Ribonuclease A template

RNase A cuts RNA after a pyrimidine: His12 removes the 2′-OH proton so the 2′-O attacks phosphorus, while His119–H⁺ protonates the 5′-oxygen of the leaving group (transphosphorylation, 2′,3′-cyclic phosphate); in the second step His119 activates water and His12–H⁺ protonates the 2′-O to give the 3′-phosphate.

Mechanism step by step

His12 (base) and His119 (acid) make a 2′,3′-cyclic phosphate, then reverse roles to hydrolyse it

1. Transphosphorylation – His12 takes the 2′-OH proton; 2′-O attacks P in line; His119–H⁺ protonates the leaving 5′-O
2. Hydrolysis – His119 activates water to attack P; His12–H⁺ protonates the leaving 2′-O

Key points

• The 2′-OH is essential: DNA, which lacks it, is not cut by RNase A (and RNA is far less stable than DNA in base).
• Lys41 and backbone NHs stabilise the extra negative charge of the pentacoordinate phosphorane transition state.
• The bell-shaped pH–rate profile (optimum near pH 6–7) reflects one His as base and the other as acid.
• Anfinsen used RNase A to show that sequence alone determines the fold (reduced, denatured RNase refolds to full activity).

Drawn with 152 atoms, 11 arrows, 6 shapes – every one editable in MolDraw.

Sources

• Voet & Voet, Biochemistry, 4th ed., Wiley 2011, ch. 15 Textbook
• Raines, Chem. Rev. 1998, 98, 1045 Review