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B vitamins function as coenzymes essential for energy production from carbohydrates, fats, and amino acids. Thiamine forms thiamine pyrophosphate for pyruvate dehydrogenase and transketolase; riboflavin forms FAD/FADH2 for oxidative phosphorylation; niacin forms NAD+/NADH for glycolysis and the citric acid cycle. A deficiency in any B vitamin disrupts multiple metabolic pathways and impairs ATP production.
Map each B vitamin to its specific coenzyme form and identify all the metabolic enzymes that require it. Compare the metabolic consequences of thiamine, riboflavin, and niacin deficiency to understand their distinct roles.
From your study of enzyme cofactors and coenzymes, you know that many enzymes cannot catalyze reactions alone — they need small non-protein molecules to carry functional groups, electrons, or other chemical species from one reaction to another. B vitamins are the dietary precursors to the most important of these carriers in energy metabolism. The body cannot synthesize them in sufficient quantities, so they must come from food. Without them, the metabolic pathways you studied in glucose metabolism — glycolysis, the pyruvate dehydrogenase reaction, the citric acid cycle, and oxidative phosphorylation — stall at specific steps.
Thiamine (B1) is converted to thiamine pyrophosphate (TPP), which is required by enzymes that cleave carbon-carbon bonds adjacent to carbonyl groups. The most important are pyruvate dehydrogenase (converting pyruvate to acetyl-CoA), α-ketoglutarate dehydrogenase (a citric acid cycle step), and transketolase (the pentose phosphate pathway). All three are at critical metabolic junctions. When thiamine is deficient (as in beriberi, historically from polished-rice diets, or in alcoholics with poor nutrition), pyruvate cannot enter the citric acid cycle and accumulates — lactate and pyruvate levels rise in blood. Brain and heart tissue, which are entirely dependent on aerobic glucose metabolism, are most vulnerable: Wernicke's encephalopathy (ophthalmoplegia, ataxia, confusion) reflects the acute neurological crisis of thiamine deficiency, and it responds dramatically to IV thiamine.
Riboflavin (B2) forms two coenzymes: FAD and FMN. These accept two hydrogen atoms (two electrons + two protons) at their flavin ring during oxidation reactions and donate them during reduction, functioning as electron carriers. FAD appears in the citric acid cycle (succinate dehydrogenase, which is also Complex II of the electron transport chain), in fatty acid β-oxidation at each cycle's first oxidation step, and in the electron transport chain itself. Unlike NAD⁺/NADH (which you studied in glucose metabolism), FAD is tightly bound to the enzymes it works with — it rarely dissociates freely. Riboflavin deficiency impairs multiple pathways simultaneously but rarely in isolation; in practice it occurs alongside other B-vitamin deficiencies.
Niacin (B3) forms NAD⁺ and NADP⁺, the most abundant redox coenzymes in the cell. NAD⁺ accepts a hydride ion (H⁻, equivalent to 2 electrons + 1 proton) to become NADH, and NADH delivers electrons to Complex I of the electron transport chain to drive ATP synthesis. Because NAD⁺/NADH is involved in glycolysis (two NADH per glucose), the pyruvate dehydrogenase reaction (one NADH per pyruvate), and three steps of the citric acid cycle, niacin deficiency broadly impairs energy production. NADP⁺/NADPH has a different role: it drives biosynthetic reactions (fatty acid synthesis, cholesterol synthesis) and maintains glutathione in its reduced (antioxidant) form. Niacin deficiency causes pellagra — the "3 D's": dermatitis, diarrhea, dementia — reflecting its essential role in high-turnover tissues like skin, gut epithelium, and neurons.
The broader principle: each B vitamin's metabolic role is defined by the chemical reaction its coenzyme form catalyzes. Thiamine handles carbon-carbon bond cleavage near carbonyls, riboflavin handles two-electron transfers via the flavin ring, and niacin handles hydride transfer via the nicotinamide ring. Deficiency in any one creates a specific metabolic bottleneck — identifiable by which reactions are blocked and which substrates accumulate. This is why B-vitamin deficiencies produce distinctive clinical syndromes rather than generic malnutrition, and why understanding coenzyme chemistry lets you predict and reason through those syndromes rather than merely memorizing them.