Ribonuclease A, a well-characterized enzyme from beef pancreas, catalyzes the hydrolysis of RNA into its component ribonucleic acids. When this reaction is studied with an isotopically chiral sulfur analog, as shown in Fig. P25.34, the reaction occurs with overall retention of stereochemistry. No phosphoenzyme intermediate is formed. Give a curved-arrow mechanism that explains this stereochemical result. (Assume that acidic and basic groups are present in the enzyme active site as needed.)

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HO 18 O P=S HO uracil (base) OH 16 + H₂O cytosine (base) OH a thio-substituted ribodinucleotide Figure P25.34 ribonuclease A HO 18 O 16. uracil (base) OH P =S HO- HO + cytosine (base) OH