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![[Figure 5]](ba5290fig5mag.jpg)
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Figure 5
Outline of the intended MMX approach for identifying coupled conformational motions. This manuscript discusses a new paradigm in structural biology: multitemperature multiconformer crystallography (MMX). Future approaches based on MMX will identify residues whose conformational ensembles change concertedly with respect to temperature, which could predict energetically coupled residues that are key to allosteric communication through a protein structure. (a) To ensure that future MMX-based approaches compare related data sets in an unbiased way, it will be important to build a sufficiently complete multiconformer model at each temperature. This may be improved by `cross-pollinating' conformers between models at different temperatures. Some of the occupancies of these conformers may refine to low but appreciable values, which will aid in identifying coordinated changes in mixtures of states (Smith et al., 2015  ). (b) Conformational changes will be monitored by changes in the electron-density map or refined occupancies as a function of temperature. In the schematic example depicted here, the side chains of two residues on adjacent helices in the tertiary structure have mutually exclusive conformations, and the helix–helix interface reconfigures as the populations of the side chains shift from one collective state to another with temperature. Similar analyses could also be performed with other experimental perturbations such as humidity, pH, pressure, ligand concentration etc. in future MMX experiments. (c) To capture the more complex conformational transitions involving subtle distributed backbone motions that occur in proteins (Deis et al., 2014), the principles of MMX can be used to superpose maps in real space based on models (Pearce, Krojer, Bradley et al., 2017) and to examine not just arbitrary volumes of space, but rather structural elements that may move as a cooperative unit – for example, the volume around (dotted rectangle) an α-helix whose conformational ensemble shifts from ordered to quasi-disordered (semi-transparent rectangle), or β-sheets, loops and other `fragments' that compose protein structure (Rohl et al., 2004). |
![[Figure 5]](ba5290fig5.jpg)
 | STRUCTURAL BIOLOGY |
ISSN: 2059-7983
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