BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 8. VITAMINS

8.3. Water-soluble vitamins

8.3.1. Vitamin B₁ — Thiamine

Structure and physicochemical properties. Vitamin B₁ (thiamine) was discovered during The Study of beriberi, a disease prevalent in Asian countries. Chemically, it consists of two cyclic systems, pyrimidine and thiazole, connected by a methylene group. The synthesis of both cyclic structures pro

ceeds separately in the form of phosphorylated derivatives. The vitamin is stable to heat in an acidic medium, but rapidly decomposes in an alkaline one. Upon oxidation of thiamine, thiochrome is formed, which exhibits blue fluorescence under ultraviolet irradiation. This reaction serves as the basis for the Quantitative determination of thiamine.

The active form of vitamin B₁ is thiamine pyrophosphate (TPP, TRL), which is formed through the action of the ATP-dependent enzyme thiamine pyrophosphokinase by attaching a pyrophosphate group from ATP to the hydroxyl group of thiamine (Fig. 8.9).

Biochemical Functions. Reactions Catalyzed by thiamine are types of α-Condensation and α-Cleavage. They share the common feature that the C–C bond acted upon by thiamine is located directly adjacent to the carbonyl group.

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Fig. 8.9. Synthesis of thiamine pyrophosphate

The best-studied thiamine-dependent enzyme is Yeast Pyruvate decarboxylase, which catalyzes The conversion of pyruvate into acetaldehyde during Alcoholic Fermentation.

TPP is also a component of two major enzyme systems: the pyruvate and α-ketoglutarate dehydrogenase complexes, which catalyze The oxidative decarboxylation of the corresponding keto acids. A key intermediate in these reactions is the thiazolium zwitterion, to which the substrate binds. For instance, the initial step in the conversion of pyruvate to acetyl-CoA is The addition of the enol form of pyruvate to TPP (Fig. 8.10).

Fig. 8.10. The first step in the conversion of pyruvate to acetyl-CoA

Transketolase, an enzyme of the Pentose Phosphate Pathway of Carbohydrate METABOLISM, also requires TPP. Transketolase catalyzes a two-substrate reaction: The transfer of a glycolaldehyde moiety from a ketose (donor substrate) to an aldose (acceptor substrate):

Nutritional requirement and natural occurrence. Vitamin B₁ deficiency still occurs today as Wernicke's encephalopathy, as well as Wernicke–Korsakoff syndrome (or cardiac beriberi), which primarily affects The Cardiovascular system. The most characteristic and specific sign of avitaminosis is polyneuritis. This condition manifests as painful sensations along nerve trunks, culminating in a loss of Skin sensation and paralysis. Another major sign of deficiency is impaired cardiac function. Characteristic features also include disturbances in the motor and secretory Functions of the digestive tract, memory impairment, shortness of breath, and precordial pain. Biochemical abnormalities include The Development of a negative nitrogen balance and the accumulation of α-keto acids in the Blood.

Humans obtain most of their thiamine from meat and legume products. Yeast, whole-grain flour products, Liver, and Kidneys are particularly rich in this vitamin. The daily requirement is 2–3 mg. A predominantly carbohydrate-rich diet increases the body's need for the vitamin. Thiamine withstands high temperatures only in an acidic environment. In neutral and especially alkaline environments, it is rapidly destroyed upon heating.