Why Small Molecules Matter Enormously
Vitamins and minerals are micronutrients — substances required in small quantities for normal physiological function. Unlike carbohydrates, proteins, and fats (macronutrients), micronutrients do not provide energy directly. Instead, they serve as enzyme cofactors, antioxidants, structural components, and regulators of gene expression and cellular signaling. Their absence in the diet causes deficiency diseases, some of which have shaped human history.
Vitamins: Organic Micronutrients
Vitamins are organic compounds that the body cannot synthesize in adequate amounts and must obtain through food. They are classified into two groups based on solubility.
Fat-Soluble Vitamins (A, D, E, K)
These vitamins dissolve in lipids and are stored in adipose tissue and the liver. Because they accumulate, both deficiency and toxicity (hypervitaminosis) are possible.
Vitamin A (Retinol)
Chemical form: Retinol, retinal, retinoic acid (all derived from β-carotene, a provitamin).
Retinal is the light-absorbing chromophore in rhodopsin, the visual pigment of rod cells in the retina. When photons strike 11-cis-retinal, it isomerizes to all-trans-retinal, triggering the nerve impulse underlying vision. Vitamin A deficiency causes night blindness and, in severe cases, xerophthalmia (corneal ulceration).
Retinoic acid acts as a nuclear receptor ligand, regulating gene expression programs essential for cell differentiation, embryonic development, and immune function.
Vitamin D (Calciferols)
Chemical form: Cholecalciferol (D₃) from animal sources; ergocalciferol (D₂) from plants.
Vitamin D is unique: it is synthesized in the skin when UV-B radiation (290–315 nm) converts 7-dehydrocholesterol to previtamin D₃. Subsequent hydroxylation in the liver (→ 25-hydroxyvitamin D) and kidneys (→ 1,25-dihydroxyvitamin D, calcitriol) produces the active hormone.
Calcitriol regulates calcium and phosphorus absorption in the intestine, bone mineralization, and immune function. Deficiency causes rickets in children (soft, deformed bones) and osteomalacia in adults.
Vitamin E (Tocopherols)
Chemical form: α-Tocopherol (most biologically active).
Vitamin E is the body's primary fat-soluble antioxidant. It protects polyunsaturated fatty acids in cell membranes from peroxidation by donating a hydrogen atom to lipid peroxyl radicals, terminating chain reactions:
R–OO• + α-Tocopherol–OH → R–OOH + α-Tocopherol–O•
The resulting tocopheryl radical is relatively stable and regenerated by vitamin C or glutathione.
Vitamin K (Phylloquinone / Menaquinones)
Chemical form: K₁ (phylloquinone, from leafy greens), K₂ (menaquinones, from fermented foods and gut bacteria).
Vitamin K is a coenzyme for γ-glutamyl carboxylase, an enzyme that adds carboxyl groups to specific glutamate residues in clotting factors (II, VII, IX, X) and other proteins. This carboxylation is essential for calcium binding and activation. Without vitamin K, blood coagulation fails. Warfarin (a blood thinner) works by antagonizing vitamin K recycling.
Water-Soluble Vitamins (B-group and C)
These vitamins are not stored in large amounts; excess is excreted in urine, making toxicity rare but requiring regular dietary intake.
Vitamin B₁ (Thiamine)
As thiamine pyrophosphate (TPP), B₁ is an essential coenzyme for oxidative decarboxylation reactions, including pyruvate dehydrogenase and α-ketoglutarate dehydrogenase in the Krebs cycle. Deficiency causes beriberi (peripheral neuropathy and heart failure) or Wernicke's encephalopathy in alcoholics.
Vitamin B₂ (Riboflavin)
Precursor of FAD (flavin adenine dinucleotide) and FMN, electron carrier coenzymes critical in the electron transport chain (Complex II) and many oxidation-reduction reactions throughout metabolism.
Vitamin B₃ (Niacin)
Precursor of NAD⁺ and NADP⁺ — among the most important coenzymes in metabolism, carrying electrons in glycolysis, the Krebs cycle, fatty acid oxidation, and biosynthesis. Deficiency causes pellagra (dermatitis, diarrhea, dementia). The body can synthesize niacin from tryptophan, but this conversion is inefficient.
Vitamin B₅ (Pantothenic Acid)
Essential component of coenzyme A (CoA). CoA carries acetyl groups in the Krebs cycle (acetyl-CoA), fatty acid synthesis and oxidation, and the synthesis of cholesterol and steroid hormones.
