Figure 1. Structures of (a) thiamin (vitamin B₁); (b) thiamin diphosphate (ThDP); (c) the conjugate base of ThDP resulting from ionization of its C–H bond at C2; and (d) the ‘second carbanion’ (or zwitterion), formed by C–H fission in the attached fragment of an adduct with a carbonyl compound.

Figure 2. Chemical conversions in the cell that depend on ThDP as an enzyme cofactor. Only the chemically reactive thiazolium ring of ThDP is shown. (a) The decarboxylation of pyruvic acid. The conversion of pyruvic acid (or the equivalent of pyruvate anion and a proton) to carbon dioxide and acetaldehyde occurs in yeast and bacteria at the branch point between the glycolytic pathway and fermentation to produce ethanol. The addition of ThDP to the carbonyl group of pyruvate is followed by loss of carbon dioxide to produce the intermediate shown. Carbonyl-forming elimination then generates acetaldehyde and regenerates ThDP for a new catalytic cycle. (b) The formation of acetyl–coenzyme A. This is a related reaction performed in the mitochondria of higher organisms, e.g. in neurons of the human brain. The cofactor-bearing enzyme is the E₁ subunit of the pyruvate dehydrogenase enzyme complex. In this case, departure of carbon dioxide from the adduct of ThDP is followed by reaction with lipoic acid to cleave its sulfur–sulfur bond and generate the intermediate shown. In a series of subsequent steps, acetyldihydrolipoic acid is formed and reacts with coenzyme A to transfer the acetyl group and produce dihydrolipoic acid. Here also ThDP has been generated for a new catalytic cycle. (c) The transketolase reaction, described in the text.

Figure 3. Schematic presentation of the properties of many ThDP-dependent enzymes. The domain organization of one subunit of a typical enzyme is shown (cf. Linqvist and Schneider, 1993).

Figure 4. Molecular details of the events in carbon–carbon bond cleavage and in electron transfer in oxidoreductases, assisted by the cofactor ThDP. The pathways are illustrated by the catalytic mechanisms for pyruvate decarboxylase and for the E₁ subunit of the pyruvate dehydrogenase enzyme complex. (a) Events that occur in both enzyme systems: ThDP undergoes ionization by C–H fission, its conjugate base adds to the pyruvate carbonyl group, and the adduct loses carbon dioxide to form a ‘second carbanion’. (b) The two pathways diverge by reaction of the ‘second carbanion’ with different electrophiles. In the action of pyruvate decarboxylase, the electrophile is a proton, and the final elimination reaction thus generates acetaldehyde. In the action of the pyruvate dehydrogenase enzyme complex, the electrophile is lipoic acid, a disulfide. Overall, the sulfur–sulfur bond is broken and the adduct shown is formed, but the exact route remains unclear: either direct nucleophilic displacement at sulfur or electron transfer followed by C–S bond formation are consistent with available data. The final elimination generates acetyldihydroipoic acid. With pyruvate decarboxylase in the fermentative pathway of microorganisms, the acetaldehyde product of the ThDP-dependent reaction is reduced to ethanol through the action of alcohol dehydrogenase. In the action of the pyruvate dehydrogenase enzyme complex, acetyldihydrolipoic acid transfers its acetyl group to coenzyme A to form acetyl–coenzyme A. Acetyl–coenzyme A enters the tricarboxylic acid cycle for energy generation or, in nerve cells, serves as a substrate for the synthesis of the neurotransmitter acetylcholine.