Protein Structure

Lecture 11 PTM

3. PTM Analysis by Mass Spectrometry 🧬✨

This chapter is all about how proteins are chemically modified after they are made, and how mass spectrometry (MS) can detect those changes.

This is extremely important because proteins are not static molecules.

Even if two cells express the same protein, that protein can behave completely differently depending on its modifications.

Think of PTMs as molecular “switches”, “labels”, or “decorations” added onto proteins.

These modifications can change:

activity
folding
stability
localization
binding partners
degradation
signaling

This is why the proteome is much more complex than the genome.

A single gene → one protein backbone → many modified versions.

That is one major reason proteomics is so powerful.

Why PTMs matter 🧠

The text starts by explaining something very important:

The eukaryotic proteome is far more diverse than the genome.

Why?

Because regulation happens at two levels:

1) Post-transcriptional regulation

This happens at the RNA level.

For example:

alternative splicing
RNA editing

This creates different mRNA forms.

2) Post-translational regulation

This happens after translation.

Once the protein is made, chemical groups can be attached.

Examples:

phosphate groups
sugars
ubiquitin
methyl groups
acetyl groups

This creates different functional versions of the same protein.

This is one of the central ideas in molecular biology.

Why MS is used for PTMs ⚡

Mass spectrometry detects PTMs by measuring mass shifts.

This is the key principle.

If a modification adds mass, MS can detect it.

For example:

phosphorylation → +80 Da
acetylation → +42 Da
ubiquitination signature → +114 Da

This makes MS ideal for PTM mapping.

The workflow is usually:

digest proteins into peptides
enrich modified peptides
LC-MS/MS
identify mass shifts
localize modified residue

3.1 Phosphorylation ⚡🧪

This is one of the most important PTMs in biology.

Phosphorylation means adding a phosphate group (PO₄³⁻).

Usually on:

serine (S)
threonine (T)
tyrosine (Y)

These are often called:

phosphoserine
phosphothreonine
phosphotyrosine

Why phosphorylation is important

This is basically the language of cell signaling.

Phosphorylation can control:

enzyme activity
protein folding
localization
interaction
degradation

Examples:

cell cycle
apoptosis
differentiation
immune signaling
receptor signaling

Cancer biology heavily depends on phosphorylation.

Kinases and phosphatases regulate this.

Think:

kinase = ON switch
phosphatase = OFF switch

Very simplified, but useful.

Important challenge: low abundance

A key concept here is:

phosphorylation is often substoichiometric.

This means not every copy of a protein is phosphorylated.

For example:

Only 2% of protein X molecules may be phosphorylated.

That makes detection hard.

This is why enrichment is essential.

Enrichment of phosphopeptides 🎯

This section is very important for exams.

Before MS, phosphorylated peptides are enriched.

IMAC

Immobilized Metal Ion Affinity Chromatography

This is one of the most common methods.

It uses metal ions like:

Fe³⁺
Ga³⁺
Al³⁺

Phosphate groups bind strongly to metal ions.

So phosphorylated peptides stick to the column.

Non-phosphorylated peptides are washed away.

Then phosphopeptides are eluted.

This is a classic selective chemistry principle.

TiO₂ enrichment

Another major method.

Titanium dioxide beads bind phosphate groups strongly.

This is extremely common in phosphoproteomics.

Often even better than IMAC.

Important concept:

selective adsorption

Phosphate-containing peptides preferentially bind.

Why SCX / HILIC fractionation is also used

This is about reducing complexity before MS.

Because cell lysates are extremely complex.

Methods mentioned:

SCX
SAX
HILIC
ERLIC

These separate peptides based on:

charge
hydrophilicity

Phosphorylated peptides behave differently because phosphate is negatively charged.

Very important theoretical point.

MS analysis of phosphopeptides ⚡

This section explains why phosphopeptides are harder to analyze.

Two big reasons:

1) Poor ionization

Phosphate groups are highly electronegative.

This can reduce ionization efficiency.

2) Labile bond

The phosphoester bond is fragile.

