic acid.

Comparison of Metabolic Pathways

Regulation and Integration of Metabolism

Metabolic Regulation

• Cells regulate gene expression for metabolic enzymes based on substrate availability.

• Anabolic pathways are suppressed if the end product is available in the environment.

• Some reactions are amphibolic (can be catabolic or anabolic).

Key Terms and Concepts

• Activation Energy: The minimum energy required to initiate a chemical reaction.

• Substrate: The molecule upon which an enzyme acts.

• Allosteric Site: A site on the enzyme other than the active site, where molecules can bind and affect enzyme activity.

• ATPase: Enzyme that synthesizes ATP from ADP and Pi using the proton motive force.

• Beta-oxidation: Catabolic process by which fatty acids are broken down in mitochondria and/or peroxisomes to generate acetyl-CoA.

• Deamination: Removal of an amino group from an amino acid during protein catabolism.

Notable Scientist

Peter Mitchell

• Proposed the chemiosmotic theory, explaining how ATP is generated by the movement of protons across a membrane during oxidative phosphorylation.

Review Questions (Selected)

• What molecule is the “energy currency” of the cell? ATP

• Name the three main electron carriers in cells. NAD+, NADP+, FAD

• Compare the three types of phosphorylation in metabolism. Substrate-level (direct transfer), oxidative (ETC and chemiosmosis), photophosphorylation (light-driven, in photosynthesis).

• Why are enzymes sensitive to pH, temperature, and ionic concentration? These factors affect enzyme structure and function; extreme changes can denature enzymes.

• Compare fermentation with cellular respiration. Fermentation yields less ATP, does not use ETC, and produces organic end products; respiration yields more ATP and fully oxidizes glucose.

Additional info: Commercial products of fermentation include yogurt (lactic acid), beer and wine (ethanol), and vinegar (acetic acid).