enzyme cofactors_and_vitamins

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 1

Chapter 20 Enzymes and Vitamins

20.6Enzyme Cofactors and Vitamins


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.


Enzyme Cofactors (continued)


Function of Coenzymes

A coenzyme prepares the active site for catalytic activity.


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.


Some Enzymes and Their Cofactors


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


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


Water-Soluble Vitamins

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

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc.

Descriptions of Water-Soluble Enzymes

10


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


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


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


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


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


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


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

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc.

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

18


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


Folic Acid (Folate) (continued)


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


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


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


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


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


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


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


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


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

enzyme cofactors_and_vitamins

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