Microbial Metabolism: Enzymes, Respiration, and Fermentation

Microbial Metabolism

Overview of Metabolism

Metabolism refers to all the chemical reactions that occur within a living organism. It is divided into two main processes: anabolism and catabolism.

• Catabolism: The breakdown of large molecules into smaller ones, releasing energy. These reactions are exergonic (energy-releasing).

• Anabolism: The synthesis of complex molecules from simpler ones, requiring energy input. These reactions are endergonic (energy-consuming).

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

ATP: The Energy Currency

Adenosine triphosphate (ATP) acts as an energy intermediary between catabolic and anabolic reactions.

• Catabolism: Produces ATP by releasing energy from molecules.

• Anabolism: Consumes ATP to build complex molecules.

Example: Energy from glucose catabolism is stored in ATP, which is then used for biosynthetic processes.

Enzymes and Their Function

Enzyme Structure and Identification

• Enzymes are biological catalysts, mostly composed of proteins, that speed up chemical reactions without being consumed.

• They are highly specific for their substrates and can be reused.

• Enzyme names typically end with the suffix "-ase" (e.g., lactase, DNA polymerase).

Mechanism of Enzymatic Action

• Catalyst: A substance that increases the rate of a chemical reaction without being altered.

• Substrate: The molecule upon which an enzyme acts.

• Product: The molecule(s) produced from the enzymatic reaction.

• Coenzyme: An organic molecule (often derived from vitamins) that assists enzyme function, often as electron carriers (e.g., NAD+, NADP+).

• Cofactor: An inorganic ion (e.g., Mg2+) required for enzyme activity.

Example: The enzyme hexokinase requires Mg2+ as a cofactor to phosphorylate glucose.

Factors Influencing Enzyme Activity

• Temperature: Enzyme activity increases with temperature up to an optimum, but high temperatures cause denaturation (loss of 3D structure and function).

• pH: Each enzyme has an optimal pH (often around 7), but this varies with the organism's environment.

• Substrate Concentration: Increasing substrate increases reaction rate until the enzyme is saturated, after which the rate plateaus.

• Inhibitors: Substances that decrease enzyme activity. Types include competitive and noncompetitive inhibitors.

Example: High fever can denature enzymes, impairing metabolic processes.

Enzyme Inhibition

• Competitive Inhibition: Inhibitor resembles the substrate and binds to the active site, blocking substrate binding.

• Noncompetitive Inhibition: Inhibitor binds to a different site (allosteric site), changing the enzyme's shape and reducing activity.

Type of Inhibition	Binding Site	Effect	Example
Competitive	Active site	Blocks substrate	Sulfanilamide (competes with PABA, inhibits folic acid synthesis)
Noncompetitive	Allosteric site	Changes enzyme shape	Heavy metals (e.g., mercury)

Example: Sulfanilamide is a competitive inhibitor of the enzyme involved in folic acid synthesis in bacteria.

Ribozymes

• Ribozymes are RNA molecules with catalytic activity.

• They bind to substrates, catalyze reactions, and are not consumed in the process.

• Functions include RNA splicing and protein synthesis in ribosomes.

Cellular Respiration and Fermentation

Stages of Cellular Respiration

Cellular respiration is the process by which cells extract energy from organic molecules. It consists of four main stages:

1. Glycolysis

2. Intermediate Stage (Pyruvate to Acetyl-CoA)

3. Krebs Cycle (Citric Acid Cycle)

4. Electron Transport System (ETS)

Glycolysis

• Begins with one molecule of glucose.

• Ends with two molecules of pyruvate.

• Produces a net gain of 2 ATP and 2 NADH per glucose.

• Fate of pyruvate depends on oxygen availability:

  • With oxygen: Proceeds to cellular respiration (aerobic).

  • Without oxygen: Proceeds to fermentation (anaerobic).

Intermediate Stage

• Converts pyruvate (from glycolysis) into acetyl-CoA for entry into the Krebs cycle.

• Each glucose yields two acetyl-CoA molecules.

Krebs Cycle (Citric Acid Cycle)

• Completes the oxidation of acetyl-CoA to CO2.

• Produces NADH, FADH2, and ATP (or GTP).

• Occurs in the presence of oxygen (aerobic), but does not directly use O2.

Electron Transport System (ETS)

• Transfers electrons from NADH and FADH2 to oxygen (in aerobic respiration), generating a proton gradient used to produce ATP.

• Located in the plasma membrane of prokaryotes and the inner mitochondrial membrane of eukaryotes.

Electron Carriers: NADH and FADH2

• NADH and FADH2 are reduced coenzymes that carry electrons to the ETS.

• Each NADH typically generates 3 ATP; each FADH2 generates 2 ATP (values may vary in prokaryotes).

Overall Equation for Aerobic Respiration

The basic formula for aerobic respiration is:

$\mathrm{C_6H_{12}O_6} + 6\,\mathrm{O_2} \rightarrow 6\,\mathrm{CO_2} + 6\,\mathrm{H_2O} + \text{energy (ATP)}$

Anaerobic Respiration

• Uses an electron acceptor other than oxygen (e.g., nitrate, sulfate).

• Produces less ATP than aerobic respiration.

Fermentation

• Occurs when oxygen is absent.

• Does not use the Krebs cycle or ETS.

• Produces ATP only by substrate-level phosphorylation during glycolysis.

• Different organisms use different organic molecules as electron acceptors, resulting in various end products:

  • Lactic acid: Produced by Lactobacillus (used in yogurt production).

  • Ethanol and CO2: Produced by Saccharomyces (yeast).

Other Key Terms

• Chemoheterotroph: An organism that obtains energy by ingesting organic molecules (chemical energy) and uses organic compounds as a carbon source.

Additional info: The ATP yield per NADH and FADH2 can vary depending on the organism and the efficiency of the electron transport chain. In eukaryotes, the theoretical maximum is 3 ATP per NADH and 2 ATP per FADH2, but actual yields may be lower.