18.9 Enzyme Cofactors and Vitamins

Learning Objective

  1. Explain why vitamins are necessary in the diet.

Many enzymes are simple proteins consisting entirely of one or more amino acid chains. Other enzymes contain a nonprotein component called a cofactor that is necessary for the enzyme’s proper functioning. There are two types of cofactors: inorganic ions [e.g., zinc or Cu(I) ions] and organic molecules known as coenzymes. Most coenzymes are vitamins or are derived from vitamins.

Vitamins are organic compounds that are essential in very small (trace) amounts for the maintenance of normal metabolism. They generally cannot be synthesized at adequate levels by the body and must be obtained from the diet. The absence or shortage of a vitamin may result in a vitamin-deficiency disease. In the first half of the 20th century, a major focus of biochemistry was the identification, isolation, and characterization of vitamins.

Despite accumulating evidence that people needed more than just carbohydrates, fats, and proteins in their diets for normal growth and health, it was not until the early 1900s that research established the need for trace nutrients in the diet.

Because organisms differ in their synthetic abilities, a substance that is a vitamin for one species may not be so for another. Over the past 100 years, scientists have identified and isolated 13 vitamins required in the human diet and have divided them into two broad categories: the fat-soluble vitamins, which include vitamins A, D, E, and K, and the water-soluble vitamins, which are the B complex vitamins and vitamin C. All fat-soluble vitamins contain a high proportion of hydrocarbon structural components. There are one or two oxygen atoms present, but the compounds as a whole are nonpolar. In contrast, water-soluble vitamins contain large numbers of electronegative oxygen and nitrogen atoms, which can engage in hydrogen bonding with water. Most water-soluble vitamins act as coenzymes or are required for the synthesis of coenzymes. The fat-soluble vitamins are important for a variety of physiological functions. The key vitamins and their functions are found in Table 18.8 “Fat-Soluble Vitamins and Physiological Functions” and Table 18.9 “Water-Soluble Vitamins and Physiological Functions”.

Table 18.8 Fat-Soluble Vitamins and Physiological Functions

Vitamin Physiological Function Effect of Deficiency
vitamin A (retinol) formation of vision pigments; differentiation of epithelial cells night blindness; continued deficiency leads to total blindness
vitamin D (cholecalciferol) increases the body’s ability to absorb calcium and phosphorus osteomalacia (softening of the bones); known as rickets in children
vitamin E (tocopherol) fat-soluble antioxidant damage to cell membranes
vitamin K (phylloquinone) formation of prothrombin, a key enzyme in the blood-clotting process increases the time required for blood to clot

Table 18.9 Water-Soluble Vitamins and Physiological Functions

Vitamin Coenzyme Coenzyme Function Deficiency Disease
vitamin B1 (thiamine) thiamine pyrophosphate decarboxylation reactions beri-beri
vitamin B2 (riboflavin) flavin mononucleotide or flavin adenine dinucleotide oxidation-reduction reactions involving two hydrogen atoms —
vitamin B3 (niacin) nicotinamide adenine dinucleotide or nicotinamide adenine dinucleotide phosphate oxidation-reduction reactions involving the hydride ion (H−) pellagra
vitamin B6 (pyridoxine) pyridoxal phosphate variety of reactions including the transfer of amino groups —
vitamin B12 (cyanocobalamin) methylcobalamin or deoxyadenoxylcobalamin intramolecular rearrangement reactions pernicious anemia
biotin biotin carboxylation reactions —
folic acid tetrahydrofolate carrier of one-carbon units such as the formyl group anemia
pantothenic Acid coenzyme A carrier of acyl groups —
vitamin C (ascorbic acid) none antioxidant; formation of collagen, a protein found in tendons, ligaments, and bone scurvy

Vitamins C and E, as well as the provitamin β-carotene can act as antioxidants in the body. Antioxidants prevent damage from free radicals, which are molecules that are highly reactive because they have unpaired electrons. Free radicals are formed not only through metabolic reactions involving oxygen but also by such environmental factors as radiation and pollution.

Note

β-carotene is known as a provitamin because it can be converted to vitamin A in the body.

Free radicals react most commonly react with lipoproteins and unsaturated fatty acids in cell membranes, removing an electron from those molecules and thus generating a new free radical. The process becomes a chain reaction that finally leads to the oxidative degradation of the affected compounds. Antioxidants react with free radicals to stop these chain reactions by forming a more stable molecule or, in the case of vitamin E, a free radical that is much less reactive. (Vitamin E is converted back to its original form through interaction with vitamin C.)

Concept Review Exercises

  1. What is the difference between a cofactor and a coenzyme?

  2. How are vitamins related to coenzymes?

answers

Key Takeaways

  • Vitamins are organic compounds that are essential in very small amounts for the maintenance of normal metabolism.
  • Vitamins are divided into two broad categories: fat-soluble vitamins and water-soluble vitamins.
  • Most water-soluble vitamins are needed for the formation of coenzymes, which are organic molecules needed by some enzymes for catalytic activity.

Exercises

  1. Identify each vitamin as water soluble or fat soluble.

    1. vitamin D
    2. vitamin C
    3. vitamin B12

  2. Identify each vitamin as water soluble or fat soluble.

    1. niacin
    2. cholecalciferol
    3. biotin

  3. What vitamin is needed to form each coenzyme?

    1. pyridoxal phosphate
    2. flavin adenine dinucleotide
    3. coenzyme A
    4. nicotinamide adenine dinucleotide

  4. What coenzyme is formed from each vitamin?

    1. niacin
    2. thiamine
    3. cyanocobalamin
    4. pantothenic acid

  5. What is the function of each vitamin or coenzyme?

    1. flavin adenine dinucleotide
    2. vitamin A
    3. biotin

  6. What is the function of each vitamin or coenzyme?

    1. vitamin K
    2. pyridoxal phosphate
    3. tetrahydrofolate

Answers