MEDIUM - S9 Bioenergetics and Biochemical Reactions

ATP and Phosphoryl Group Transfers

• ATP (Adenosine Triphosphate) is the major energy currency of the cell, produced in exergonic reactions and consumed in endergonic reactions via hydrolysis.
• ATP connects catabolism and anabolism.
• ATP has three components: adenine, ribose, and triphosphate groups.

ATP Hydrolysis

• ATP hydrolysis involves charge separation, which relieves electrostatic repulsion.
• Pi is stabilized by formation of a resonance hybrid.
• ADP2- immediately ionizes at pH 7, releasing a proton into the medium, lowering pH.
• Greater solvation of products Pi and ADP relative to ATP.

The Phosphorylation Potential, ∆GP

• The standard free energy of hydrolysis of ATP is -30.5 kJ/mol.
• The actual free energy of hydrolysis of ATP in the cell (ΔGp) differs from the standard free energy change in standard conditions (ΔG´o).
• ΔGp can be calculated using the equation: ΔGp = ΔG´° + RT ln ([ADP][Pi] / [ATP]).

ATP Donates Phosphoryl, Pyrophosphoryl, and Adenylyl Groups

• Each of the three phosphates of ATP is susceptible to nucleophilic attack.
• ATP drives unfavorable reactions not by harnessing the energy of hydrolysis but through the coupling of group transfer (phosphoryl, pyrophosphoryl, or adenylyl) to a substrate or enzyme.

Phosphoryl Group Transfer

• Phosphoryl group transfer is generally a two-step reaction.
• Phosphoryl group transfer is catalyzed by ATP-dependent enzymes, such as ATP-dependent glutamine synthetase.

Free Energy Changes Are Additive

• A favorable reaction (ΔG1 < 0) can drive an unfavorable reaction (ΔG2 > 0) when ΔG1 + ΔG2 < 0.
• This principle has important consequences in cellular metabolism, such as energy coupling.

Energy Coupling

• Energy coupling allows the energy from one reaction to drive another reaction.
• Example: the synthesis of glucose 6-phosphate, where the energy from ATP hydrolysis drives the reaction.