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16.9 Enzyme Cofactors

Chapter 16 Amino Acids, Proteins, and Enzymes

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Function of Coenzymes

A coenzyme prepares the active site for catalytic activity.

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Water-Soluble Vitamins

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

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

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Fat-Soluble Vitamins

Fat-soluble vitamins• are A, D, E, and K. • are soluble in lipids, but not in aqueous solutions.• are important in vision, bone formation, antioxidants,

and blood clotting.• are stored in the body.

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

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Learning Check

Identify each compound as a

1) water-soluble vitamin 2) fat-soluble vitamin.

A. Folic acid

B. Retinol (Vitamin A)

C. Vitamin C

D. Vitamin E

E. Niacin

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Solution

Identify each compound as a

1) water-soluble vitamin 2) fat-soluble vitamin.

A. 1 Folic acid

B. 2 Retinol (Vitamin A)

C. 1 Vitamin C

D. 2 Vitamin E

E. 1 Niacin

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Thiamin (Vitamin B1)

Thiamin • was the first B vitamin identified.• is part of the coenzyme thiamin pyrophosphate (TPP), required in the decarboxylation of -keto carboxylic

acids.

• deficiency results in beriberi (fatigue, weight loss, and nerve degeneration).

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Riboflavin (Vitamin B2)

Riboflavin is• made of the sugar alcohol ribitol and flavin.• part of the coenzymes flavin adenine dinucleotide (FAD)

and flavin mononucleotide (FMN).• needed for good vision and healthy skin.

N

N N

NHH3C

H3C

CH2 CH CH CH CH2 OH

OHOHOH

O

O

D-Ribitol

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Niacin (Vitamin B3)

Niacin• is part of the coenzyme

nicotinamide adenine dinucleotide (NAD+) involved in oxidation-reduction reactions.

• deficiency can result in dermatitis, muscle fatigue, and loss of appetite.

• is found in meats, rice, and whole grains.

N

C

O

OH

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Pantothenic Acid (Vitamin B5)

Pantothenic acid• is part of coenzyme A needed for energy production

as well as glucose and cholesterol synthesis.• deficiency can result in fatigue, retarded growth,

cramps, and anemia.• is found in salmon, meat, eggs, whole grains, and

vegetables.

HO CH2 C CH C N CH2 CH2 C OH

O

H

OH OCH3

CH3

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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.

• deficiency can lead to pernicious anemia and nerve damage.

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Ascorbic Acid (Vitamin C)

Vitamin C• is required in collagen

synthesis.• deficiency can lead to

weakened connective tissue, slow-healing wounds, and anemia.

• is found in blueberries, citrus fruits, tomatoes, broccoli, red and green vegetables.

O CHOH

CH2OH

OHHO

O

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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 carbon groups and the synthesis of nucleic acids.

• deficiency can lead to abnormal red blood cells, anemia, and poor growth.

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Vitamin A

Vitamin A• is obtained from meats and beta-carotenes in plants.• has beta-carotenes that are converted by liver

enzymes to vitamin A (retinol).

H3C CH3

CH3

CH3 CH3

CH3 CH3H3C CH3

H3C

CH3 CH3

CH2OHH3C CH3

CH3

Beta-carotene

Retinol (vitamin A)

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Vitamin D

Vitamin D (D3)• is synthesized in skin

exposed to sunlight. • regulates the absorption of

phosphorus and calcium during bone growth.

• deficiency can result in weakened bones.

• sources include cod liver oil, egg yolk, and enriched milk.

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Vitamin E

Vitamin E• is an antioxidant in cells.• may prevent the oxidation of unsaturated fatty acids.• is found in vegetable oils, whole grains, and

vegetables.

O

CH3

HO

H3C

CH3

CH3

CH3

CH3 CH3 CH3

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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.

3n

CH3

CH3

O

O CH3 CH3

Vitamin K2 (menaquinone)

CH3

CH3

O

O CH3 CH3

Vitamin K1 (phylloquinone)

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Learning Check

Identify the vitamin associated with each1) Thiamin (B1) 2) Vitamin A

3) Vitamin K 4) Vitamin D5) Ascorbic Acid

A. Collagen formationB. BeriberiC. Absorption of phosphorus and calcium in bone D. VisionE. Blood clotting

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Solution

Identify the vitamin associated with each

1) Thiamin (B1) 2) Vitamin A

3) Vitamin K 4) Vitamin D

5) Ascorbic Acid

A. 5 Collagen formation

B. 1 Beriberi

C. 4 Absorption of phosphorus and calcium in bone

D. 2 Vision

E. 3 Blood clotting