Thiamine was the 1st vitamin (vital amine) ever discovered.

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Thiamine pyrophosphate
Thiamine was the 1st vitamin (vital amine) ever
discovered. Thiamine deficiency results in a
disease called beri-beri. (Beri-beri is a Malay
word meaning extreme lassitude.)
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Beri-Beri was first described by Dutch physician
Nicolaas Tulp in 1652.
Tulp also commissioned Rembrandt to paint The
Anatomy Lesson.
Thiamine pyrophosphate functions in
decarboxylation of a-ketoacids.
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Direct decarboxylation of an a-keto acid would be
difficult because there is no way of
delocalising the negative charge that would
develop on the a-carbon.
TPP facilitates decarboxylation.
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The important part of TPP is the thiazole ring.
One of the carbon atoms can easily lose a proton
to form a carbanion that is stabilised by the
ring.
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This adds to the ketone group of the a-ketoacid.
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Decarboxylation generates a carbanion on the C
atom derived from C2 of the a-keto acid.
The charge can be delocalised over the thiazole
ring.
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The hydroxyethyl carbanion (sometimes referred to
as activated acetaldehyde) can be transferred to
an acceptor molecule like pyruvate (for VAL
biosynthesis, see below), or a-ketobutyrate (for
ILE biosynthesis) or to CoA via lipoic acid.
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With pyruvate dehydrogenase, the hydroxyethyl
carbanion is transferred to a lipoic
acid cofactor.
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The acyl group is then transferred to coenzyme A.
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Finally the dihydrolipoamide is re-oxidised by
NAD .
a-ketoglutarate DH same story except that
the a-keto acid is a ketoglutarate and the
product is succinyl CoA