alterlab-glycoengineering

Glycoengineering

Overview

Glycosylation is the most common and complex post-translational modification (PTM) of proteins, affecting over 50% of all human proteins. Glycans regulate protein folding, stability, immune recognition, receptor interactions, and pharmacokinetics of therapeutic proteins. Glycoengineering involves rational modification of glycosylation patterns for improved therapeutic efficacy, stability, or immune evasion.

Two major glycosylation types:

• N-glycosylation: Attached to asparagine (N) in the sequon N-X-[S/T] where X ≠ Proline; occurs in the ER/Golgi
• O-glycosylation: Attached to serine (S) or threonine (T); no strict consensus motif; primarily GalNAc initiation

When to Use This Skill

Use this skill when:

• Antibody engineering: Optimize Fc glycosylation for enhanced ADCC, CDC, or reduced immunogenicity
• Therapeutic protein design: Identify glycosylation sites that affect half-life, stability, or immunogenicity
• Vaccine antigen design: Engineer glycan shields to focus immune responses on conserved epitopes
• Biosimilar characterization: Compare glycan patterns between reference and biosimilar
• Drug target analysis: Does glycosylation affect target engagement for a receptor?
• Protein stability: N-glycans often stabilize proteins; identify sites for stabilizing mutations

N-Glycosylation Sequon Analysis

Scanning for N-Glycosylation Sites

N-glycosylation occurs at the sequon N-X-[S/T] where X ≠ Proline.

import re
from typing import List, Tuple

def find_n_glycosylation_sequons(sequence: str) -> List[dict]:
    """
    Scan a protein sequence for canonical N-linked glycosylation sequons.
    Motif: N-X-[S/T], where X ≠ Proline.

    Args:
        sequence: Single-letter amino acid sequence

    Returns:
        List of dicts with position (1-based), motif, and context
    """
    seq = sequence.upper()
    results = []
    # Step by 1, not 3: adjacent sequons can overlap (e.g. NNST has a sequon at
    # both position 1 (N-N-S) and position 2 (N-S-T)); skipping ahead misses them.
    for i in range(len(seq) - 2):
        triplet = seq[i:i+3]
        if triplet[0] == 'N' and triplet[1] != 'P' and triplet[2] in {'S', 'T'}:
            context = seq[max(0, i-3):i+6]  # ±3 residue context
            results.append({
                'position': i + 1,   # 1-based
                'motif': triplet,
                'context': context,
                'sequon_type': 'NXS' if triplet[2] == 'S' else 'NXT'
            })
    return results

def summarize_glycosylation_sites(sequence: str, protein_name: str = "") -> str:
    """Generate a research log summary of N-glycosylation sites."""
    sequons = find_n_glycosylation_sequons(sequence)

    lines = [f"# N-Glycosylation Sequon Analysis: {protein_name or 'Protein'}"]
    lines.append(f"Sequence length: {len(sequence)}")
    lines.append(f"Total N-glycosylation sequons: {len(sequons)}")

    if sequons:
        lines.append(f"\nN-X-S sites: {sum(1 for s in sequons if s['sequon_type'] == 'NXS')}")
        lines.append(f"N-X-T sites: {sum(1 for s in sequons if s['sequon_type'] == 'NXT')}")
        lines.append(f"\nSite details:")
        for s in sequons:
            lines.append(f"  Position {s['position']}: {s['motif']} (context: ...{s['context']}...)")
    else:
        lines.append("No canonical N-glycosylation sequons detected.")

    return "\n".join(lines)

# Example: IgG1 Fc region
fc_sequence = "APELLGGPSVFLFPPKPKDTLMISRTPEVTCVVVDVSHEDPEVKFNWYVDGVEVHNAKTKPREEQYNSTYRVVSVLTVLHQDWLNGKEYKCKVSNKALPAPIEKTISKAKGQPREPQVYTLPPSREEMTKNQVSLTCLVKGFYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLYSKLTVDKSRWQQGNVFSCSVMHEALHNHYTQKSLSLSPGK"
print(summarize_glycosylation_sites(fc_sequence, "IgG1 Fc"))

Mutating N-Glycosylation Sites

def eliminate_glycosite(sequence: str, position: int, replacement: str = "Q") -> str:
    """
    Eliminate an N-glycosylation site by substituting Asn → Gln (conservative).

