Thiamine (Vitamin B1)

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Chapter: Biochemistry : Vitamins

Thiamine pyrophosphate (TPP) is the biologically active form of the vitamin, formed by the transfer of a pyrophosphate group from adenosine triphosphate (ATP) to thiamine .


THIAMINE (VITAMIN B1)

Thiamine pyrophosphate (TPP) is the biologically active form of the vitamin, formed by the transfer of a pyrophosphate group from adenosine triphosphate (ATP) to thiamine (Figure 28.11). TPP serves as a coenzyme in the formation or degradation of α-ketols by transketolase (Figure 28.12A), and in the oxidative decarboxylation of α-keto acids (Figure 28.12B).


Figure 28.11 A. Structure of thiamine and its coenzyme form, thiamine pyrophosphate. B. Structure of intermediate formed in the reaction catalyzed by pyruvate dehydrogenase. C. Structure of intermediate formed in the reaction catalyzed by α-ketoglutarate dehydrogenase. AMP = adenosine monophosphate.


Figure 28.12 Reactions that use thiamine pyrophosphate (TPP) as coenzyme. A. Transketolase. B. Pyruvate dehydrogenase and α-ketoglutarate dehydrogenase. Note that TPP is also used by branched-chain α-keto acid dehydrogenase. P = phosphate; CoA = coenzyme A.

 

A. Clinical indications for thiamine

The oxidative decarboxylation of pyruvate and α-ketoglutarate, which plays a key role in energy metabolism of most cells, is particularly important in tissues of the CNS. In thiamine deficiency, the activity of these two dehydrogenase-catalyzed reactions is decreased, resulting in decreased production of ATP and, therefore, impaired cellular function. TPP is also required by branched-chain α-keto acid dehydrogenase of muscle. [Note: It is the decarboxylase of each of these α-keto acid dehydrogenase multienzyme complexes that requires TPP.] Thiamine deficiency is diagnosed by an increase in erythrocyte transketolase activity observed on addition of TPP.]

 

1. Beriberi: This is a severe thiamine-deficiency syndrome found in areas where polished rice is the major component of the diet. Adult beriberi is classified as dry (characterized by peripheral neurologic deficits) or wet (characterized by edema due to cardiac dysfunction). Infantile beriberi is seen in nursing infants whose mothers are deficient in thiamine.

 

2. Wernicke-Korsakoff syndrome: In the United States, thiamine deficiency, which is seen primarily in association with chronic alcoholism, is due to dietary insufficiency or impaired intestinal absorption of the vitamin. Some alcoholics develop Wernicke-Korsakoff syndrome, a thiamine deficiency state characterized by confusion, ataxia, and a rhythmic to-and-fro motion of the eyeballs (nystagmus) with Wernicke encephalopathy as well as memory problems and hallucinations with Korsakoff dementia. The syndrome is treatable with thiamine supplementation, but recovery of memory is typically incomplete.

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