BMS5021 · Introduction to Bioinformatics
Proteomics & Mass Spectrometry
Week 9 of Monash University BMS5021 opens Topic 3: identifying proteins by mass spectrometry. It covers the shotgun (bottom-up) workflow - extract, trypsin-digest into peptides, separate, run MS/MS, then identify by database search - the mass-to-charge ratio m/z, peptide-mass fingerprinting, and the peptide-spectrum match (PSM) with MSFragger, illustrated by the SARS-CoV-2 protein-interaction study. These are the analysis skills the Topic 3 proteomics report is built on, so the digestion rule and the PSM concept are directly assessable.
What this chapter covers
- 01Proteomics = large-scale study of the proteome: sequence, structure, function, interactions and modifications
- 02Mass spectrometry measures the mass-to-charge ratio (m/z) of ions: source (ionisation) -> mass analyzer (sorts by m/z) -> detector
- 03Shotgun / bottom-up workflow: (1) extract protein, (2) trypsin digest to peptides, (3) LC separation, (4) MS/MS spectra, (5) database-search identification
- 04Trypsin cleaves C-terminally of arginine (R) and lysine (K); a missed cleavage changes the peptide list
- 05Peptide-mass fingerprinting (PMF): compute theoretical tryptic peptide masses in silico and match experimental masses to identify the protein
- 06Peptide-spectrum match (PSM): MS1 = precursor mass, MS2 = fragment masses; software (MSFragger) matches to the best-scoring database peptide
- 07KEY POINT: peptides are NOT sequenced directly - the sequence is INFERRED from database matches
- 08Protein structure levels (primary-quaternary) and >300 post-translational modifications; PPIs studied via bait pull-down / AP-MS (mzML files)
Trypsin digestion and peptide-mass fingerprinting (Week 9)
- +1Find the cut sites: trypsin cuts immediately AFTER each K and each R. In A-L-K | T-P-E-R | G-S-K | M the K at position 3, the R at position 7 and the K at position 10 are cleavage points. Cutting after each gives the peptides ALK, TPER, GSK and M.
- +1Explain PMF. Each peptide has a characteristic mass. In peptide-mass fingerprinting you digest every protein in the database in silico with the same trypsin rule, compute the theoretical peptide masses, then match the experimentally measured peptide masses to the database protein whose predicted tryptic masses fit best - identifying the protein without directly sequencing it.
- +1Account for a missed cleavage. If trypsin fails to cut at one site, the two flanking peptides stay joined as one larger peptide - for example a missed cut after the position-7 R would merge TPER and GSK into TPERGSK - so the observed peptide list (and the masses) differ from the fully-digested prediction, which is why search tools allow for a number of missed cleavages.
Key terms
- Mass-to-charge ratio (m/z)
- The quantity a mass spectrometer measures: an ion's mass divided by its charge. Only ionised (charged) molecules can be measured, so peptides are ionised before analysis.
- Shotgun / bottom-up proteomics
- The standard workflow: extract proteins, digest them into peptides with trypsin, separate the peptides by liquid chromatography, acquire MS/MS spectra, and identify the peptides by database search.
- Trypsin digestion
- Enzymatic cleavage C-terminal to arginine (R) and lysine (K), producing predictable peptides. A missed cleavage leaves two adjacent peptides joined, so searches allow for a few.
- Peptide-mass fingerprinting (PMF)
- Identifying a protein by comparing its experimentally measured peptide masses to the theoretical tryptic-digest masses computed in silico for every protein in a database.
- Peptide-spectrum match (PSM)
- A match between an observed spectrum (MS1 precursor mass + MS2 fragment masses) and the best-scoring peptide in a reference proteome database (found by MSFragger); the peptide sequence is inferred, not directly read.
- MS1 / MS2
- MS1 is the precursor scan giving the intact peptide's mass; MS2 (fragmentation) gives the masses of its fragments, which together resolve which peptide produced the spectrum.
Proteomics & Mass Spectrometry FAQ
How does Week 9 feed the assessment?
It opens Topic 3, whose 35% report is a quantitative-proteomics / mass-spectrometry data analysis. Week 9 gives you the workflow and vocabulary - digestion, m/z, PMF, PSM and MSFragger search output - that the report analysis and interpretation depend on. Understanding how a protein is identified from a spectrum is the foundation for everything you write up in Topic 3. Confirm the report details on Moodle.
Why can't we just read a peptide's sequence directly from the spectrum?
Because a mass spectrometer measures masses, not letters. It gives the precursor mass (MS1) and fragment masses (MS2), and software infers which peptide best explains those masses by matching against a reference proteome database. That is why the identification is called a peptide-spectrum MATCH: the sequence is inferred from the database, not spelled out by the instrument.
Why trypsin specifically?
Because trypsin cuts at predictable sites - C-terminal to arginine and lysine - so the resulting peptides have predictable masses that can be computed in silico for every database protein and matched. That predictability is what makes peptide-mass fingerprinting and database search work. Searches allow a small number of missed cleavages to account for incomplete digestion.
Can AI help me with mass-spectrometry proteomics?
Yes. Sia can walk you through the shotgun workflow, digest a sequence with you, explain the m/z principle and how MS1/MS2 resolve a peptide, and talk through what an MSFragger PSM result means - step by step, checking your reasoning. It does not perform your graded Topic 3 analysis or write the report for you, and Monash University academic-integrity rules apply.
Exam move
Learn Topic 3 as a workflow you can narrate end to end: extract, trypsin-digest, separate, MS/MS, database-search. Drill the two concrete skills - digesting a sequence at R/K (cut after each, and know what a missed cleavage does) and explaining how PMF and PSM identify a protein from masses rather than by direct sequencing. Fix the m/z principle (only charged ions are measured; source -> analyzer -> detector) and the MS1/MS2 distinction. Because the Topic 3 report is a proteomics data analysis, connect each concept to what you will actually do with the MSFragger output. Ask Sia to give you sequences to digest and PSM scenarios to interpret and to check your answers. Confirm the report scope and any tools on Moodle.
Working through Proteomics & Mass Spectrometry in BMS5021? Sia is AskSia’s AI Biology tutor — ask any BMS5021 Proteomics & Mass Spectrometry question and get a clear, step-by-step explanation grounded in how BMS5021 is taught and assessed. Read this chapter free, then take your hardest questions to Sia.
