Vitamins are small organic molecules that the body needs in tiny amounts but cannot always make in sufficient supply. Many vitamins matter because they are converted into cofactors, which help enzymes carry out chemical reactions that would otherwise be too slow or too difficult. In metabolism, these vitamin-derived helpers are essential for releasing energy from food, building biomolecules, repairing tissues, and protecting cells from damage.
A cofactor can act like a removable tool that plugs into an enzyme active site and gives the enzyme new chemical abilities. Some cofactors transfer electrons, some carry carbon groups, some stabilize charged intermediates, and some help rearrange bonds during a reaction. Water-soluble vitamins often become coenzymes that move between enzymes, while fat-soluble vitamins are often stored in membranes and fatty tissues and can act in signaling, vision, or antioxidant roles.
Key Facts
- Water-soluble vitamins include B vitamins and vitamin C, and many must be consumed regularly because excess amounts are often excreted in urine.
- Fat-soluble vitamins include A, D, E, and K, and they are absorbed with dietary fat and stored in fatty tissues or the liver.
- A coenzyme is an organic cofactor, often derived from a vitamin, that helps an enzyme catalyze a reaction.
- NAD+ + 2 e- + H+ = NADH shows how niacin-derived NAD+ carries electrons in oxidation-reduction reactions.
- FAD + 2 H+ + 2 e- = FADH2 shows how riboflavin-derived FAD carries electrons in energy metabolism.
- Reaction rate with enzyme and cofactor is usually much greater than reaction rate with enzyme alone when the cofactor is required.
Vocabulary
- Vitamin
- A vitamin is an organic nutrient required in small amounts for normal metabolism, growth, and health.
- Cofactor
- A cofactor is a nonprotein helper, either an ion or an organic molecule, that an enzyme needs for activity.
- Coenzyme
- A coenzyme is an organic cofactor that helps enzymes by carrying electrons, atoms, or chemical groups.
- Active site
- The active site is the region of an enzyme where substrates bind and the chemical reaction takes place.
- Prosthetic group
- A prosthetic group is a cofactor that remains tightly bound to an enzyme during its catalytic cycle.
Common Mistakes to Avoid
- Treating vitamins as energy sources is wrong because vitamins do not directly provide calories like carbohydrates, fats, or proteins.
- Confusing cofactors with enzymes is wrong because cofactors assist enzymes but are not usually protein catalysts by themselves.
- Assuming all vitamins leave the body quickly is wrong because fat-soluble vitamins can accumulate in tissues and may become toxic in excess.
- Thinking an enzyme always works without its cofactor is wrong because many enzymes require the correct cofactor to bind substrates, transfer electrons, or stabilize reaction intermediates.
Practice Questions
- 1 A student consumes 60 mg of vitamin C and excretes 25 mg in urine. How many milligrams remain available for use or storage in the body during that period?
- 2 One molecule of NAD+ accepts 2 electrons during a metabolic reaction. How many electrons are accepted by 150 molecules of NAD+?
- 3 Explain why a deficiency of a B vitamin can slow a metabolic pathway even if the enzyme proteins in that pathway are present.