Download High-Energy Phosphate Compounds

High-Energy Phosphate Compounds:
Free Energy Sources in Biological Systems
How can intrinsically unfavorable reactions (∆Go > 0) be driven?
Coupled Reactions:
For
A
B (∆Go = +10 kJ/mol) and
C
D (∆Go = -30kJ/mol),
A+C
B + D (∆Go = +10 - 30 = -20 kJ/mol)
The overall ∆Go for the coupled reactions is negative. Therefore,
despite the uphill conversion of A to B this reactions proceeds.
High-Energy Phosphate Compounds as Energy Shuttles
High-Energy Phosphate Compounds:
Free Energy Sources in Biological Systems
High-Energy Phosphate Compounds as Energy Shuttles
Adenine
triphosphate
Ribose
Adenosine
Adenosine triphosphate
(ATP)
ATP is the most important high energy compound that has a very negative standard state
free energy of hydrolysis.
High-Energy Phosphate Compounds:
Free Energy Sources in Biological Systems
High-Energy Phosphate Compounds as Energy Shuttles
High-Energy Phosphate Compounds:
Free Energy Sources in Biological Systems
High-Energy Phosphate Compounds as Energy Shuttles
High-Energy Phosphate Compounds:
Free Energy Sources in Biological Systems
High-Energy Phosphate Compounds as Energy Shuttles
∆Go
Transfer
(kJ/mol) potential
High-Energy Phosphate Compounds:
Free Energy Sources in Biological Systems
1. Resonance Stabilization of the Phosphate Products
Not all of these resonance forms are possible when the phosphate, Pi, is bound in
an ester, therefore release of Pi upon hydrolysis results in an entropy increase and
is therefore favored.
High-Energy Phosphate Compounds:
Free Energy Sources in Biological Systems
2. Additional Hydration of the Hydrolysis Products
Release of the phosphate group allows greater opportunities for hydration,
especially when both products are charged.
3. Electrostatic Repulsion Between Charged Products
When both products of hydrolysis are negatively charged, the repulsion between
the two species favors the hydrolysis reaction.
4. Enhanced Resonance Stabilization of Product Molecules
Creatine has three resonance forms:
High-Energy Phosphate Compounds:
Free Energy Sources in Biological Systems
5. Release of a Proton in Buffered Solutions
ATP4- + H2O
ADP3- + HPO42- + H+
Hydrolysis reactions in vivo occur at a constant pH, near 7 ([H+]=10-7 M). The
hydrolysis of ATP for example occurs in the presence of a vast excess of H2O, and
under conditions in which the H+ concentration is kept constant and low. These
conditions drive the reaction to the right.
High-Energy Phosphate Compounds:
Free Energy Sources in Biological Systems
Real Examples:
From Figure 3.7
Using the phosphate transfer potentials, you can determine which compounds can
phosphorylate others. For example,
(1) Hydrolysis of phosphoenolpyruvate
(2) Phosphorylation of adenosine diphosphate
PEP
pyruvate + Pi
ADP + Pi
ATP
(1)+(2) Coupled phosphorylation of ADP by PEP
PEP + ADP
∆Go = -62 kJ/mol
∆Go = +31kJ/mol
pyruvate + ATP
∆Go = -31 kJ/mol
High-Energy Phosphate Compounds:
Free Energy Sources in Biological Systems
ATP
ATP is probably the single most important substance in biochemistry!
Energy obtained from photosynthesis or the breakdown of foodstuffs (catabolism) is stored
in ATP. This energy can then be used, via coupled hydrolysis, to drive life processes.
It is essential that ATP not be wasted. That is, ATP should NOT be hydrolyzed randomly in
the cell. Fortunately, hydrolysis of ATP is kinetically slow (although thermodynamically
favored). Enzymes are needed to catalyze the hydrolysis ATP as well as to couple the
hydrolysis of ATP to other reactions.