Adenosine 5’-triphosphate (ATP) serves as the main source of free energy in living cells. The energy stored by ATP can be liberated through direct hydrolysis or by group transfer.
Direct hydrolysis of ATP consists of nucleophilic attack by water and has a highly negative free energy change (ΔG° = -30.5 kJ/mol). The energy is mainly released in the form of heat. This heat energy can be used to cycle proteins through different conformations (for instance, in muscle contraction).
Group transfer reactions involve the covalent transfer of a portion of the ATP molecule to a substrate (e.g. an enzyme active site), which in turn makes subsequent metabolic reactions involving this substrate more thermodynamically favorable. The substrates are commonly coupled to biological reactions that have a large positive standard free energy change. A common example of this when terminal hydroxyl groups in amino acids or alcohols attack phosphate groups via a nucleophilic mechanism resulting in the exchange of hydrogen atoms on a hydroxyl group for a phosphate group from ATP.
Practice Questions
Khan Academy
Basic concepts in bioenergetics: phosphoryl group transfers and ATP hydrolysis
The thermodynamics of ATP hydrolysis in living cells
MCAT Official Prep (AAMC)
Practice Exam 4 B/B Section Passage 1 Question 2
Key Points
• Adenosine 5’-triphosphate (ATP) serves as the main source of free energy in living cells.
• ATP can be hydrolyzed and release mainly heat.
• Groups from ATP can be transferred to substrates, which will make their future reactions more favourable.
Key Term
Adenosine 5’-triphosphate (ATP): main source of free energy in living cells