# Essential Site Scanning Analysis Tutorial¶

This tutorial demonstrates how to use ESSA for determining the essential sites that would significantly alter the global/functional dynamics of a protein upon ligand binding. ESSA emulates ligand binding by increasing the node density around each scanned residue. This is achieved by adding extra nodes at the positions of the specific residue’s heavy atoms (other than the C-alpha atoms that define the reference network in GNM or ANM)

Let's import all the required packages!

# Generation of ESSA profile¶

First, we need to parse the structure on which we want to perform ESSA.

In this tutorial, we will fetch the TEM-1 beta-lactamase (1pzo) from the PDB.

ESSA is implemented as a ProDy class, so we can instantiate an ESSA object.

Before starting residue scanning, we first need to set the system. Even though the ligand(s) are not included in the scanning, they can be specified for further analysis. For this purpose, you can simply give the chain ids and residue numbers as a string shown in the following. The protein residues interacting with each ligand can be determined within a specific cutoff distance (default dist=4.5). ESSA will use the title of the atomic object in the names of the output files.

ESSA, by default, generates a ModeEnsemble containing the modes resulting from the perturbation of each residue. However, if the user does not have enough memory resources for conducting computation on a large structure, then lowmem=True should be chosen, thus only storing eigenvalues/eigenvectors for each perturbed model.

In the structure 1pzo, there are two identical allosteric inhibitors (residues 300 and 301 of chain A). ESSA can be informed about them when setting the system as follows:

Scanning can be done using scanResidue(). Here you can specify the number of softest modes, the type of ENM (GNM or ANM), and the cutoff, which by default are n_modes=10, enm='gnm', cutoff=None, respectively. If cutoff is not specified, its value is adopted from the default value of the specified ENM. During the scanning, the progress will also be displayed.

The generated ESSA profile can be shown by showESSAProfile(). On this profile, the residues interacting with the previously specified ligand(s) are automatically highlighted. The blue dashed baseline shows the q-th quantile of the profile, which is by default q=0.75, representing the top quartile.

Other residues of interest can also be indicated with their single-letter code and residue number on this plot if specified by a ProDy selection string provided as a parameter.

We can dynamically customize the properties of the plot using the matplotlib context manager as shown below.

Other residues of interest can also be indicated with their single-letter code and residue number on this plot if specified by a ProDy selection string provided as a parameter.

ESSA z-scores can be obtained as a NumPy array using getESSAZscores(), and saved with saveESSAZscores(). Let’s have the z-scores of the first ten residues:

In order to visualize the essential residues, a PDB file can be generated, in which the z-scores are written in the B-factor column. Later, this file can be opened in a molecular graphics program such as PyMOL or VMD, where the structure can be colored according to the B-factors.

# Prediction of allosteric pockets¶

Allosteric pocket prediction requires Fpocket 3.0 and Pandas installed in your system. The first step is the pocket hunting, which is automatically carried out in the background, by calling scanPockets(). This method parses the pocket features provided by Fpocket, and all identified pockets are stored in a folder ending with _out in the current working directory. Additionally, maximum/median ESSA scores are assigned to each pocket based on the ESSA scores of the residues forming it.

Pocket features that are stored in a Pandas DataFrame can be obtained by getPocketFeatures(), and saved as a Python pickle file by savePocketZscores().

Key features of the pockets to be used in the prediction, namely ESSA and local hydrophobic density (LHD) z-scores, can be listed by getPocketZscores().

The prediction protocol ranks the pockets with respect to their ESSA and LHD z-scores. Concurrently, the pockets with negative LHD z-scores are filtered out as allosteric sites are known to have relatively higher LHD.

Ranking of the pockets can be performed and obtained by rankPockets() and getPocketRanks(), respectively.

Pocket 6 with the top ESSA_max score has been identified as the only allosteric pocket in this structure. Interestingly, other pockets have been filtered out due to their negative LHD z-scores. Pocket 6 is a large pocket that includes CBT allosteric ligands at A300 and A301.

In order to visualize the pockets, the pqr, .sh, or .tcl file, an output of Fpocket needs to be opened by PyMOL or VMD together with the original pdb file.

Pocket z-scores and ranks can be saved by savePocketZscores() and writePocketRanksToCSV(), respectively.