Handling Non-standard Amino Acids

Tutorial Created with Software Release: 2026-3
Topics: Biologics Drug Discovery
Products Used: BioLuminate

Tutorial files

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Abstract:

This tutorial demonstrates how to design custom non-standard amino acid residues, add them to the Schrödinger non-standard residue database from the command line, set up and run an MM-GBSA residue scanning calculation that incorporates these custom residues, and analyze results to identify mutations that improve binding affinity or stability.

Tutorial Content
  1. Introduction
  2. Creating Projects and Importing Structures
  3. Designing Custom Non-standard Amino Acid Residues
  4. Processing Custom Non-standard Amino Acid Residues
  5. Adding Custom Non-standard Residues to the Database
  6. Running and Analyzing an MM-GBSA Residue Scanning Job
  7. Conclusion and References
  8. Glossary of Terms

1. Introduction

Incorporating non-standard amino acids (NSAAs) into proteins and peptides is a powerful strategy to improve stability, selectivity, binding affinity, membrane permeability, and resistance to enzyme degradation. This makes NSAAs particularly valuable in peptide therapeutics and protein engineering applications. Standard amino acids can be modified into NSAAs by chemically altering their side chains, backbone atoms, stereochemistry, or terminal groups to introduce new structural and functional properties. Common modifications include side-chain substitutions such as halogenation, methylation, phosphorylation, acetylation, as well as backbone modifications like N-methylation or incorporation of β- and D-amino acids to improve conformational stability and proteolytic resistance.

In computational workflows, these modifications are typically generated by editing or mutating the existing amino acid residues. Such modifications allow you to explore how changes in size, hydrophobicity, flexibility, and hydrogen-bonding properties influence molecular recognition, stability, and biological activity.

This tutorial uses the 6SBA structure, which is a tertiary structure of the transcription factor TEAD bound to a macrocyclic two-helix proteomimetic peptide designed to mimic the natural binding mode of the Hippo pathway co-repressor VGL4.

In this tutorial, you will learn how to how to design custom non-standard amino acid residues, add them to the Schrödinger non-standard residue database from the command line, set up and run an MM-GBSA residue scanning calculation that incorporates these custom residues, and analyze results to identify mutations that improve binding affinity or stability. This workflow is essential for peptide engineering, stapled peptide design, and biologics optimization where canonical amino acids are insufficient.

2. Creating Projects and Importing Structures

  1. Open BioLuminate and create a new project named nonstandard_residues_tutorial.prj for this tutorial.
  2. Download the tutorial zip file including input files and reference outputs here: https://www.schrodinger.com/sites/default/files/s3/release/current/Tutorials/zip/handling_nsaas.zip
  3. After downloading the zip file, unzip the contents in your Working Directorythe location where files are saved for ease of access throughout the tutorial.

3. Designing Custom Non-standard Amino Acid Residues

The following interactive tutorials guide you to design NSAAs using the 3D Builder panel and the Non-standard Residues panel and export the designed residues as an SDF file.

4. Processing Custom Non-standard Amino Acid Residues

In this section, you will process the designed non-standard amino acid residues using the process_residue_db.py script (command help).

The script validates and standardizes your custom residues for use in residue scanning workflows through a rigorous five-stage processing pipeline:

  1. Title Assignment – The script ensures each structure has a human-readable title derived from its existing title, an external database ID, or its PDB title. Structures lacking any valid identification are automatically rejected.
  2. Chirality Verification: The script reviews stereochemistry annotations to filter out racemic mixtures or ambiguous structures. Only single known enantiomers, achiral molecules, or structures without explicit chirality tags are accepted.
  3. Name Collision Check: To maintain database integrity, the script cross-references the input’s three letter PDB residue name against standard canonical (20) and shipped non-standard (48) databases, rejecting any duplicates.
  4. Subclass Validation: By default, the script confirms that the input molecule is specifically an alpha-amino acid.
  5. Structural Standardization: This stage performs the core structural optimization through four sequential substeps. It first validates the molecular backbone against an alpha-amino acid chemical pattern, then generates optimized 3D coordinates. Next, it standardizes the structure by assigning proper PDB atom names, adjusting hydrogens, and running a force-field energy minimization. Finally, it executes a mutability test by embedding the candidate NSAA into a standard tripeptide model to confirm the structure successfully survives biological incorporation.

Once all residues pass through the initial pipeline, the script executes a database-level post-processing step. It calculates chemical similarity scores between the new custom residues and the 20 standard amino acids. This assigns a natural analog code and a similarity score that the residue scanning engine uses later for initial backbone placement during mutation.

If you are using custom residues other than those provided in this tutorial, please ensure they are alpha-amino acids with a standard peptide backbone (N – Cα – C(=O) – O). However, this constraint can be relaxed via a command-line flag (--allow-non-alpha-aa) to permit the inclusion of beta- or gamma-amino acids. Please also note that only alpha-amino acids are currently supported for downstream modeling tasks like MM-GBSA Residue Scanning, FEP+ Residue Scanning, and Protein FEP+.

