enzyme cofactors_and_vitamins

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 1
Chapter 20 Enzymes and Vitamins
20.6
Enzyme 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 α-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 30µ 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 µ 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 µ 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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