574
THE ORGANIC CHEMISTRY OF INTERMEDIARY METABOLISM
usually depicted. This is done primarily to simplify structures, to eliminate the need for counterions, and to avoid mechanistic confusion. Likewise,
amino acids are usually shown in non-ionized form,
although they will typically exist as zwitterions (see
Section 4.11.3):
R
CO 2
R
CO 2 H
NH 2
NH 3
H
H
zwitterion
Ionized and non-ionized forms of many compounds are regarded as synonymous in the text, thus
citrate/citric acid, acetate/acetic acid or others may
be used according to the author’s whim and context,
and should not be considered as having any especial
relevance.
15.1.1 Oxidation reactions and ATP
The currency unit for energy in biochemical
reactions is the nucleotide derivative ATP, adenosine
triphosphate. We have already discussed this
molecule in Chapter 7, where we rationalized many
of the reactions of phosphates in terms of them
being analogues of carbonyl compounds. Only the
triphosphate portion of ATP is involved chemically
in the energy processes; the remaining complex part
of the molecule is a recognizable feature that allows
binding to the enzyme.
The triphosphate portion can be visualized as containing two anhydride functions and one ester function. We have seen that hydrolysis of anhydrides
is achieved much more easily than is hydrolysis of
esters, an observation that can be related to the nature
of the leaving group (see Section 7.8). Accordingly,
hydrolysis of the anhydride bond liberates considerably more energy than does hydrolysis of the ester
bond, and it is anhydride hydrolysis that is crucial to ATP’s role in biochemistry. Hydrolysis of
ATP to ADP liberates energy, which can be coupled
to energy-requiring processes. Alternatively, energyreleasing processes can be coupled to the synthesis
of ATP from ADP.
ATP
ADP +
ADP
AMP +
both represent hydrolysis of anhydride
bond
− release significant amount of energy
ΔG –34 kJ / mole
AMP
+
hydrolysis of ester bond
− releases smaller amount of energy
ΔG –9 kJ / mole
hydrolysis of ATP to ADP
− coupled to energy-requiring processes
synthesis of ATP from ADP
− coupled to energy-releasing processes
currency unit for energy is ATP, adenosine triphosphate
Ad = adenosyl
ester
anhydride
ester
ATP
ADP
AMP
P
P
P
N
N
N
NH 2
O
HO
OH
N
CH 2
anhydride
adenosine triphosphate; ATP
adenosine
adenine
D-ribose
anhydride
ester
O
P
HO
O
P
O
O
OH OH
P
O
O
OH
Ad
anhydride
anhydride
ester
O
P
OH
O
P
O
O
OH
OH
P
O
O
OH
Ad
OH
P
O
O
OH
P
O
O
OH
Ad
OH
P
O
O
OH
A
THE ORGANIC CHEMISTRY OF INTERMEDIARY METABOLISM
usually depicted. This is done primarily to simplify structures, to eliminate the need for counterions, and to avoid mechanistic confusion. Likewise,
amino acids are usually shown in non-ionized form,
although they will typically exist as zwitterions (see
Section 4.11.3):
R
CO 2
R
CO 2 H
NH 2
NH 3
H
H
zwitterion
Ionized and non-ionized forms of many compounds are regarded as synonymous in the text, thus
citrate/citric acid, acetate/acetic acid or others may
be used according to the author’s whim and context,
and should not be considered as having any especial
relevance.
15.1.1 Oxidation reactions and ATP
The currency unit for energy in biochemical
reactions is the nucleotide derivative ATP, adenosine
triphosphate. We have already discussed this
molecule in Chapter 7, where we rationalized many
of the reactions of phosphates in terms of them
being analogues of carbonyl compounds. Only the
triphosphate portion of ATP is involved chemically
in the energy processes; the remaining complex part
of the molecule is a recognizable feature that allows
binding to the enzyme.
The triphosphate portion can be visualized as containing two anhydride functions and one ester function. We have seen that hydrolysis of anhydrides
is achieved much more easily than is hydrolysis of
esters, an observation that can be related to the nature
of the leaving group (see Section 7.8). Accordingly,
hydrolysis of the anhydride bond liberates considerably more energy than does hydrolysis of the ester
bond, and it is anhydride hydrolysis that is crucial to ATP’s role in biochemistry. Hydrolysis of
ATP to ADP liberates energy, which can be coupled
to energy-requiring processes. Alternatively, energyreleasing processes can be coupled to the synthesis
of ATP from ADP.
ATP
ADP +
ADP
AMP +
both represent hydrolysis of anhydride
bond
− release significant amount of energy
ΔG –34 kJ / mole
AMP
+
hydrolysis of ester bond
− releases smaller amount of energy
ΔG –9 kJ / mole
hydrolysis of ATP to ADP
− coupled to energy-requiring processes
synthesis of ATP from ADP
− coupled to energy-releasing processes
currency unit for energy is ATP, adenosine triphosphate
Ad = adenosyl
ester
anhydride
ester
ATP
ADP
AMP
P
P
P
N
N
N
NH 2
O
HO
OH
N
CH 2
anhydride
adenosine triphosphate; ATP
adenosine
adenine
D-ribose
anhydride
ester
O
P
HO
O
P
O
O
OH OH
P
O
O
OH
Ad
anhydride
anhydride
ester
O
P
OH
O
P
O
O
OH
OH
P
O
O
OH
Ad
OH
P
O
O
OH
P
O
O
OH
Ad
OH
P
O
O
OH
A