To save time, we have provided an SDF file (custom_TRP-derivatives.sdf) with 7 custom non-standard residues in the tutorial files. To demonstrate that the script only processes alpha-amino acids by default, we have included a structure that lacks a canonical alpha-amino acid backbone in addition to those with a standard alpha-amino acid backbone.

The following interactive tutorial guides you on how to process the custom non-standard residues using the process_residue_db.py script via the command line interface. To follow along with this tutorial, you need to open a terminal window with a properly configured $SCHRODINGER environment on your system. The command used in this tutorial is:

$SCHRODINGER/run process_residue_db.py custom_TRP-derivatives.sdf

Warning

Handling chirality requires particular attention when importing non-standard amino acid residues from 2D sources – whether drawn in the 2D sketcher, provided as SMILES, or read from an SDF file. The process_residue_db.py script relies on stereochemistry being explicitly defined in the input to generate 3D coordinates. In practice, you must ensure every chiral center in your 2D input has explicitly defined stereochemistry before processing.

In cases where chirality is not known or clear, we recommend preparing the residues using the LigPrep panel before processing.

5. Adding Custom Non-standard Residues to the Database

The following interactive tutorial guides you to create a new database that adds your custom residues to the built-in standard and non-standard residues using the Nonstandard Residues panel.

6. Running and Analyzing an MM-GBSA Residue Scanning Job

In this section, you will set up and run MM-GBSA Residue Scanning calculations. You will mutate the LYS 15 residue in the macrocyclic peptide (chain B in the 6SBA structure) to both standard and custom non-standard TRP derivatives and evaluate if these mutations enhance the binding affinity with the transcription factor or stability of the overall structure.

6.1 Set up and Run an MM-GBSA Residue Scanning Job

The following interactive tutorial guides you through setting up and running the MM-GBSA Residue Scanning calculations.

6.2 Analyze the MM-GBSA Residue Scanning Results

The properties in the MM-GBSA Residue Scanning results are frequently most helpful in assessing the potential impact of a mutation are ∆ Affinity and ∆ Stability (solvated). See the documentation for an in-depth explanation of all properties in the table and how they are calculated. These values are the relative change in free energy (in arbitrary energy units) under the MM-GBSA approximations, so negative values correspond to favorable mutations. Mutations with ∆ Stability < 0 improve the stability of the folded mutant compared to its unfolded state, whereas mutations with ∆ Affinity < 0 improve the strength of the interactions.

Due to the underlying approximations, MM-GBSA Residue Scanning is helpful for predicting mutations as significantly improving binding ( ∆ Affinity < -3 energy units), neutral (-3 energy units < ∆ Affinity < +3 energy units), or reducing binding affinity significantly (∆ Affinity > +3 energy units). These scores are best used as criteria for prioritizing mutants for further investigation with FEP+ residue scanning.

The following interactive tutorial guides you to analyze the MM-GBSA Residue Scanning results.

This tutorial is focussed on incorporating custom non-standard amino acids into the MM-GBSA Residue Scanning workflow, which offers a fast and computationally inexpensive way to triage large numbers of mutations to identify those with the most promising effects on stability or binding affinity.

Protein FEP+ is a more rigorous approach used when more quantitative accuracy is required. It captures the free energy change between wild-type and mutant states by alchemically transforming one residue into another through a series of intermediate, non-physical states (lambda-windows), with each window sampled extensively using molecular dynamics in explicit solvent. This rigorous sampling captures conformational flexibility, side-chain reorganization, and solvent reorganization effects that MM-GBSA's single-structure approximation cannot, yielding predicted free energy changes that typically agree with experimental values to within about 1 kcal/mol. The tradeoff is computational cost: Protein FEP+ calculations require GPU resources and run substantially longer than MM-GBSA Residue Scanning calculations.

In practice, the two methods are best used in sequence: MM-GBSA Residue Scanning serves as an efficient filter to narrow a large mutation space, and Protein FEP+ is then applied when prioritizing candidates for experimental synthesis, resolving closely ranked mutations that MM-GBSA cannot distinguish, or validating a small set of high-impact designs before committing resources. 

7. Conclusion and References

In this tutorial, you learned how to design custom non-standard amino acid residues, add them to the Schrödinger non-standard residue database from the command line. You also learned how to set up and run an MM-GBSA residue scanning calculation that incorporates these custom residues, and analyze results to identify mutations that improve binding affinity or stability.

8. Glossary of Terms

Entries - a simplified view of the Project Table that allows you to perform basic operations such as selection and inclusion

Included - the entry is represented in the Workspace, the circle in the In column is blue

Project Table - displays the contents of a project and is also an interface for performing operations on selected entries, viewing properties, and organizing structures and data

Recent actions - This is a list of your recent actions, which you can use to reopen a panel, displayed below the Browse row. (Right-click to delete.)

Scratch Project - a temporary project in which work is not saved, closing a scratch project removes all current work and begins a new scratch project

Selected - (1) the atoms are chosen in the Workspace. These atoms are referred to as "the selection" or "the atom selection". Workspace operations are performed on the selected atoms. (2) The entry is chosen in the Entries (and Project Table) and the row for the entry is highlighted. Project operations are performed on all selected entries

Working Directory - the location where files are saved

Workspace - the 3D display area in the center of the main window, where molecular structures are displayed