During CID fragmentation, phosphate often falls off.

This creates neutral loss peaks.

This is one of the most classic phosphoproteomics concepts.

Loss of:

H₃PO₄
metaphosphoric acid

instead of backbone fragmentation.

Better fragmentation methods

This is extremely important.

Instead of only CID, they use:

ECD
ETD

These preserve PTMs much better.

Because they fragment the peptide backbone while keeping phosphate attached.

This helps identify the exact site.

This is one of the biggest practical advances in PTM proteomics.

Site localization 🎯

Finding that a peptide is phosphorylated is not enough.

You must know which residue.

Example:

AASTYGG

Is it:

S phosphorylated?
T phosphorylated?
Y phosphorylated?

This is called site localization.

Very important concept.

Software:

Mascot
SEQUEST
Ascore

But the paper strongly emphasizes manual validation.

This is good scientific practice.

3.2 Ubiquitination 🏷️

This is another hugely important PTM.

Ubiquitin is a small 76 amino acid protein.

Instead of adding a small chemical group, the cell adds an entire protein.

This is amazing biologically.

What does ubiquitination do?

Most famous role:

protein degradation

It labels proteins for destruction by the proteasome.

Think of it as a molecular “trash label”.

But it also regulates:

DNA repair
signaling
apoptosis
immune response
trafficking

How attachment works

Ubiquitin attaches to lysine residues.

Specifically to the ε-amino group.

This forms an isopeptide bond.

Important enzyme cascade:

E1 = activating enzyme
E2 = conjugating enzyme
E3 = ligase

Very important to remember.

Mono vs polyubiquitination

mono = one ubiquitin
poly = chain of ubiquitins

This chain itself carries information.

Example:

K48-linked chains → degradation
K63-linked chains → signaling / DNA repair

This is a very important biological distinction.

MS signature

This is a classic exam question.

After trypsin digestion, ubiquitin leaves behind a diglycine remnant.

This adds:

+114.043 Da

This is the signature used in MS.

Very important.

Important caveat ⚠️

The paper highlights false positives.

Excellent scientific point.

A +114 Da shift can come from other artifacts.

Examples:

missed cleavage
alkylation artifacts
isotope peaks

So high mass accuracy and manual validation are essential.

This is exactly why Orbitrap accuracy matters.

3.3 Glycosylation 🍬🧬

This is one of the most biologically complex PTMs.

Glycosylation = attachment of sugar chains.

Very important in:

membrane proteins
secreted proteins
antibodies
receptors

Two main types

N-linked glycosylation

Attached to asparagine (N)

Consensus motif:

Asn-X-Ser/Thr

This motif is extremely important.

Likely exam topic.

O-linked glycosylation

Attached to:

serine
threonine

via hydroxyl groups.

Biological importance

Glycosylation affects:

folding
stability
turnover
immune recognition
cell-cell interactions

This is huge in therapeutic antibodies.

For example Fc glycosylation changes antibody function.

Why glycosylation is difficult in MS

This is one of the hardest PTMs.

Because glycans are:

heterogeneous
branched
variable length

This makes spectra very complicated.

Much harder than phosphorylation.

This is a major concept.

Enrichment methods

The main one mentioned is:

Lectin affinity

Lectins bind specific sugars.

Examples:

ConA
WGA

This is extremely widely used.

Very important technique.

Two MS strategies

1) Intact glycopeptide analysis

Analyze peptide + glycan together.

Best information.

But hardest.

2) Deglycosylation first

Remove glycan using PNGase F

Then analyze peptide.

Very common.

Important detail:

Asparagine becomes aspartic acid

This gives +1 Da

Or +3 Da in ¹⁸O water

This helps site localization.

Very high-yield exam concept.

3.4 Other PTMs 🧪

This section gives additional important modifications.

Acetylation ✨

Mass shift:

+42 Da

Usually on:

N-terminus
lysine residues

Very important in histones and epigenetics.

Controls gene expression.

Histone acetylation often opens chromatin.

This promotes transcription.

