Comprehensive Biochemistry Study Guide: Vitamins and Coenzymes

General Introduction to Vitamins

• Definition: Vitamins are complex organic compounds present in natural foods either as active vitamins or as utilizable precursors. They are required in small quantities for normal growth, maintenance, and reproduction, which are essential components of normal nutrition and health.

• Unique Characteristics: Vitamins differ from other biomolecules (like carbohydrates, proteins, and lipids) in several key ways:

  • They do not enter into tissue structures.

  • They do not undergo metabolism to generate energy.

  • They function primarily as coenzymes in cellular metabolic reactions.

• Vitamins vs. Hormones: Unlike hormones, vitamins are generally not synthesized within the organism (with a few exceptions) and must be obtained from the diet.

Classification of Vitamins

Vitamins are divided into two main groups based on their solubility:

• Fat-Soluble Vitamins:

  • Vitamin A

  • Vitamin D

  • Vitamin E

  • Vitamin K

• Water-Soluble Vitamins:

  • Vitamin C (Ascorbic acid)

  • Vitamin B Complex Group:

    • Vitamin $B_1$ (Thiamine)

    • Vitamin $B_2$ (Riboflavin)

    • Niacin (Nicotinic acid)

    • Vitamin $B_6$ (Pyridoxine)

    • Pantothenic acid

    • $\alpha$-Lipoic acid

    • Biotin

    • Folic acid group

    • Vitamin $B_{12}$ (Cyanocobalamine)

Detailed Profiles of Water-Soluble Vitamins

Biotin

• Coenzyme: Biotin.

• Associated Enzymes: Pyruvate carboxylase, Acetyl CoA carboxylase, Propionyl CoA carboxylase.

• Metabolic Pathways: Gluconeogenesis, Fatty acid synthesis, metabolism of odd-carbon fatty acids, Valine ($Val$), Methionine ($Met$), Isoleucine ($Ile$), and Threonine ($Thr$).

• Deficiency Details:

  • Most Common Cause (MCC): Excessive consumption of raw eggs because they contain avidin, a biotin-binding protein. It can also be caused by biotinidase deficiency.

  • Symptoms: Alopecia (hair loss), bowel inflammation, and muscle pain.

Thiamine (Vitamin $B_1$)

• Coenzyme: Thiamine pyrophosphate (TPP).

• Associated Enzymes: Pyruvate dehydrogenase (PDH), $\alpha$-Ketoglutarate dehydrogenase, Transketolase, Branched chain ketoacid dehydrogenase.

• Metabolic Pathways: TCA cycle, HMP shunt (Pentose Phosphate Pathway), metabolism of Valine, Isoleucine, and Leucine.

• Deficiency Details:

  • MCC: Alcoholism, as alcohol interferes with absorption.

  • Wernicke Syndrome: Characterized by ataxia, nystagmus, and ophthalmoplegia.

  • Korsakoff Syndrome: Characterized by confabulation and psychosis.

  • Wet Beri-beri: High-output cardiac failure, fluid retention, and vascular leak.

  • Dry Beri-beri: Peripheral neuropathy.

Niacin (Vitamin $B_3$)

• Coenzyme: $NAD(H)$, $NADP(H)$.

• Associated Enzymes: Many dehydrogenases.

• Deficiency Details:

  • Pellagra: Symptoms are the "Four Ds" — Diarrhea, Dementia, Dermatitis, and eventually Death if untreated.

  • Etiology: May be related to a deficiency of Tryptophan (e.g., corn-heavy diets which are low in Tryptophan), as Tryptophan supplies a portion of the niacin requirement.

Folic Acid

• Coenzyme: Tetrahydrofolate ($THF$).

• Associated Enzymes: Thymidylate synthase and various enzymes in purine synthesis.

• Metabolic Pathways: Thymidine (pyrimidine) synthesis and purine synthesis.

• Deficiency Details:

  • MCC: Alcoholism and pregnancy (body stores are depleted in about 3 months); also hemodyalisis.

  • Clinical Consequences: Megaloblastic (macrocytic) anemia. Deficiency in early pregnancy causes neural tube defects in the fetus.

Cyanocobalamin (Vitamin $B_{12}$)

• Associated Enzymes: Homocysteine methyltransferase, Methylmalonyl CoA mutase.

