Recent advances in cryo-electron microscopy (cryo-EM)
have enabled
modeling macromolecular complexes that are essential components of
the cellular machinery. The density maps derived from cryo-EM experiments
are often integrated with manual, knowledge-driven or artificial intelligence-driven
and physics-guided computational methods to build, fit, and refine
molecular structures. Going beyond a single stationary-structure determination
scheme, it is becoming more common to interpret the experimental data
with an ensemble of models that contributes to an average observation.
Hence, there is a need to decide on the quality of an ensemble of
protein structures on-the-fly while refining them against the density
maps. We introduce such an adaptive decision-making scheme during
the molecular dynamics flexible fitting (MDFF) of biomolecules. Using
RADICAL-Cybertools, the new RADICAL augmented MDFF implementation
(R-MDFF) is examined in high-performance computing environments for
refinement of two prototypical protein systems, adenylate kinase and
carbon monoxide dehydrogenase. For these test cases, use of multiple
replicas in flexible fitting with adaptive decision making in R-MDFF
improves the overall correlation to the density by 40% relative to
the refinements of the brute-force MDFF. The improvements are particularly
significant at high, 2–3 Å map resolutions. More importantly,
the ensemble model captures key features of biologically relevant
molecular dynamics that are inaccessible to a single-model interpretation.
Finally, the pipeline is applicable to systems of growing sizes, which
is demonstrated using ensemble refinement of capsid proteins from
the chimpanzee adenovirus. The overhead for decision making remains
low and robust to computing environments. The software is publicly
available on GitHub and includes a short user guide to install R-MDFF
on different computing environments, from local Linux-based workstations
to high-performance computing environments.