    Args:
        sequence: Protein sequence
        position: 1-based position of the Asn to mutate
        replacement: Amino acid to substitute (default Q = Gln; similar size, not glycosylated)

    Returns:
        Mutated sequence
    """
    seq = list(sequence.upper())
    idx = position - 1
    assert seq[idx] == 'N', f"Position {position} is '{seq[idx]}', not 'N'"
    seq[idx] = replacement.upper()
    return ''.join(seq)

def add_glycosite(sequence: str, position: int, flanking_context: str = "S") -> str:
    """
    Introduce an N-glycosylation site by mutating a residue to Asn,
    and ensuring X ≠ Pro and +2 = S/T.

    Args:
        position: 1-based position to introduce Asn
        flanking_context: 'S' or 'T' at position+2 (if modification needed)
    """
    seq = list(sequence.upper())
    idx = position - 1

    # Mutate to Asn
    seq[idx] = 'N'

    # Ensure X+1 != Pro (mutate to Ala if needed)
    if idx + 1 < len(seq) and seq[idx + 1] == 'P':
        seq[idx + 1] = 'A'

    # Ensure X+2 = S or T
    if idx + 2 < len(seq) and seq[idx + 2] not in ('S', 'T'):
        seq[idx + 2] = flanking_context

    return ''.join(seq)

O-Glycosylation Analysis

Heuristic O-Glycosylation Hotspot Prediction

def predict_o_glycosylation_hotspots(
    sequence: str,
    window: int = 7,
    min_st_fraction: float = 0.4,
    disallow_proline_next: bool = True
) -> List[dict]:
    """
    Heuristic O-glycosylation hotspot scoring based on local S/T density.
    Not a substitute for NetOGlyc; use as fast baseline.

    Rules:
    - O-GalNAc glycosylation clusters on Ser/Thr-rich segments
    - Flag Ser/Thr residues in windows enriched for S/T
    - Avoid S/T immediately followed by Pro (TP/SP motifs inhibit GalNAc-T)

    Args:
        window: Odd window size for local S/T density
        min_st_fraction: Minimum fraction of S/T in window to flag site
    """
    if window % 2 == 0:
        window = 7
    seq = sequence.upper()
    half = window // 2
    candidates = []

    for i, aa in enumerate(seq):
        if aa not in ('S', 'T'):
            continue
        if disallow_proline_next and i + 1 < len(seq) and seq[i+1] == 'P':
            continue

        start = max(0, i - half)
        end = min(len(seq), i + half + 1)
        segment = seq[start:end]
        st_count = sum(1 for c in segment if c in ('S', 'T'))
        frac = st_count / len(segment)

        if frac >= min_st_fraction:
            candidates.append({
                'position': i + 1,
                'residue': aa,
                'st_fraction': round(frac, 3),
                'window': f"{start+1}-{end}",
                'segment': segment
            })

    return candidates

External Glycoengineering Tools

1. NetOGlyc 4.0 (O-glycosylation prediction)

Web service for high-accuracy O-GalNAc site prediction:

• URL: https://services.healthtech.dtu.dk/services/NetOGlyc-4.0/
• Input: FASTA protein sequence
• Output: Per-residue O-glycosylation probability scores
• Method: Neural network trained on experimentally verified O-GalNAc sites

NetOGlyc 4.0 has no stable public REST API — the CGI submission endpoint and its form parameters change between web-service revisions. For reliable results, submit FASTA at the web interface and download the result table:

• NetOGlyc 4.0 (O-GalNAc): https://services.healthtech.dtu.dk/services/NetOGlyc-4.0/
• NetNGlyc 1.0 (N-glyc): https://services.healthtech.dtu.dk/services/NetNGlyc-1.0/

The standalone packages are also downloadable from those pages for offline/batch runs. Use the inline find_n_glycosylation_sequons above as a fast pre-screen.

