Glycoengineering

概述

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

使用時機

適用於以下情境：

• 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

Canonical N-glycosylation candidates follow N-X-[S/T] where X ≠ Proline. Scan every residue: NNST contains candidates at positions 1 and 2. These are sequence motifs, not proof of occupancy; NetNGlyc also warns that cellular topology and sequence context affect interpretation.

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 = []
    i = 0
    while i <= len(seq) - 3:
        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'
            })
        # Advance one residue so overlapping sequons (NNST) are retained.
        i += 1
    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

import requests

def submit_netoglycv4(fasta_sequence: str) -> str:
    """
    Submit sequence to NetOGlyc 4.0 web service.
    Returns the job URL for result retrieval.

    Note: This uses the DTU Health Tech web service. Results take ~1-5 min.
    """
    url = "https://services.healthtech.dtu.dk/cgi-bin/webface2.cgi"
    # NetOGlyc submission (parameters may vary with web service version)
    # Recommend using the web interface directly for most use cases
    print("Submit sequence at: https://services.healthtech.dtu.dk/services/NetOGlyc-4.0/")
    return url

# Also: NetNGlyc for N-glycosylation prediction
# URL: https://services.healthtech.dtu.dk/services/NetNGlyc-1.0/

2. GlycoSHIELD (Glycan Shielding Analysis)

GlycoSHIELD grafts libraries of pre-simulated glycan conformers onto a static protein structure and scores how much of the protein surface the glycans shield, without running new MD (Tsai et al., Cell 2024, doi:10.1016/j.cell.2024.01.034):

• URL: https://gitlab.mpcdf.mpg.de/dioscuri-biophysics/glycoshield-md/ (web app: https://glycoshield.eu)
• Use: Model the glycan shield on a glycoprotein and map per-residue shielding
• Output: Glycosylated PDB/XTC ensembles per site, per-residue shielding plot, PDB with shielding in the B-factor column

GlycoSHIELD is not on PyPI — uv pip install glycoshield fails. It ships as three scripts on top of a small glycoshield package (needs numpy, scipy, matplotlib, MDAnalysis; GlycoSASA.py also needs gmx from GROMACS on PATH). Install from the checkout:

# Installation (GPL-3.0). Glycan conformer libraries are downloaded separately —
# see glycan_library_downloader.py and GLYCAN_LIBRARY/ in the repository.
git clone https://gitlab.mpcdf.mpg.de/dioscuri-biophysics/glycoshield-md.git
cd glycoshield-md
uv pip install -e .

# 1. Graft glycan conformers onto each sequon listed in the input file.
#    One line per site: <chain> <res-1,res,res+1> <1,2,3> <glycan.pdb> <glycan.xtc> <out.pdb> <out.xtc>
python GlycoSHIELD.py --protpdb protein.pdb --inputfile sequons_input \
    --threshold 3.5 --mode CG --shuffle-sugar

# 2. Per-residue shielding score across the grafted ensembles (probe radii in nm)
python GlycoSASA.py --pdblist A_463.pdb,A_492.pdb --xtclist A_463.xtc,A_492.xtc \
    --probelist 0.14,0.25 --plottrace

Illustrative: the flags come from the scripts' argparse definitions and the upstream tutorial (N-cadherin EC5 with Man5 glycans); they were not run here. --mode CG checks clashes against Cα atoms only and pairs with --threshold 3.5; --mode All with --threshold 0.7 is the all-atom setting.

3. GlycoWorkbench (Glycan Structure Drawing/Analysis)

• URL: https://www.eurocarbdb.org/project/glycoworkbench
• 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

Common Mutations Used

Glycan Notation

IUPAC Condensed Notation (Monosaccharide abbreviations)

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)

最佳實踐

• 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

延伸資源

• 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: 10564035
• Therapeutic glycoengineering review: Jefferis R (2009) Nature Reviews Drug Discovery. PMID: 19448661