Dehydrogenase: An enzyme that oxidizes a molecule by the removal of a
pair of electrons and one or two protons.
Nucleophilic addition: The process of forming a bond between an electron-
rich group (nucleophile) and an electron-deficient atom (electrophile).
Oxidation: The removal of electrons from an atom or compound.
Thiamine: Vitamin B1, a water-soluble vitamin containing a thiazolium
ring. The active form of the vitamin is thiamine pyrophosphate, which is
an important coenzyme for many biochemical reactions.
D ISC U SSIO N
Thiamine (vitamin B1) is an important water-soluble vitamin that, in its
active form of thiamine pyrophosphate, is used as a cofactor in enzymatic
reactions that involve the transfer of an aldehyde group. Thiamine can be
synthesized by plants and some microorganisms, but not usually by animals.
Hence, humans must obtain thiamine from the diet, though small amounts
may be obtained from synthesis by intestinal bacteria. Because of its importance
in metabolic reactions, it is present in large amounts in skeletal muscle, heart,
liver, kidney, and brain. Thus, it has a widespread distribution in foods, but there
can be a substantial loss of thiamine during cooking above 100°C (212°F).
Thiamine is absorbed in the intestine by both active transport mechanisms
and passive diffusion. The active form of the cofactor, thiamine pyrophos-
phate (thiamine diphosphate, TPP), is synthesized by an enzymatic trans-
fer of a pyrophosphate group from ATP to thiamine (Figure 15-1). The
resulting TPP has a reactive carbon on the thiazole ring that is easily ionized
to form a carbanion, which can undergo nucleophilic addition reactions.
T P P synthetase
Thiamine pyrophosphate (TPP)
Figure 15-1. Activation of thiamine (vitamin B
) to the active cofactor, thiamine
pyrophosphate (TPP) by the enzyme TPP synthetase.