2. GlycoShield-MD (Glycan Shielding Analysis)

GlycoShield-MD analyzes how glycans shield protein surfaces during MD simulations:

• URL: https://gitlab.mpcdf.mpg.de/dioscuri-biophysics/glycoshield-md/
• Use: Map glycan shielding on protein surface over MD trajectory
• Output: Per-residue shielding fraction, visualization

# Installation
pip install glycoshield

# Basic usage: analyze glycan shielding from glycosylated protein MD trajectory
glycoshield \
    --topology glycoprotein.pdb \
    --trajectory glycoprotein.xtc \
    --glycan_resnames BGLCNA FUC \
    --output shielding_analysis/

3. GlycoWorkbench (Glycan Structure Drawing/Analysis)

• URL: https://github.com/glycoinfo/eurocarbdb
• Use: Draw glycan structures, calculate masses, annotate MS spectra
• Format: GlycoCT, IUPAC condensed glycan notation

4. GlyConnect (Glycan-Protein Database)

• URL: https://glyconnect.expasy.org/
• Use: Find experimentally verified glycoproteins and glycosylation sites
• Query: By protein (UniProt ID), glycan structure, or tissue

import requests

def query_glyconnect(uniprot_id: str) -> dict:
    """Query GlyConnect for glycosylation data for a protein."""
    url = f"https://glyconnect.expasy.org/api/proteins/uniprot/{uniprot_id}"
    response = requests.get(url, headers={"Accept": "application/json"})
    if response.status_code == 200:
        return response.json()
    return {}

# Example: query EGFR glycosylation
egfr_glyco = query_glyconnect("P00533")

5. UniCarbKB (Glycan Structure Database)

• URL: https://unicarbkb.org/
• Use: Browse glycan structures, search by mass or composition
• Format: GlycoCT or IUPAC notation

Key Glycoengineering Strategies

For Therapeutic Antibodies

| Goal | Strategy | Notes | |------|----------|-------| | Enhance ADCC | Defucosylation at Fc Asn297 | Afucosylated IgG1 has ~50× better FcγRIIIa binding | | Reduce immunogenicity | Remove non-human glycans | Eliminate α-Gal, NGNA epitopes | | Improve PK half-life | Sialylation | Sialylated glycans extend half-life | | Reduce inflammation | Hypersialylation | IVIG anti-inflammatory mechanism | | Create glycan shield | Add N-glycosites to surface | Masks vulnerable epitopes (vaccine design) |

Common Mutations Used

| Mutation | Effect | |----------|--------| | N297A/Q (IgG1) | Removes Fc glycosylation (aglycosyl) | | N297D (IgG1) | Removes Fc glycosylation | | S298A/E333A/K334A | Increases FcγRIIIa binding | | F243L (IgG1) | Increases defucosylation | | T299A | Removes Fc glycosylation |

Glycan Notation

IUPAC Condensed Notation (Monosaccharide abbreviations)

| Symbol | Full Name | Type | |--------|-----------|------| | Glc | Glucose | Hexose | | GlcNAc | N-Acetylglucosamine | HexNAc | | Man | Mannose | Hexose | | Gal | Galactose | Hexose | | Fuc | Fucose | Deoxyhexose | | Neu5Ac | N-Acetylneuraminic acid (Sialic acid) | Sialic acid | | GalNAc | N-Acetylgalactosamine | HexNAc |

Complex N-Glycan Structure

Typical complex biantennary N-glycan:
Neu5Ac-Gal-GlcNAc-Man\
                       Man-GlcNAc-GlcNAc-[Asn]
Neu5Ac-Gal-GlcNAc-Man/
(±Core Fuc at innermost GlcNAc)

Best Practices

• Start with NetNGlyc/NetOGlyc for computational prediction before experimental validation
• Verify with mass spectrometry: Glycoproteomics (Byonic, Mascot) for site-specific glycan profiling
• Consider site context: Not all predicted sequons are actually glycosylated (accessibility, cell type, protein conformation)
• For antibodies: Fc N297 glycan is critical — always characterize this site first
• Use GlyConnect to check if your protein of interest has experimentally verified glycosylation data

Additional Resources

• GlyTouCan (glycan structure repository): https://glytoucan.org/
• GlyConnect: https://glyconnect.expasy.org/
• CFG Functional Glycomics: http://www.functionalglycomics.org/
• DTU Health Tech servers (NetNGlyc, NetOGlyc): https://services.healthtech.dtu.dk/
• GlycoWorkbench: https://glycoworkbench.software.informer.com/
• Review: Apweiler R et al. (1999) Biochim Biophys Acta. PMID: 10580125
• Therapeutic glycoengineering review: Jefferis R (2009) Nature Reviews Drug Discovery. PMID: 19247305