NADH (Nicotinamide adenine dinucleotide) and FADH2 (Flavin adenine dinucleotide) are crucial electron carriers in microbial metabolism, playing vital roles in various metabolic pathways.
Production:
* Glycolysis: Both NADH and FADH2 are produced during the breakdown of glucose in glycolysis.
* Krebs Cycle (Citric Acid Cycle): The Krebs cycle generates significant amounts of NADH, while FADH2 is produced in a single step.
* Fatty Acid Oxidation: The breakdown of fatty acids generates NADH and FADH2 as well.
* Other pathways: Some other metabolic pathways, like the pentose phosphate pathway, also produce NADH.
Usage:
* Electron Transport Chain (ETC): Both NADH and FADH2 donate their electrons to the ETC, powering the generation of ATP through oxidative phosphorylation.
* Anaerobic Respiration: In the absence of oxygen, some microbes use NADH or FADH2 to reduce alternative electron acceptors, like nitrate or sulfate, for energy production.
* Biosynthetic Reactions: NADH and FADH2 can be used as reducing agents in various biosynthetic pathways, such as the synthesis of amino acids and fatty acids.
Specific examples:
* Aerobic respiration: In aerobic respiration, NADH and FADH2 are produced during glycolysis and the Krebs cycle, then donate their electrons to the ETC, leading to the production of ATP.
* Fermentation: Some microbes utilize NADH to reduce pyruvate into lactate or ethanol during fermentation, regenerating NAD+ for glycolysis to continue.
* Methanogenesis: Methanogenic archaea use FADH2 to reduce CO2 to methane, a major process in the global carbon cycle.
Summary:
NADH and FADH2 are essential electron carriers in microbial metabolism, playing critical roles in energy production, biosynthesis, and various metabolic pathways. Their production and usage vary depending on the specific metabolic pathways employed by different microbes and their environmental conditions.
