VITAMINE B1
Vitamin B1 is also known as thiamine, thiamin and aneurin. Thiamine is the currently accepted name for vitamin B1 in the US. Aneurin is still widely used in Europe, especially in the United Kingdom. The chemical name for this water-soluble vitamin is 3-[(4-amino-2-methyl-5-pyrimidinyl) methyl]- 5-(2-hydroxyethyl)-4-methylthiazolium. Thiamine consists of a pyrimidine ring and a thiazole ring connected by a one-carbon link. The nitrogen in the thiazole ring has a charge of +1. This nitrogen atom serves as an important electron sink in thiamine pyrophosphate mediated reactions.
BICHEMICAL FUNCTION OF THIAMINE
THIAMINE PRYOPHOSPHATE
A major biologically active form of thiamine is thiamine pyrophosphate (TPP), sometimes called thiamine diphosphate (TDP) and cocarboxylase. In thiamine pyrophosphate the hydroxyl group of thiamine is replaced by a diphosphate ester group. The reaction site of TPP is carbon 2 of the thiazole ring. The proton on this carbon is rather acidic. When this proton dissociates a carbanion is formed which readily undergoes nucleophilic addition carbonyl groups. TPP is a coenzyme for two types of enzymes, alpha-ketoacid dehydrogenases and transketolases, both of which cleave a C-C bond adjacent to a carbonyl group releasing either carbon dioxide or an aldehyde. The resulting product is then transferred to an acceptor molecule. alpha-Ketoacid dehydrogenases decarboxylate alpha-ketoacids. The decarboxylation product is then transferred to coenzyme A (CoA). Transketolases cleaves the C-C bond adjacent to the carbonyl group of an alpha-ketosugar to give an activated glycoaldehyde. The glycoaldehyde is then combined with an aldose to give a new ketose. All known TPP dependent enzymes also require a divalent cation, commonly Mg2+.
The Krebs cycle (also called the citric acid cycle and the tricarboxylic acid cycle) is very important in extracting energy from fuel molecules. TPP is the coenzyme for alpha-ketoacid dehydrogenases which catalyze two reactions of the Krebs cycle.
In each of these reactions NAD+ is reduced to NADH and a molecule of CO2 is liberated. The first step in the Krebs cycle is the addition of a 2-carbon acetyl group (from acetyl CoA) to the 4-carbon oxaloacetate to give the 6-carbon citric acid. The acetyl CoA is produced by the oxidative decarboxylation of pyruvate (an alpha-ketoacid) and the covalent bonding of the decarboxylation product to CoA. This conversion of pyruvate to acetyl CoA is catalyzed by a multiprotein assembly called pyruvate dehydrogenase complex. TPP is the cofactor for the pyruvate dehydrogenase component of the complex. Pyruvate dehydrogenase catalyzes the oxidative decarboxylation of pyruvate. Other components of the complex complete the conversion of pyruvate to acetyl CoA. After 3 steps in the Krebs cycle CO2 is released (TPP independent) to give alpha-ketoglutarate (another alpha-ketoacid). alpha-Ketoglutarate's fate is similar to pyruvate's. alpha-Ketoglutarate is decarboxylated and the product is transferred to CoA to give succinyl CoA. alpha-Ketoglutarate dehydrogenase complex, also a multiprotein complex, catalyzes the conversion of alpha-ketoglutarate to succinyl CoA. As with pyruvate dehydrogenase complex, TPP is the coenzyme for the alpha-ketoglutarate dehydrogenase part of the complex.
The mechanism is identical for both the conversion of pyruvate to acetyl CoA and the conversion of alpha-ketoglutarate to succinyl CoA. In the reaction, the proton on C2 of TPP dissociates to give a carbanion. Nucleophilic addition by the carbanion to the carbonyl group of the alpha-ketoacid (i. e. pyruvate or alpha-ketoglutarate) followed by protonation forms an activated alpha-hydroxyacid. The hydroxyacid then undergoes decarboxylation. The positively charged nitrogen of TPP serves as a critical electron sink during the decarboxylation step and contributes to the resonance stabilization of the hydroxyalkyl-TPP decarboxylation product. The hydroxyalkyl group is transferred by other proteins in the complex to CoA to produce acetyl CoA from pyruvate or succinyl CoA from alpha-ketoglutarate.
The pentose phosphate pathway harvests energy from fuel molecules and stores it in the form of NADPH. NADPH (reduced nicotinamide adenine dinucleotide phosphate) is an important electron donor in reductive biosynthesis. The pentose phosphate pathway also produces 5-carbon sugars such as ribose which is used in the synthesis of DNA and RNA. TPP is the coenzyme for the enzyme transketolase. Transketolase transfers a 2-carbon unit from an alpha-ketose (a sugar with a carbonyl group at position 2) to an aldose. In the reaction below a 2-carbon unit from the 5-carbon alpha-ketose xylulose 5-phosphate is transferred to the 4-carbon aldose erythrose 4-phosphate to make the 6 carbon alpha-ketose fructose 6-phosphate. Glyceraldehyde 3-phosphate results from the 3-carbon fragment that is cleaved from xylulose 5-phosphate. Note that fructose 6-phosphate and glyceraldehyde 3-phosphate (the products of the forward reaction) are an alpha-ketose and an aldose and that the reaction is reversible.
