HYDROLYSIS REACTIONS OF N-PHOSPHOAMINO ACIDS—A MODEL FOR PROTEIN DEPHOSPHORYLATION

Abstract

Phosphoamino acids are the smallest units of phosphoprotein. In Tris-HCl buffer (pH7.5)/DMSO(v/v=1:1) mixed solvent at 40°C the kinetic rates of N-phosphoamino acid hydrolysis reactions were studied. The reactions were pseudo first-order. and the hydrolysis rates of the N-phosphoamino acids were much faster than those of simple phosphoamidates under the same conditions. The kinetic results suggested that the side chain groups of the amino acids played significant roles on the rate of the hydrolysis reaction. For example, the hydrolysis rate constant of a phosphohistidine that had a polar imidazole side chain group was k=32.5 × 10⁻⁶ sec⁻¹, whereas for phosphoglycine without a side chain group k=3.3 × 10⁻⁶ sec⁻¹. The hydrolysis products were identified. The coparticipation of the phosphoryl, carboxyl and amino acids' side chain groups to form a penta-coordinate phosphorus transition state is proposed for this reaction. The analogues of the proposed transition state were synthesized. The dephosphorylation and phosphotransfer mechanisms of phosphoprotein are proposed to occur through a penta-coordinate transition state.

Key Words:

• phosphoamino acids
• hydrolysis reaction
• kinetic study
• penta-coordinate phosphorus
• dephosphorylation
• phosphotransfer