Catalytically perfect enzyme
A catalytically perfect enzyme or kinetically perfect enzyme is an enzyme that catalyzes so efficiently, that almost every time enzyme meets its substrate, the reaction occurs. kcat/Km factor of such enzyme is of order 108 to 109 M-1 s-1. Such reaction is only limited by substrate diffusion rate.
Some enzymes operate with kinetics which are faster than diffusion rates, which would seem to be impossible. Several mechanisms have been invoked to explain this phenomenon. Some proteins are believed to accelerate catalysis by drawing their substrate in and preorienting them by using dipolar electric fields. Some invoke a quantum-mechanical tunneling explanation whereby a proton or an electron can tunnel through activation barriers, although for protons tunneling remains somewhat controversial. 
- Mireia Garcia-Viloca, Jiali Gao, Martin Karplus, Donald G. Truhlar Science 9 January 2004: Vol. 303. no. 5655, pp. 186 - 195
- Mats M. H. Olsson, Per E. M. Siegbahn, Warshel A. J Am Chem Soc. 2004 Mar 10;126(9):2820-2828.