Conformational changes of proteins
upon ligand binding are usually
explained in terms of several mechanisms including the induced fit,
conformational selection, or their mixtures. Due to the slow time
scales, conventional molecular dynamics (cMD) simulations based on
the atomistic models cannot easily simulate the open-to-closed conformational
transition in proteins. In our previous study, we have developed an
enhanced sampling scheme (generalized replica exchange with solute
tempering selected surface charged residues: gREST_SSCR) for multidomain
proteins and applied it to ligand-mediated conformational changes
in the G134R mutant of ribose-binding protein (RBPG134R) in solution. The free-energy landscape (FEL) of RBPG134R in the presence of a ribose at the binding site included the open
and closed states and two intermediates, open-like and closed-like
forms. Only the open and open-like forms existed in the FEL without
a ribose. In the current study, the coupling between the conformational
changes and ligand binding is further investigated using coarse-grained
MD, multiple atomistic cMD, and free-energy calculations. The ribose
is easily dissociated from the binding site of wild-type RBP and RBPG134R in the cMD simulations starting from the open and open-like
forms. In contrast, it is stable at the binding site in the simulations
from the closed and closed-like forms. The free-energy calculations
provide the binding affinities of different structures, supporting
the results of cMD simulations. Importantly, cMD simulations from
the closed-like structures reveal transitions toward the closed one
in the presence of a bound ribose. On the basis of the computational
results, we propose a molecular mechanism in which conformational
selection and induced fit happen in the first and second halves of
the open-to-closed transition in RBP, respectively.