General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 1
Chapter 20 Enzymes and Vitamins
20.6Enzyme Cofactors and Vitamins
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Enzyme Cofactors
A simple 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.
A coenzyme is a cofactor that is a small organic molecule such as a vitamin.
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Enzyme Cofactors (continued)
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Function of Coenzymes
A coenzyme prepares the active site for catalytic activity.
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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.
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Some Enzymes and Their Cofactors
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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
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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
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Water-Soluble Vitamins
Water-soluble vitamins are soluble in aqueous solutions cofactors for many enzymes not stored in the body
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Descriptions of Water-Soluble Enzymes
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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
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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
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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
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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
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Pyridoxine (Vitamin B6)
Pyridoxine and 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
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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 2.0–2.6 μg deficiencies are pernicious
anemia, nerve damage, and malformed red blood cells
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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
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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
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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
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Folic Acid (Folate) (continued)
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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
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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
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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
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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
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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
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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
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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
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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
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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
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