Phosphate Esters
Phosphoric acid (H₃PO₄) is a triprotic acid that forms the backbone of DNA, the high-energy bonds of ATP, and the head groups of phospholipids. Its three acidic protons ionize at different pKa values, giving rise to pH-dependent behavior central to biochemistry.
Phosphoric Acid Structure
Phosphoric acid has a central phosphorus atom bonded to four oxygens: three as -OH groups (ionizable) and one as a P=O double bond. The structure is tetrahedral around P (sp³).
Three pKa values:
| Ionization | pKa |
|---|---|
| H₃PO₄ ⇌ H₂PO₄⁻ | 2.1 |
| H₂PO₄⁻ ⇌ HPO₄²⁻ | 7.2 |
| HPO₄²⁻ ⇌ PO₄³⁻ | 12.7 |
At physiological pH 7.4, phosphoric acid is between pKa₂ (7.2) and pKa₃ (12.7). The dominant species are roughly 50-50 H₂PO₄⁻ and HPO₄²⁻, with a small amount of each at the far ends. The ~1:1 mix is a key biological buffer (phosphate buffer).
Phosphate Esters
A phosphate ester replaces one or more of phosphoric acid’s -OH groups with -OR. Three types:
- Monoester (R-O-PO₃²⁻): one OR, two ionizable -OH. Example: glucose-6-phosphate.
- Diester ((RO)₂-PO₂⁻): two OR groups, one ionizable -OH. Example: DNA phosphodiester backbone, phospholipids.
- Triester ((RO)₃-PO): three OR groups, no ionizable -OH. Example: organophosphate pesticides (mostly synthetic, rare in biology).
Phosphodiesters are essential in biology: DNA’s phosphate backbone connects one sugar’s 3’-OH to the next sugar’s 5’-OH via a phosphodiester. Phospholipids have one phosphodiester connecting glycerol’s 3-position to a polar head group (choline, serine, etc.).
Hydrolysis
Phosphate esters hydrolyze to the parent alcohol + phosphoric acid (or phosphate anion). Uncatalyzed hydrolysis is slow (half-life of years to millennia at neutral pH). Biology uses phosphatases to accelerate hydrolysis billions of times.
Phosphodiesters (like DNA) are especially slow to hydrolyze uncatalyzed - this is why DNA can preserve genetic information for millions of years in fossil samples. RNA hydrolyzes faster than DNA because the 2’-OH of ribose can attack the phosphate internally (enzymatic or spontaneous).
Phosphate as a Leaving Group
Phosphate can be a good leaving group in biological reactions. Examples:
- In glycolysis, a phosphate group leaves when sugar-phosphate becomes pyruvate.
- In ATP hydrolysis, the terminal phosphate leaves as Pi.
- In protein phosphorylation/dephosphorylation, kinases and phosphatases transfer phosphate between ATP and serine/threonine/tyrosine residues.
The phosphate’s leaving ability is enhanced by its resonance stabilization over four oxygens (when it leaves as HPO₄²⁻ or PO₄³⁻, the charge is delocalized).
Phosphate Transfer Reactions
Many enzymes transfer a phosphate from one molecule to another via a nucleophilic substitution at phosphorus. The mechanism can go through a trigonal bipyramidal intermediate (SN2-like at P) or through a metaphosphate-like intermediate (SN1-like).
For MCAT purposes, recognize that “phosphate transfer” is a type of nucleophilic acyl substitution at phosphorus, analogous to carbonyl NAS but with P instead of C as the central atom.
Phosphate in Buffers
The HPO₄²⁻/H₂PO₄⁻ pair has pKa 7.2, very close to physiological pH. This makes phosphate an excellent biological buffer - small changes in [H⁺] are absorbed by shifting the equilibrium between the two forms without dramatic pH changes.
Inside cells, phosphate buffer maintains cytoplasmic pH near 7.2. In blood, the main buffer is actually bicarbonate (HCO₃⁻/CO₂), but phosphate is a backup.