Markov state models
(MSMs) based on molecular dynamics (MD) simulations
are routinely employed to study protein folding, however, their application
to functional conformational changes of biomolecules is still limited.
In the past few years, the field of computational chemistry has experienced
a surge of advancements stemming from machine learning algorithms,
and MSMs have not been left out. Unlike global processes, such as
protein folding, the application of MSMs to functional conformational
changes is challenging because they mostly consist of localized structural
transitions. Therefore, it is critical to properly select a subset
of structural features that can describe the slowest dynamics of these
functional conformational changes. To address this challenge, we recommend
several automatic feature selection methods such as Spectral-OASIS.
To identify states in MSMs, the chosen features can be subject to
dimensionality reduction methods such as TICA or deep learning based
VAMPNets to project MD conformations onto a few collective variables
for subsequent clustering. Another challenge for the application of
MSMs to the study of functional conformational changes is the ability
to comprehend their biophysical mechanisms, as MSMs built for these
processes often require a large number of states. We recommend the
recently developed quasi-MSMs (qMSMs) to address this issue. Compared
to MSMs, qMSMs encode the non-Markovian dynamics via the generalized
master equation and can significantly reduce the number of states.
As a result, qMSMs can be built with a handful of states to facilitate
the interpretation of functional conformational changes. In the wake
of machine learning, we believe that the rapid advancement in the
MSM methodology will lead to their wider application in studying functional
conformational changes of biomolecules.