• Metabolic Pathways: Synthesis of Methionine and SAM ($S$-adenosylmethionine); metabolism of odd-carbon fatty acids, Valine, Methionine, Isoleucine, and Threonine.

• Deficiency Details:

  • MCC: Pernicious anemia. Also associated with aging (poor nutrition), bacterial overgrowth of the terminal ileum, terminal ileum resection (secondary to Crohn's disease), chronic pancreatitis, vegans (rare), or infection with $D. latum$.

  • Clinical Consequences: Homocystinemia (risk of deep vein thrombosis and atherosclerosis), Megaloblastic (macrocytic) anemia, and progressive peripheral neuropathy.

Pyridoxine (Vitamin $B_6$)

• Coenzyme: Pyridoxal-P ($PLP$).

• Associated Enzymes: Aminotransferases (AST/GOT, ALT/GPT) and $\delta$-Aminolevulinate synthase.

• Metabolic Pathways: Protein catabolism, Heme synthesis.

• Deficiency Details:

  • MCC: Isoniazid therapy.

  • Symptoms: Sideroblastic anemia, Cheilosis or stomatitis (cracking/scaling of lips), and convulsions.

Riboflavin (Vitamin $B_2$)

• Coenzyme: $FAD(H_2)$.

• Associated Enzymes: Various dehydrogenases.

• Deficiency Details: Corneal neovascularization, Cheilosis, and a distinctive magenta-colored tongue.

Ascorbate (Vitamin C)

• Associated Enzymes: Prolyl and lysyl hydroxylases, Dopamine hydroxylase.

• Metabolic Pathways: Collagen synthesis, catecholamine synthesis, and iron absorption in the GI tract.

• Deficiency Details:

  • MCC: Diet deficient in citrus fruits and green vegetables.

  • Scurvy: Poor wound healing, easy bruising (perifollicular hemorrhage), bleeding gums, increased bleeding time, painful glossitis, and anemia.

Pantothenic acid

• Coenzyme: $CoA$.

• Associated Enzymes: Fatty acid synthase, Fatty acyl CoA synthetase, Pyruvate dehydrogenase, $\alpha$-Ketoglutarate dehydrogenase.

• Metabolic Pathways: Fatty acid metabolism, PDH, and TCA cycle. Deficiency is rare.

Fat-Soluble Vitamins: General Principles

• Vitamins A, D, E, and K: All four act through specialized mechanisms.

• Hormonal Action: Vitamins A and D work through enhancer mechanisms similar to lipid-soluble hormones, involving gene transcription regulation.

Vitamin A (Retinoids and Carotenoids)

• Chemical Forms:

  • $3$ structures differ by the functional group on $C-1$: Retinol (hydroxyl group), Retinoic acid (carboxyl group), and Retinal (aldehyde group).

  • $\beta$-carotene: A precursor (carotene) converted to active forms.

• Metabolic Interconversion:

  • $\beta$-carotene is converted by $\beta$-carotene dioxygenase (utilizing $O_2$ and bile salts) into Retinaldehyde.

  • Retinaldehyde can be reduced to Retinol via Retinaldehyde reductase ($NADPH + H^+$ required).

  • Retinaldehyde can be oxidized to Retinoic acid.

• Maintenance of Epithelium: Retinol and Retinoic acid are required for growth and differentiation. They bind to intracellular receptors of the Zinc-finger protein family and regulate transcription via specific response elements.

• Role in Vision (Rhodopsin Cycle):

  • Retina has two receptor cells: Cones (iodopsin for color/bright light) and Rods (rhodopsin for dim light/night vision).

  • Rhodopsin: Consists of Opsin (apoprotein) and Retinine (11-cis-retinal prosthetic group).

  • Mechanism: The aldehyde group of 11-cis-retinal binds to the $\epsilon$-$NH_2$ group of lysine in Opsin. Light waves striking the rods cause chemical changes (splitting rhodopsin into opsin and all-trans-retinal), creating nerve impulses. All-trans-retinal must be converted back to 11-cis-retinal to sustain vision.

Vitamin D (Cholecalciferol)

• Chemistry and Precursors: Produced in human skin via UV light or obtained from fish liver.

  1. Ergosterol (Provitamin $D_2$): Found in plants; poorly absorbed.

  2. 7-dehydrocholesterol (Provitamin $D_3$): Formed from cholesterol in the liver/intestinal mucosa and passed to the skin.

