Allostery is the process by which action at one site, such as the binding of an effector molecule, causes a functional effect at a distant site. Allosteric mechanisms are important for the regulation of cellular processes, altering the activity of a protein, or the whole biosynthetic pathway. Triggers for allosteric action include binding of small molecules, protein-protein interaction, phosphorylation events and modification of disulphide bonds. These triggers can lead to changes in accessibility of the active site, through large or small motions, such as hinge motion between two domains, or the motion of a single side chain.

Figure 1 from [1]: Rearrangement of a residue–residue interaction in phosphofructokinase. Left panel: interaction between E241 and H160 of chain A in the inactive state; right: this interaction in the active state. Red circles mark six atoms unique to the residue–residue interface in the I state, green circles mark four atoms unique to the A state, and yellow circles mark three atoms present in both states. In these two residues, there are a total of 19 atoms, so the rearrangement factor R(i,j) = max(6, 4)/19 = 0.32

One way to consider allostery is as signal propagation from one site to another, as a change in residue to residue contacts. Networks provide a way to represent these changes. Daily et al [1] introduce the idea of contact rearrangement networks, constructed from a local comparison of the protein structure with and without molecules bound to the allosteric site. These are referred to as the active and inactive structures respectively. To measure the whether a residue to residue contact is changed between the active and inactive states, the authors use a rearrangement factor (R(i,j)). This is the ratio of atoms which are within a threshold distance (5 angstroms) in only one of the active or inactive states (whichever is greater), to the total number of atoms in the two residues.The rearrangement factor is distributed such that the large majority of residues have low rearrangement factors (as they do not change between the active and inactive state). To consider when a rearrangement is significant the authors use a benchmark set of non allosteric proteins to set a threshold for the rearrangement factor. The residues above this threshold form the contact rearrangement network, which can be analysed to assess whether the allosteric and functional sites are linked by residue to residue contacts. In the paper 5/15 proteins analysed are found to have linked functional and allosteric sites.

Adaption of Figure 2 from [1]. Contact rearrangement network for phosphofructokinase. Circles in each graph represent protein residues, and red and green squares represent substrate and effector molecules, respectively. Lines connect pairs of residues with R(i,j) ≥ 0.3 and residues in the graph with any ligands which are adjacent (within 5.0 Å) in either structure. All connected components which include at least one substrate or effector molecule are shown.

Collective rigid body domain motion was not initially analysed by these contact rearrangement networks, however a later paper [2], discusses how considering these motions alongside the contact rearrangement networks can lead to a detection of allosteric activity in a greater number of proteins analysed. These contact rearrangement networks provide a way to assess the residues that are likely to be involved in allosteric signal propagation. However this requires a classification of allosteric and non-allosteric proteins, to undertake the thresholding for significance of the change in contacts, as well as multiple structures that have and do not have a allosteric effector molecule bound.

Figure 1 from [3]. (a) X-ray electron density map contoured at 1σ (blue mesh) and 0.3σ (cyan mesh) of cyclophilin A (CYPA) fit with discrete alternative conformations using qFit. Alternative conformations are colored red, orange or yellow, with hydrogen atoms added in green. (b) Visualizing a pathway in CYPA: atoms involved in clashes are shown in spheres scaled to van der Waals radii, and clashes between atoms are highlighted by dotted lines. This pathway originates with the OG atom of Ser99 conformation A (99A) and the CE1 atom of Phe113 conformation B (113B), which clash to 0.8 of their summed van der Waals radii. The pathway progresses from Phe113 to Gln63, and after the movement of Met61 to conformation B introduces no new clashes, the pathway is terminated. A 90° rotation of the final panel is shown to highlight how the final move of Met61 relieves the clash with Gln63. (c) Networks identified by CONTACT are displayed as nodes connected by edges representing contacts that clash and are relieved by alternative conformations. The node number represents the sequence number of the residue. Line thickness between a pair of nodes represents the number of pathways that the corresponding residues are part of. The pathway in b forms part of the red contact network in CYPA. (d) The six contact networks comprising 29% of residues are mapped on the three-dimensional structure of CYPA.

Alternatively, Van den Bedem et al [3]  define contact networks of conformationally coupled residues, in which movement of an alternative conformation of a residue likely influences the conformations of all other residues in the contact network. They utilise qFit, a tool for exploring conformational heterogeneity in a single electron density map of a protein, by fitting alternate conformations to the electron density.  For each conformation of a residue, it assesses whether it is possible to reduce steric clashes with another residue, by changing conformations. If a switch in conformations reduces steric clashes, then a pathway is extend to the neighbours of the residue that is moved. This continued until no new clashes are introduced. Pathways that share common members are considered as conformationally coupled, and grouped into a single contact network. As this technique is suitable for a single structure, it is possible to estimate residues which may be involved in allosteric signalling without prior knowledge of the allosteric binding region.

These techniques show two different ways to locate and annotate local conformational changes in a protein, and determine how they may be linked to one another. Considering whether these, and similar techniques highlight the same allosteric networks within proteins will be important in the integration of many data types and sources to inform the detection of allostery. Furthermore, the ability to compare networks, for example finding common motifs, will be important as the development of techniques such as fragment based drug discovery present crystal structures with many differently bound fragments.

[1] Daily, M. D., Upadhyaya, T. J., & Gray, J. J. (2008). Contact rearrangements form coupled networks from local motions in allosteric proteins. Proteins: Structure, Function and Genetics. http://doi.org/10.1002/prot.21800

[2] Daily, M. D., & Gray, J. J. (2009). Allosteric communication occurs via networks of tertiary and quaternary motions in proteins. PLoS Computational Biology. http://doi.org/10.1371/journal.pcbi.1000293

[3] van den Bedem, H., Bhabha, G., Yang, K., Wright, P. E., & Fraser, J. S. (2013). Automated identification of functional dynamic contact networks from X-ray crystallography. Nature Methods, 10(9), 896–902. http://doi.org/10.1038/nmeth.2592