The rate of protein association places an upper limit on the response
time due to protein interactions, which, under certain circumstances,
can be diffusion-controlled. Simulations of model proteins show that d
iffusion-limited association rates are similar to 10(6)-10(7) M-1 s(-1
) in the absence of long-range forces (Northrup, S. H., and H. P. Eric
kson. 1992. Kinetics of protein-protein association explained by Brown
ian dynamics computer simulations. Proc. Natl. Acad. Sci. U.S.A. 89:33
38-3342), The measured association rates of barnase and barstar are 10
(8)-10(9) M-1 s(-1) at 50 mM ionic strength, and depend on ionic stren
gth (Schreiber, G., and A. R. Fersht. 1996. Rapid, electrostatically a
ssisted association of proteins. Nat. Struct Biol. 3:427-431), implyin
g that their association is electrostatically facilitated. We report B
rownian dynamics simulations of the diffusional association of barnase
and barstar to compute association rates and their dependence on ioni
c strength and protein mutation. Crucial to the ability to reproduce e
xperimental rates is the definition of encounter complex formation at
the endpoint of diffusional motion. Simple definitions, such as a requ
ired root mean square (RMS) distance to the fully bound position, fail
to explain the large influence of some mutations on association rates
. Good agreement with experiments could be obtained if satisfaction of
two intermolecular residue contacts was required for encounter comple
x formation. In the encounter complexes, barstar tends to be shifted f
rom its position in the bound complex toward the guanine-binding loop
on barnase.