Certain enzymes need very small amounts of nonprotein components.
These cofactors may be metal ions or they may be organic molecules
called coenzymes.
Vitamins are organic nutrients required in very small quantities;
many vitamins are known to function as coenzymes.
- Cofactors: Many enzymes (and also hemoglobin and chlorophyll)
cannot function properly unless a particular metallic ion is present as a cofactor.
Examples: iron (Fe3+) in hemoglobin;
iron (Fe3+) in the bacterial enzyme catalase;
magnesium (Mg2+) in chlorophyll.
- Coenzymes: Some enzymes need organic cofactors, called coenzymes.
Examples: malate dehydrogenase (in the Krebs cycle) requires the coenzyme NAD+;
hemoglobin requires a ring-like "heme" group.
- Vitamins are organic nutrients needed in our diets in very small quantities.
Many vitamins function as coenzymes.
- Fat-soluble vitamins can accumulate in the body's fat reserves;
chronic overdoses can therefore build up to toxic levels.
- Vitamin A is derived from carotene pigment. It forms rhodopsin,
a light-collecting (visual) pigment in the retina.
- Vitamin D prevents rickets, and aids in calcium and phosphorus metabolism.
The final step in its synthesis uses the ultraviolet rays present in sunlight.
- Vitamin E is an antioxidant that protects membranes.
- Vitamin K binds calcium ions, and is needed in blood coagulation (clotting).
- Water-soluble vitamins circulate in body fluids but do not accumulate because
they are excreted in urine; they must therefore be replenished frequently, and overdoses
are much less likely.
- Ascorbic acid (vitamin C) is an antioxidant. It promotes healthy mucous
membranes and collagen; it thus prevents scurvy.
- Thiamin (Vitamin B1) is needed in carbohydrate metabolism.
- Riboflavin (Vitamin B2) is part of coenzymes FMN and FAD.
- Niacin (Vitamin B3) helps make NAD and NADP coenzymes.
- Pantothenic acid is needed to make Coenzyme A.
- Other B-group vitamins include folate (folic acid or folacin),
biotin, pyridoxine (vitamin B6), and cyanocobalamin (vitamin B12).
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