Albuminomics

Top-Down Proteomics

3 min read

Top-Down Proteomics

Definition

Top-down proteomics analyzes intact proteins without prior enzymatic digestion. The protein is ionized whole, its exact mass is measured, and fragmentation (MS/MS) can be used to localize PTMs at the amino acid level.

Contrast with Bottom-up proteomics (digest first → identify peptides → infer protein), which loses isoform connectivity.

Why it matters for PTM research

  • Preserves PTM combinations: you see which modifications co-exist on the same molecule
  • Resolves isoforms: distinct molecular mass for each PTM combination → separable peaks
  • No protein inference problem: you measure the actual protein, not a reconstructed composite
  • Quantitative: peak areas reflect relative abundance of each isoform

Workflow

Plasma sample
    ↓ Protein enrichment / depletion (optional)
    ↓ LC separation (size-exclusion, reversed-phase)
    ↓ ESI-MS (electrospray ionization)
    ↓ Charge state deconvolution → monoisotopic or average mass
    ↓ Mass matching to theoretical PTM combinations
    ↓ Quantification (peak area integration)
    ↓ Results: isoform profile (% of each form)

Mass accuracy requirements

  • For intact proteins ~66 kDa (HSA): need mass accuracy ~1–5 ppm
  • Distinguish +162 Da (glycation) from +163 Da (other): requires high-res MS (Orbitrap, Q-TOF)
  • Mass accuracy scales with protein size — harder for large proteins

Instruments in use

  • Bruker timsTOF Pro2 — Platform 1 in ALBOM study; used in CQFD-PTM pipeline
  • Sciex TripleTOF 5600+ — Platform 2 in ALBOM study (cross-platform validation)
  • Thermo Orbitrap (Fusion, Eclipse) — standard for top-down fragmentation
  • Waters Q-TOF (Synapt)

ALBOM cross-platform result

Both Bruker and Sciex instruments produced equivalent diagnostic classifications (McNemar p=0.149, Jaccard error index 0.696) confirming the method is not tied to a specific manufacturer. ⚠️ Bruker Platform 1 baseline correction algorithm attenuates high-mass peaks >67,500 Da (poly-glycated species) — needs optimization for this specific spectral region.

Limitations

  • Lower sensitivity than bottom-up (fewer ion copies detected per protein)
  • Challenging for proteins >100 kDa or very heterogeneous glycoproteins
  • Complex spectra deconvolution — requires specialized software (Xtract, mMass, Protein Deconvolution)
  • Not yet routine in clinical labs

In our research

Top-down MS is the core analytical strategy of the ALBOM study and CQFD-PTM pipeline, enabling intact HSA isoform profiling.

ALBOM method details (el-balkhi-2025)

The ALBOM study provides a detailed clinical application of top-down LC-HR-MS for HSA isoform profiling:

  • Sample prep: 1:50 dilution in 0.9% NaCl; equine myoglobin (4 g/L final concentration) as IS
  • Purpose of Mb IS: mass recalibration + absolute quantification
  • Column: C4 reverse-phase; gradient elution
  • Ionization: ESI
  • Mass range monitored: 66,000–68,000 Da (deconvoluted)
  • Isoforms resolved: 10 HSA isoforms (native + 9 modified)
  • Quantification: absolute (g/L); high-MW glycated species (HSA+2GLYC, HSA+CYS+2GLYC) calibrated using slope from nearest quantifiable related species
  • Spectral feature extraction: TIC normalization → PQN normalization → 75-feature selection for ML input

Key references

  • el-balkhi-2025 — ALBOM clinical application; cross-platform validation (Bruker timsTOF Pro2 vs Sciex TripleTOF 5600+)
  • Lakis et al. [ref 22 in el-balkhi-2025] — original validated LC-QTOF-MS method for absolute HSA isoform quantification

Graph View