Download Structure of Coenzyme A

22.2 Important Coenzymes in Metabolic
Pathways
Coenzyme A (CoA) is made up of several components: pantothenic
acid (vitamin B5), phosphorylated ADP, and aminoethanethiol.
Learning Goal Describe the components and functions of the
coenzymes NAD+, NADP+, FAD, and coenzyme A.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Metabolic Reactions
Metabolic reactions that extract energy from food involve
both oxidation and reduction reactions.
• Oxidation involves the loss of hydrogen or electrons
by a substance or an increase in the number of bonds
to oxygen.
• Reduction is the gain of hydrogen ions and electrons
or a decrease in the number of bonds to oxygen.
In both types of reactions, coenzymes are required to
carry the hydrogen ions and electrons from or to the
reacting substrate.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Metabolic Reactions
A coenzyme that gains hydrogen ions and electrons is
reduced, whereas a coenzyme that loses hydrogen
ions and electrons to a substrate is oxidized.
Core Chemistry Skill Identifying Important Coenzymes
in Metabolism
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Structure of Coenzyme NAD+
NAD+, nicotinamide adenine dinucleotide,
• is an important coenzyme in which the vitamin niacin
provides the nicotinamide group, which is bonded to
ribose and the nucleotide adenosine diphosphate (ADP).
• is required in dehydrogenation reactions that produce
carbon–oxygen double bonds, such as the oxidation of
alcohols to aldehydes and ketones.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Coenzyme NAD+
The oxidized form of
NAD+ undergoes
reduction when a
carbon atom in the
nicotinamide ring
reacts with 2H (two
hydrogen ions and
two electrons),
leaving one H+.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Coenzyme NADP+
NADP+, nicotinamide
adenine dinucleotide
phosphate,
• is used in anabolic
reactions, such as
lipid and nucleic acid
synthesis.
• is similar to NAD+
except that a 2′ OH
group is replaced by
a phosphate group.
• is reduced to form
NADPH.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Structure, Coenzyme FAD
FAD, flavin adenine dinucleotide,
• contains ADP and riboflavin (vitamin B2).
• undergoes reduction when the two nitrogen atoms
in the flavin part of the FAD coenzyme react with
two hydrogen atoms (2H+ + 2 e−), reducing it to
FADH2.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Structure of Coenzyme FAD
The coenzyme FAD (flavin adenine dinucleotide) made from riboflavin (vitamin B2) and
adenosine diphosphate is reduced to FADH2 by adding two hydrogen atoms.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Function, Coenzyme FAD
FAD, flavin adenine dinucleotide,
• participates in reactions that produce a carbon-carbon
double bond.
• is reduced to FADH2 with the aide of enzyme succinate
dehydrogenase.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Structure of Coenzyme A
Coenzyme A is derived from a phosphorylated ADP and
pantothenic acid bonded by an amide bond to
aminoethanethiol, which contains the —SH reactive part
of the molecule.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Function, Coenzyme A
Important functions of coenzyme A include
• preparation of small acyl groups such as acetyl for
reactions with enzymes.
• production of the energy-rich thioester acetyl CoA.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Types of Metabolic Reactions
Metabolic reactions, which take place at body temperature
and physiological pH, require enzymes and often
coenzymes.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Study Check
Match the following terms to their description.
1) NAD+
2) FAD
3) NADH + H+
4) FADH2
5) coenzyme A
A. coenzyme used in oxidation of
carbon-oxygen bonds
B. reduced form of flavin adenine
dinucleotide
C. used to transfer acetyl groups
D. contains riboflavin
E. the coenzyme after C O bond
formation
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Solution
Match the following terms to their description.
1) NAD+
2) FAD
3) NADH + H+
4) FADH2
5) coenzyme A
A. coenzyme used in oxidation of
carbon-oxygen bonds
B. reduced form of flavin adenine
dinucleotide
C. used to transfer acetyl groups
D. contains riboflavin
E. the coenzyme after C O bond
formation
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
1) NAD+
4) FADH2
5) coenzyme A
2) FAD, 4) FADH2
3) NADH + H+
© 2016 Pearson Education, Inc.