MECHANISM OF UREA PARTICIPATION IN PHOSPHONATE ESTER HYDROLYSIS - MECHANISTIC AND STEREOCHEMICAL CRITERIA FOR ENZYMIC FORMATION AND REACTION OF PHOSPHORYLATED BIOTIN

The mechanism of participation of a urea group in hydrolysis of a phosphonate ester in acidic solution was investigated. Participation of the oxygen center of the ureido group of 1 and 3 at the phosphonate center is consistent with (1) the high degree of facilitation of ester hydrolysis compared to molecules lacking the adjacent functional group, (2) the observation that species resulting from attack of proximal nitrogen atoms of 1 and 3 are more stable than their precursors, and (3) the observation that acidic, anhydrous media containing alcohols cause rapid transesterification. A detailed mechanistic examination suggests that the participation reaction involves initial formation of a pentacovalent phosphorus intermediate with the rate-determining step overall being breakdown of the intermediate to expel the leaving alcohol. The potential involvement of O-phosphobiotin in adenosine 5’-triphosphate (ATP) dependent carboxylations in enzymic systems is considered with regard to these results, enzymic results from other laboratories, and stereochemical restrictions. It is concluded that all available experimental evidence is consistent with, but does not require, intermediate formation of 0-phosphobiotin. Enzymic carboxylation of 6>-phosphobiotin, leading to formation of A’-carboxybiotin and phosphate, is considered in terms of the stereochemistry of displacement at the phosphorus center of phosphobiotin. An “adjacent” mechanism, not involving free carboxyphosphate, and an “in-line” mechanism, involving free carboxyphosphate are possible. Reactions not involving 0-phosphobiotin are likely to involve “in-line” displacement by bicarbonate at the terminal phosphate residue of ATP, requiring inversion of configuration at phosphorus. Since formation of 0-phosphobiotin should occur with inversion at phosphorus, and transfer to bicarbonate with inversion or retention, the route via 0-phosphobiotin can account for net inversion or net retention at phosphorus. © 1979, American Chemical Society. All rights reserved.