ATP and Coulpled Reactions
13.2
Phosphoryl Group Transfers and ATP
p. 496/501
13.2.1 The Free-Energy Change for ATP Hydrolysis is large and Negative
p. 496/501
13.2.2 Other Phosphorylated Compounds and Thioesters Also Have Large Free Energies of
Hydrolysis
p. /504
13.2.3 ATP Provides Energy by Group Transfers, Not by Simple Hydrolysis
p. /506
13.2.4 ATP Donates Phosphoryl, Pyrophosphoryl, and Adenylyl Groups
p. 502/507
13.2.5 Assembly of Informational Macromolecules Requires Free Energy
p. 504/508
13.2.6 ATP Energizes Active Transport and Muscle Contraction
p. 504/509
13.2.7 Transphosphorylations between Nucleotides Occur in all Types of Cells
p. 505/510
13.2.8 Inorganic Polyphosphate Is a Potential Phosphoryl Group Donor OMIT
13.2.9 Biochemical and Chemical Equations are not Identical OMIT
Suggested Problems in Chapter 13 of 4th ed.: 8, 9, 10, 11, 12, 13, 14, 15, 16.
Suggested Problems in Chapter 13 of 5th ed.: 9, 11, 12, 13, 14, 15, 19, 20, 21.
The Free-Energy Change for ATP Hydrolysis is large and Negative
o
Rationalizing the relatively large negative ΔG ' values of phosphoanhydride hydrolysis:
1.
2.
3.
-
++
Electrostatic Repulsion of O . Reduced by Mg .
Resonance stabilization (electron delocalization) in products.
+
Law of mass Action based on [H ].
Phosphorylation Potential
⎡ ADP ⎤⎦ ⎡⎣ Pi ⎤⎦
ΔG p = ΔG o ' + RT ln ⎣
⎡⎣ ATP ⎤⎦
Other Phosphorylated Compounds and Thioesters Also Have Large Free Energies of
Hydrolysis
Thioesters
High energy thioester in Acetyl CoA can be
exchanged for an ATP:
Acetyl~SCoA + Pi
Acetyl~P + ADP
-----> Acetyl~P + CoA
-----> ATP + Acetate
Acetyl~SCoA + ADP +Pi
-----> ATP + Acetate
ATP Provides Energy by Group Transfers, Not by Simple Hydrolysis
Coupled Reactions
Coupled reactions are frequently written as two "half-reactions", for the sake of bookeeping The half
reactions used for the sake of bookeeping are often not true representations of the enzyme
mechanism.
Example:
→
→
→
Glutamate + NH3
ATP
Glutamate ATP + NH3
Glutamine
ADP + Pi
Glutamine + ADP + Pi
Fig 13-8 shows the true nature of this reaction.
ATP Donates Phosphoryl, Pyrophosphoryl, and Adenylyl Groups
o
-4
-3
-2
+
ATP + H2O ↔ ADP + HPO4 + H
-4
-2
-3
+
ATP + H2O ↔ AMP + HP2O7 + H
-3
-2
+
HP2O7 + H2O ↔ 2 HPO4 + H
-2
-2
AMP + H2O ↔ A + HPO4
ΔG '
-1
-30 kJ mol
-1
-32 kJ mol
-1
-33 kJ mol
-1
-14 kJ mol
Inorganic Pyrophosphatase Catalyzes Additional Phosphoanhydride Bond Cleavage
The book uses fatty acid activation as an example of how adenylylation (α-β cleavage), followed by
pyrophosphate cleavage, can drive reactions that are too endergonic to be driven by typical γ-β bond
cleavage. Another example, more relevant to molecular biology types is amino acylation ("charging")
of tRNAs:
Amino Acid +ATP
→
Amino Acyl-AMP + PPi
Amino Acyl-AMP + tRNA
→ AA-tRNA + AMP
PPi + H2O
→ 2 Pi
AA + tRNA + ATP <---> AA-tRNA + AMP + 2 Pi
o
ΔG ' = ([+36]+[-46]) = -10 kJ mol
o
-1
ΔG ' = 0 kJ mol
o
-1
ΔG ' = -19 kJ mol
o
-1
ΔG ' = -29 kJ mol
-1
Assembly of Informational Macromolecules Requires Free Energy
The importance of ATP (and other rNTP's) in energy metabolism is viewed by some as a legacy of
the RNA world. Note in this context that ATP is also a precursor for RNA synthiesis.
ATP Energizes Active Transport and Muscle Contraction
Transphosphorylations between Nucleotides Occur in all Types of Cells
Kinases Interconvert Nucleoside Phosphates
Nucleoside monophosphate kinase
NMP + ATP -----> NDP + ADP
o
ΔG ’ = ~0 kJ/mole
Nucleoside diphosphate kinase
N1DP + N2TP -----> N1TP + N2DP
o
ΔG ’ = ~0 kJ/mole
Adenylate kinase
ATP + AMP -----> 2 ADP
o
ΔG ’ = ~0 kJ/mole