pyrophosphate
Summary
Pyrophosphate (PPi, P₂O₇⁴⁻) is two phosphate groups joined by a phosphoanhydride bond. It is released when ATP is cleaved to AMP and when a nucleotide is added to DNA or RNA. Its hydrolysis to two Pi is strongly exergonic, so it helps drive otherwise unfavorable reactions forward.
Detail
Meaning: Pyrophosphate (PPi, P₂O₇⁴⁻) is two phosphate groups joined by a phosphoanhydride (P–O–P) bond. It is released when a nucleophile attacks the alpha phosphate of ATP, giving AMP and PPi, and when a nucleoside triphosphate is added to a growing DNA or RNA chain. Hydrolysis of PPi is strongly favorable because the products have less charge repulsion, more resonance stabilization and better solvation.
Equation: PPi + H₂O → 2 Pi, ΔG°' < 0, where PPi is pyrophosphate, Pi is inorganic phosphate (HPO₄²⁻ and H₂PO₄⁻ near pH 7), and ΔG°' is the standard free energy change at pH 7 in J/mol.
On the MCAT: A passage describes a step with an unfavorable ΔG, such as fatty acid activation or DNA synthesis, and asks why it proceeds. The answer is that PPi is hydrolyzed by pyrophosphatase. This coupled, exergonic step makes the overall process favorable and effectively irreversible.
Common confusion: The phosphoanhydride bond is called "high-energy," but breaking any bond costs energy. The free energy released comes from the greater stability of the products, not from energy stored in the bond. Also keep PPi (two phosphorus atoms) separate from Pi (one).
Sources
- Organic Chemistry (Klein)
- Organic Chemistry (McMurry)
- Lehninger Principles of Biochemistry
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.