Vitamin B₆ (Pyridoxine)
Active form: pyridoxal-5'-phosphate (PLP), a coenzyme for over 100 enzyme reactions involving amino acids — transamination, deamination, decarboxylation. Essential for synthesis of neurotransmitters (serotonin from tryptophan, dopamine from tyrosine), heme, and glycogen phosphorylase.
Vitamin B₇ (Biotin)
Coenzyme for carboxylase enzymes (e.g., pyruvate carboxylase, acetyl-CoA carboxylase), which fix CO₂ into organic molecules. Critical for gluconeogenesis and fatty acid synthesis. Raw egg whites contain avidin, a protein that tightly binds biotin and prevents its absorption — a hazard of consuming large amounts of raw eggs.
Vitamin B₉ (Folate / Folic Acid)
Active form: tetrahydrofolate (THF), a carrier of one-carbon units used in synthesis of purines, thymine, and the amino acid methionine. Deficiency during early pregnancy causes neural tube defects (spina bifida). Folate deficiency also causes megaloblastic anemia.
Methotrexate, a cancer drug and rheumatoid arthritis treatment, inhibits dihydrofolate reductase, blocking folate activation and thereby halting DNA synthesis in rapidly dividing cells.
Vitamin B₁₂ (Cobalamin)
The only vitamin with a metal ion (Co³⁺) at its core. B₁₂ is a coenzyme for two reactions: methylmalonyl-CoA mutase (fatty acid and amino acid metabolism) and methionine synthase (methionine and folate cycle). Deficiency causes pernicious anemia and neurological damage. B₁₂ is found only in animal products; strict vegetarians require supplementation.
Vitamin C (Ascorbic Acid)
Vitamin C is a powerful water-soluble antioxidant and essential coenzyme for: - Collagen synthesis: prolyl and lysyl hydroxylases require ascorbate as a reducing agent to hydroxylate proline and lysine residues, cross-linking collagen fibers - Carnitine synthesis: essential for fatty acid transport into mitochondria - Norepinephrine synthesis from dopamine - Iron absorption: reduces Fe³⁺ to Fe²⁺ in the gut
Deficiency causes scurvy (impaired collagen synthesis → bleeding gums, joint pain, poor wound healing), historically a major cause of death among sailors. Humans, unlike most animals, cannot synthesize ascorbic acid due to a non-functional copy of the gene encoding L-gulonolactone oxidase.
Minerals: Inorganic Micronutrients
Minerals are inorganic elements required for structural and functional roles. They are divided into macrominerals (required in >100 mg/day) and trace elements.
Key Macrominerals
- Calcium (Ca²⁺): bone and tooth structure (hydroxyapatite: Ca₁₀(PO₄)₆(OH)₂), muscle contraction, nerve signaling, enzyme activation
- Phosphorus (PO₄³⁻): backbone of DNA and RNA, ATP, phospholipid membranes, hydroxyapatite
- Magnesium (Mg²⁺): cofactor for >300 enzymes including all ATP-utilizing reactions (MgATP²⁻ is the true substrate), chlorophyll center
- Sodium and Potassium: maintain membrane potential and osmotic balance; Na⁺/K⁺-ATPase keeps cells polarized
Key Trace Elements
- Iron (Fe): center of heme in hemoglobin (O₂ transport), myoglobin, cytochromes (electron transport). Deficiency causes iron-deficiency anemia.
- Zinc (Zn²⁺): structural component of zinc finger proteins (transcription factors), cofactor for ~300 enzymes (carbonic anhydrase, carboxypeptidase, alcohol dehydrogenase)
- Iodine (I⁻): essential for synthesis of thyroid hormones (T₃ and T₄), which regulate basal metabolic rate. Deficiency causes goiter and cretinism.
- Selenium (Se): incorporated as selenocysteine into glutathione peroxidase (antioxidant) and iodothyronine deiodinases (thyroid hormone activation)
- Copper (Cu): cofactor for cytochrome c oxidase (Complex IV), superoxide dismutase, and lysyl oxidase (collagen/elastin crosslinking)
- Manganese (Mn): cofactor for mitochondrial superoxide dismutase, arginase, and enzymes in gluconeogenesis
Vitamins, Minerals, and Disease Prevention
The relationship between micronutrients and health extends far beyond simple deficiency diseases. Research continues to explore how optimal micronutrient status influences cancer prevention, cardiovascular disease, immune function, and cognitive health. The precision with which vitamins and minerals perform their molecular roles — as enzyme cofactors, electron carriers, and structural atoms — illustrates how intimately linked chemistry and health truly are.