Chapter 20 Enzymes and Vitamins

1. Chapter 20 Enzymes and Vitamins 20.6Enzyme Cofactors and Vitamins

2. Enzyme Cofactors • Asimple enzyme is an active enzyme that consists only of protein. • Many enzymes are active only when they combine with cofactors such as metal ions or small molecules. • Acoenzymeis a cofactor that is a small organic molecule such as a vitamin.

3. Enzyme Cofactors (continued)

4. Function of Coenzymes • A coenzyme prepares the active site for catalytic activity.

5. Metal Ions as Cofactors Many active enzymes require a metal ion. For example, Zn2+, a cofactor for carboxypeptidase, stabilizes the carbonyl oxygen during the hydrolysis of a peptide bond.

6. Some Enzymes and Their Cofactors

7. Learning Check Identify each enzyme as 1) a simple enzyme 2) an enzyme that required a cofactor A. requires Mg2+ for hydrolysis of phosphate esters B. requires vitamin B3 to transfer an acetyl group C. is active with four polypeptide subunits

8. Solution Identify each enzyme as 1) a simple enzyme 2) an enzyme that required a cofactor 2 A. requires Mg2+ for hydrolysis of phosphate esters 2 B. requires vitamin B3 to transfer an acetyl group 1 C. is active with four polypeptide subunits

9. Water-Soluble Vitamins Water-soluble vitamins are • soluble in aqueous solutions • cofactors for many enzymes • not stored in the body

10. Descriptions of Water-Soluble Enzymes

11. Thiamin (Vitamin B1) Thiamin • was the first B vitamin identified • is part of the coenzyme thiamin pyrophosphate (TPP) • is used to decarboxylate -keto carboxylic acids • has a recommended daily allowance (RDA) of 1.2 mg; deficiencies include fatigue, poor appetite, weight loss, nerve degeneration, and heart failure

12. Riboflavin (Vitamin B2) Riboflavin • is found in the coenzymes flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN) • is needed for good vision and healthy skin • has an RDA of 1.2–1.8 mg; deficiencies include dermatitis, dry skin, tongue inflammation, and cataracts

13. Niacin (Vitamin B3) Niacin • is part of the coenzyme nicotinamide adenine dinucleotide (NAD+) involved in oxidation–reduction reactions • has an RDA of 14–18 mg • deficiency can result in dermatitis, muscle fatigue, and loss of appetite

14. Pantothenic Acid (Vitamin B5) Pantothenic acid • is part of coenzyme A needed for energy production • is involved in glucose and cholesterol synthesis • has an RDA of 5 mg • deficiency can result in fatigue, retarded growth, cramps, and anemia

15. Pyridoxine (Vitamin B6) Pyridoxineand pyridoxal are • two forms of vitamin B6, which are converted to the coenzyme pyridoxal phosphate (PLP) • required in the transamination of amino acids and decarboxylation of carboxylic acids • has an RDA of 1.3–2.0 mg; deficiency may lead to dermatitis, fatigue, and anemia

16. Cobalamin (Vitamin B12) Cobalamin • consists of four pyrrole rings with a Co2+ • is a coenzyme for enzymes that transfer methyl groups and produce red blood cells • has an RDA of 2.0–2.6μg • deficiencies are pernicious anemia, nerve damage, and malformed red blood cells

17. Ascorbic Acid (Vitamin C) Vitamin C • is required in collagen synthesis and healing of wounds • has an RDA of 60–95 mg • deficiencies are scurvy, weakened connective tissue, slow-healing wounds, and anemia

18. Biotin Biotin • is a coenzyme for enzymes that transfer carboxyl groups • has an RDA of 30 g • deficiencies include dermatitis, loss of hair, fatigue, and anemia

19. Folic Acid (Folate) Folic acid (folate) • consists of pyrimidine, p-aminobenzoic acid, and glutamate • forms the coenzyme THF used in the transfer of methyl groups and is involved in the synthesis of nucleic acids • has an RDA of 400  g • deficiencies are abnormal red blood cells, anemia, and poor growth

20. Folic Acid (Folate) (continued)

21. Fat-Soluble Vitamins Fat-soluble vitamins are • vitamins A, D, E, and K • soluble in lipids but not in aqueous solutions • stored in the body • important in vision, bone formation, antioxidants, and blood clotting

22. Retinol (Vitamin A) Vitamin A • is needed for retinol (vision); synthesis of RNA • has an RDA of 800  g • deficiencies include night blindness, immune system repression, and slowed growth

23. Cholecalciferol (Vitamin D) Vitamin D (D3) • is synthesized in skin exposed to sunlight • regulates the absorption of phosphorus and calcium during bone growth • has an RDA of 5–10 μg • deficiency includes weakened bones

24. Tocopherol (Vitamin E) Vitamin E • is an antioxidant in cells • may prevent the oxidation of unsaturated fatty acids • is found in whole grains and vegetables • has an RDA of 15 mg • deficiencies are hemolysis and anemia

25. Menaquinon (Vitamin K) • Vitamin K1 in plants has a saturated side chain. • Vitamin K2 in animals has a long unsaturated side chain. • Vitamin K2 is needed for the synthesis of zymogens for blood clotting. • Vitamin K has an RDA of 90–120 μg. • Deficiencies are prolonged bleeding time and bruising

26. Learning Check Identify each of the following as a water-soluble vitamin (WS) or fat-soluble vitamin (FS). A. folic acid B. retinol (vitamin A) C. vitamin C D. vitamin E E. niacin

27. Solution Identify each of the following as a water-soluble vitamin (WS) or fat-soluble vitamin (FS) WS A. folic acid FS B. retinol (vitamin A) WS C. vitamin C FS D. vitamin E WS E. niacin

28. Learning Check Identify the vitamin associated with each 1) riboflavin (B2) 2) vitamin A 3) vitamin K 4) vitamin D 5) ascorbic acid A. collagen formation B. part of the coenzymes FAD and FMN C. absorption of phosphorus and calcium in bone D. vision E. blood clotting

29. Solution Identify the vitamin associated with each 1) riboflavin (B2) 2) vitamin A 3) vitamin K 4) vitamin D 5) ascorbic acid 5 A. collagen formation 1 B. part of the coenzymes FAD and FMN 4 C. absorption of phosphorus and calcium in bone 2 D. vision 3 E. blood clotting