• Structural Features: Hydroxyl group at $C3$, conjugated double bonds between $C5-C6$ and $C7-C8$, and a hydrocarbon chain at $C17$.

• Activation Pathway:

  1. Liver: Undergoes hydroxylation at $C-25$ by 25-hydroxylase to form 25-OH-$D_3$ (Calcidiol). This is the major storage and circulation form.

  2. Kidney: Undergoes hydroxylation at the 1-position by $1 \alpha$-hydroxylase in the proximal convoluted tubules to form 1,25-di-OH-$D_3$ (Calcitriol), the biologically active form.

• Regulation: Synthesis is inhibited by its own concentration (feedback), and regulated by Parathyroid hormone (PTH) and serum phosphate levels.

• Mode of Action: Calcitriol binds specifically to steroid-hormone-like receptors on DNA (containing Zinc-finger motifs). Thus, Vitamin D is a "prohormone" and Calcitriol is the "hormone."

• Functions: Intestinal absorption of Calcium and Phosphate; mineralization of bones.

• Clinical Deficiency: Rickets (children), Osteomalacia (adults), Renal Osteodystrophy. Also associated with diabetes and heart disease.

Vitamin E (Tocopherols)

• Forms: $\alpha$, $\beta$, $\gamma$, and $\delta$-tocopherol. The $\alpha$-tocopherol form is the most active.

• Chemical Property: The phenolic $-OH$ group on the 6th carbon of the chromane ring provides antioxidant activity.

• Metabolic Role: Eliminates free radicals to prevent peroxidative damage to unsaturated lipids in cell membranes.

• Regeneration: The process forms a tocopheroxy-free radical ($Toc.O\bullet$), which is reduced back to $\alpha$-tocopherol by Vitamin C or reduced glutathione ($G-SH$).

• Sources: Cottonseed, corn, sunflower, and wheat germ oils.

Vitamin K

• Variants:

  • $K_1$: Phylloquinone (natural, from plants like alfalfa and spinach).

  • $K_2$: Menaquinones (produced by intestinal bacteria).

  • $K_3$: Menadione (synthetic, water-soluble, 3 times more potent than natural forms).

• Function in Coagulation: Promotes post-transcriptional modification of factors II, VII, IX, and X.

  • Mechanism: Vitamin K is converted to hydroquinone in liver microsomes ($NADPH$ dependent). It acts as a coenzyme for carboxylase, which adds a $-COOH$ group to Glutamate residues, forming $\gamma$-carboxyglutamate. These residues provide Calcium binding sites necessary for coagulation factor activation.

• Deficiency: Unusual but seen with prolonged antibiotics, malabsorption, or in newborns (immediate post-natal period).

Vitamin C (Ascorbic Acid) Chemistry and Metabolism

• Structure: An enediol-lactone of an acid similar to L-glucose. Only the L-form is active.

• Properties: Strong reducing agent due to enediol $-OH$ groups at $C2$ and $C3$.

• Redox Cycle: L-ascorbic acid (Reduced) $\rightleftharpoons$ L-dehydroascorbic acid (Oxidized). Irreversible degradation leads to Diketogulonic acid, then Oxalic acid and L-Threonic acid.

• Human Synthesis: Humans lack the necessary enzymes to synthesize Ascorbic acid.

• Metabolic Functions:

  1. Hydrogen transport in cellular oxidation-reduction.

  2. Synthesis of hydroxyproline and hydroxylysine for collagen fiber maturity.

  3. Maintains Folic acid in its reduced form ($FH_4$).

  4. Roles in Tryptophan and Tyrosine metabolism.

• Fact: Vitamin C levels are lower in women taking oral contraceptive pills.

B-Complex: Thiamine ($B_1$) Details

• Structure: Composed of a pyrimidine ring and a thiazole ring. Biologically active form is Thiamine Pyrophosphate ($TPP$).

• Metabolism: Free thiamine is absorbed in the small intestine; the ester/pyrophosphate form is not. It is phosphorylated into $TPP$ mainly in the liver, muscles, and brain.

• Deficiency (Beriberi):

  • CV manifestations: Cardiac issues.

  • Neurological: Nervous system impairments.

  • GI symptoms: Digestive upset.