Albuminomics

Bottom-Up Proteomics

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Bottom-Up Proteomics

Definition

Bottom-up proteomics (BU) is the standard paradigm for protein analysis by mass spectrometry in which proteins are enzymatically digested into peptides before analysis. Unlike Top-down proteomics (intact protein MS), BU analyzes peptide fragments and infers protein identity and modification sites from these fragments.

Mechanism

Workflow

Sample (serum/plasma/tissue)
  ↓ Protein extraction + denaturation (urea, detergent)
  ↓ Reduction (DTT/TCEP) + alkylation (iodoacetamide) of disulfide bonds
  ↓ Enzymatic digestion (trypsin: cleaves at K/R C-terminal)
  → Peptide mixture (typical: 10–100 kDa protein → 6–25 amino acid peptides)
  ↓ LC separation (nano-LC or standard C18 RP)
  ↓ MS analysis (DDA, DIA/SWATH, MRM, or PRM)
  ↓ Database search (Mascot, Sequest, MaxQuant, etc.)
  → Protein identification + PTM site localization

Trypsin digestion

  • Cleaves C-terminal to Lys (K) and Arg (R) — except when followed by Pro
  • For HSA (585 aa): generates ~80–100 tryptic peptides covering the full sequence
  • Missed cleavages (1–2) expand the list; non-specific cleavage adds more

DDA vs DIA/SWATH approaches

ModeDescriptionStrengthsLimitations
DDA (Data-Dependent Acquisition)MS selects most intense precursor ions for fragmentation in real timeDiscovers novel peptides/PTMs; flexibleLimited reproducibility; misses low-abundance peptides
DIA/SWATHAll precursors in defined m/z windows fragmented simultaneouslyReproducible; all peptides sampled; good for targeted + untargetedRequires spectral library; complex deconvolution
MRM/SRMPre-selected transitions; single/triple quadAbsolute quantification; highest sensitivity for known targetsOnly known peptides; no discovery

Application to HSA — in our research

rahali-2022 (J Chromatography B)

  • Phase 1: DDA spectral library of 127 HSA peptides built from trypsin digest + SWATH analysis
  • Phase 2: DIA validation of all CLD-relevant isoforms confirmed by BU → site-level PTM localization
  • BU-only PTMs found: carbamylation (+43 Da), deamidation (+1 Da), amino acid substitution — not detectable by top-down
  • ⚠️ All clinically relevant CLD isoforms (cysteinylation, glycation, sulfonylation, truncation) confirmed by BU → validates Top-down proteomics approach for CLD biomarker use

BU advantages for PTM research

  • Site-level resolution: determines exactly which amino acid is modified (e.g., Lys525 glycation vs Lys439)
  • Complete sequence coverage: finds modifications across the entire protein
  • Discovers novel PTMs: carbamylation, deamidation, substitutions — missed by TD
  • Lower MW constraint: works for any protein size; TD struggles >100 kDa

BU limitations vs TD

  • Loses isoform context: cannot determine if two PTMs co-occur on the same molecule
    • Example: BU can detect +Cys and +GLYC separately but cannot confirm they are on the same protein molecule (co-occurrence isoform → relevant for multi-PTM combinatorial isoforms)
  • Sample prep intensive: denaturation, reduction, alkylation, digestion → alters protein state; pre-analytical artifact risk
  • Slower: cannot achieve 3-minute runtime of Top-down proteomics
  • Quantification challenges: isoforms cannot be quantified at protein level; relative peptide abundance ≠ protein abundance; modification stoichiometry calculation complex

Role in our pipeline

The CQFD-PTM pipeline and ALBOM study primarily use Top-down proteomics for clinical HSA isoform profiling. BU serves as:

  1. Validation layer (as in rahali-2022): confirms TD identifications at site level
  2. Discovery tool: finds novel PTMs not resolvable at intact protein mass level
  3. Reference library (spectral library for SWATH): enables DIA-based targeted BU if needed

Decision tree:

Routine clinical profiling → TD (fast, isoform-aware, quantitative)
PTM site validation → BU (DDA → spectral library → DIA/SWATH)
Novel PTM discovery → BU (DDA; open modification search)
Absolute quantification → TD (with myoglobin IS; [[lakis-2024]]) or MRM (known peptides)

Key references

  • rahali-2022 — TD vs BU validation for HSA in CLD; 127-peptide BU library; BU-only PTMs
  • naldi-2017-review — comprehensive review including BU analytical approaches for HSA
  • Top-down proteomics — complementary method page
  • SWATH — DIA-based BU approach page

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