Mechanism of tyrosine phosphorylation catalyzed by the insulin receptor tyrosine kinase: A semiempirical PM3 study

The phosphorylation of tyrosine catalyzed by the tyrosine kinase domain of the insulin receptor has been investigated by means of semiempirical (PM3) molecular orbital calculations. A mechanism comprising ATP protonation followed by a proton shift within ATP and subsequent elimination-addition of the metaphosphate anion (PO₃^-) is proposed. Both the proton shift and elimination steps are endoergonic, with associated enthalpies of 17 and 18 kcal/mol, respectively. On the other hand, the addition of PO₃^- to tyrosine is exoergonic, with an associated enthalpy of 15 kcal/mol. Furthermore, the possible structural and catalytic roles of the two Mg²⁺ ions experimentally located in the active site of the insulin kinase domain have also been critically examined.