In the reaction, a carbanion at C-2 of TPP is first produced. This carbanion attacks the carbonyl carbon of the alpha-ketose to give an addition product. After deprotonation of the appropriate hydroxyl group an aldose (in this case glyceraldehyde 3-phosphate) is released and an activated glycoaldehyde bound to TPP is produced. As with the decarboxylation mechanism discussed above, the thiazole nitrogen serves as electron sink in the reaction and contributes to resonance stabilization of the resulting product (activated glycoadehyde). This glycoaldehyde is said to be activated because it is also a carbanion and readily undergoes nucleophilic addition to the carbonyl group of an aldose (erythrose 4-phosphate here). Following another deprotonation the nascent alpha-ketose is released from TPP.
Transketolase from baker's yeast (Saccharomyces cerevisiae) is shown to the right. The coloring scheme highlights the secondary structure and reveals that transketolase is a dimer In this structure TPP has been substituted by 2,3'-deazo-thiamin diphosphate which is shown as a CPK colored space filling model. Ca2+ (blue-gray) can be seen complexed with the diphosphates.
It is evident from the neurological disorders caused by thiamine deficiency that this vitamin plays a vital role in nerve function. It is unclear, however, just what that role is. Thiamine is found in both the nerves and brain. The concentration of thiamine in the brain seems to be resistant to changes dietary concentration. Electrical or chemical (e.g., acetylcholine) stimulation of nerves results in the release of thiamine monophosphate and free thiamine into the medium with accompanying decrease of cellular thiamine pyrophosphate and thiamine triphosphate. This observation suggest that thiamine has a role in the nervous system independent of its coenzyme roles. One theory is that thiamine triphosphate is involved with nerve impulses via the Na+ and K+ gradient. |
Thiamine deficiency usually causes weight loss, cardiac abnormalities, and neuromuscular disorders. The classic thiamine deficiency syndrome in humans is beri-beri (sometimes called Kakke). Thiamine is abundant in whole grains, usually in the scutellum (the thin covering of the starchy interior endosperm), but is scarce in the endosperm. Unfortunately beri-beri is still common in parts of southeast Asia where polished rice is a staple and thiamine enrichment programs are not fully in place. Beri-beri is characterized by anorexia (loss of appetite) with subsequent weight loss, enlargement of the heart, and neuromuscular symptoms such as paresthesia (spontaneous sensations, such as itching, burning, etc.), muscle weakness, lassitude (weariness, general weakness), and foot and wrist droop. There are three main types of beri-beri: (1) dry (also neuritic, paraplegic, and pernicious) beri-beri; (2) wet (also edematous or cardiac) beri-beri; (3) and infantile (also acute) beri-beri.
Dry beri-beri usually inflicts older adults and affects mainly the peripheral nerves with little cardiac involvement. It is characterized by atrophy (wasting away) and peripheral neuritis (inflammation of nerves) of the legs and paraplegia (paralysis of the lower extremities). In contrast wet beri-beri displays substantial cardiac involvement especially tachycardia (rapid heart beat) in addition to peripheral neuropathy. Edema progresses from the feet upwards to the heart causing congestive heart failure in severe cases. Infantile beri-beri is usually seen in breast-feeding infants whose mothers are thiamine deficient (but not necessarily showing signs of beri-beri). These infants are usually anoretic and often have trouble keeping the milk down. Once the disease begins it moves rapidly causing heart failure in a matter of hours.
Wernicke-Korsakoff Syndrome or Wernicke's encephalopathy is the thiamine deficient disease seen most often in the Western hemisphere. It mainly affects alcoholics due to three reasons: (1) the diets of alcoholics are usually poor; (2) diets rich in carbohydrates (e.g., alcohol or rice) increase the metabolic demands of thiamine; and (3) alcohol inhibits intestinal ATPase which is involved in the uptake of thiamine. Two observations suggest a genetic invovlement with Wernicke-Korsakoff Syndrome: (1) it is much higher in among Europeans than non-Europeans; and (2) transketolase (see above under " Thiamine in thePentose Phosphate Pathway") from Wernicke-Korsakoff Syndrome patients binds TPP 10 time less strongly than normal transketolase. The symptoms of
Wernicke-Korsakoff syndrome include confusion, sixth nerve damage resulting in ophthalmoplegia (paralysis of an eye motor nerve) and nystagmus (rhythmical oscillation of the eyes), psychosis, confabulation, and impaired retentive memory and cognitive function. In severe cases the patient may slip into a coma. A congenital defect in transketolase which causes a low binding affinity for TPP increases the chances of acquiring Wernicke-Korsakoff syndrome.
Information on Thiamine and Alzheimers
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