Polyadenylate polymerase (PAP) catalyzes the synthesis of 3'-polyadenylate tails onto mRNA.A comprehensive steady-state kinetic analysis of PAP was conducted which included initial velocity studiesof the forward and reverse reactions, inhibition studies, and the use of alternative substrates. The reaction(A
n + ATP
![](/images/entities/harr.gif)
A
n+1 + PP
i) is adequately described by a rapid equilibrium random mechanism. Severalthermodynamic parameters for the reaction were determined or calculated, including the overall equilibriumconstant (
Keq = 84) and the apparent equilibrium constant of the internal step (
Kint = 4) which involvesthe rate-determining interconversion of central complexes. A large (100-fold) difference in
Vmax accountsfor nucleotide specificity (ATP vs CTP), despite an only 3-fold difference in
Km. Comparison of thesulfur elemental effect on
Vmax for ATP and CTP suggests that the chemical step is rate-determining forboth reactions. Comparison of the sulfur elemental effect on
Vmax/
Km revealed differences in the mechanismby which either nucleotide is incorporated. Consistent with these data, an induced fit mechanism fornucleotide specificity is proposed whereby PAP couples a uniform binding mechanism, which selects forATP, with a ground-state destabilization mechanism, which serves to accelerate the velocity for the correctsubstrate.