Metabolism: Enzymes and reactions involved in fermentation

Pyruvate decarboxylase and alcohol dehydrogenase are key enzymes in alcoholic fermentation, a metabolic pathway used by yeast and some bacteria to convert pyruvate into ethanol and CO${}_2$‍. The process is summarized in Figure 1.

Figure 1 Alcohol fermentation in yeast

Pyruvate decarboxylase requires two co-factors during catalysis: magnesium and thiamine pyrophosphate (TPP), a vitamin B1 derivative. TPP has a thiazole ring, which is capable of forming a carbanion that nucleophilically attacks the substrate during pyruvate decarboxylase catalysis, as shown in Figure 2. The products of the decarboxylation reaction are acetaldehyde and carbon dioxide, which is released as a by-product of the reaction.

Figure 2 Nucleophilic attack of pyruvate and subsequent steps to produce acetaldehyde

Acetaldehyde is subsequently reduced to ethanol by alcohol dehydrogenase. This step requires the co-factor nicotinamide adenine dinucleotide (NADH), which gets oxidized to NAD+ in the process.

Humans also undergo fermentation in anaerobic conditions - for example, when muscles run out of oxygen during exercise. However, unlike in yeast cells, pyruvate gets catalyzed into lactic acid by the enzyme lactate dehydrogenase. This reaction also oxidizes NADH to NAD+.

UV absorption spectroscopy of NAD+ and NADH provides a valuable tool for monitoring and characterizing fermentation reactions. It offers real-time insights into the redox state of the reaction and can be used to assess the progress, kinetics, and endpoint of fermentation processes, making it a useful analytical technique in biotechnology, microbiology, and biochemistry. The absorption spectra of these molecules is shown in Figure 3.

Figure 3 UV absorption spectra of NAD+ and NADH