Methylation 🧬

Can be:

mono
di
tri

Often on:

lysine
arginine

Extremely important in histone regulation.

Another epigenetic PTM.

Cysteine redox modifications 🔥

Very important in oxidative stress biology.

Cysteine thiol can form:

disulfide bonds
sulfenic acid
sulfinic acid
sulfonic acid
S-nitrosylation

This is crucial in protein folding and signaling.

Especially disulfide bonds for secreted proteins.

Example:

antibodies insulin

Very relevant for structural biology.

S-nitrosylation

Very important disease-related PTM.

This is labile.

So direct MS is difficult.

The paper mentions biotin switch assay.

This is an important indirect detection strategy.

Big picture takeaway 🎯

This chapter teaches one major concept:

Mass spectrometry can identify PTMs by measuring characteristic mass shifts and localizing modified residues after enrichment and fragmentation.

The core workflow is:

enrichment → LC-MS/MS → fragmentation → site localization

The most important PTMs here are:

phosphorylation
ubiquitination
glycosylation
acetylation
methylation
redox modifications

Each one changes protein biology dramatically.

Quiz

Score: 0/30 (0%)

Q0. Why does the chapter say the eukaryotic proteome is much more diverse than the genome?

Because DNA replication introduces many permanent protein variants

Because of post-transcriptional regulation and post-translational modification

Because every protein is always glycosylated and ubiquitinated

Because proteins are translated from multiple genomes in the same cell

Q1. What is the main mass spectrometry principle used to detect post-translational modifications in this chapter?

Changes in UV absorbance of modified residues

Loss of all peptide charge after fragmentation

Detection of characteristic mass shifts caused by modifications

Direct visualization of amino acid side chains by microscopy

Q2. Why are enrichment techniques commonly required before PTM analysis by mass spectrometry?

Because modified peptides are generally too large to enter the instrument

Because PTMs are often present at relatively low levels

Because trypsin cannot digest modified proteins

Because LC-MS/MS only works on enriched samples

Q3. Which statement best describes the biological role of phosphorylation according to the chapter?

It is mainly a structural modification with little regulatory importance

It only affects protein degradation

It influences folding, activity, localization, interactions, and degradation

It occurs only in nuclear proteins

Q4. Which enrichment strategy uses metal ions such as Fe(III), Ga(III), Al(III), and Zr(III) to capture phosphopeptides?

Hydrazide chemistry

Lectin affinity

IMAC

Biotin switch chemistry

Q5. What is a major advantage of TiO2 in phosphopeptide enrichment?

It selectively enriches only tyrosine-phosphorylated proteins

It has shown higher capacity and better selectivity for phosphopeptides

It fragments phosphopeptides during enrichment to simplify MS/MS

It eliminates the need for any sample fractionation

Q6. Why is MS analysis of phosphopeptides more difficult than analysis of unmodified peptides?

Phosphopeptides cannot be separated by liquid chromatography

Phosphopeptides are always too hydrophobic for ionization

The phosphoryl group reduces ionization efficiency and the phosphoester bond is labile

Phosphopeptides never produce informative fragment ions

Q7. Which fragmentation approaches are highlighted as useful for preserving phosphorylation while generating backbone fragments?

ECD and ETD

Only low-energy CID

MALDI and SELDI

Edman degradation and amino acid analysis

Q8. Why can phosphorylation site assignment be ambiguous in some phosphopeptides?

Because phosphorylation prevents database searching entirely

Because all serine, threonine, and tyrosine residues have identical fragment ions in every spectrum

Because fragment ions may not define the site adequately or multiple phosphoisomers may coelute and cofragment

Because phosphopeptides cannot be sequenced by MS/MS

Q9. What is the key mass spectrometric signature of a ubiquitinated peptide after trypsin digestion?

A phosphate group adding 80 Da

An acetyl group adding 42 Da

A diglycine remnant adding 114.043 Da

A methyl group adding 14 Da

Q10. Why does the chapter warn that ubiquitination assignments can generate false positives?

Because ubiquitin is too small to be detected by mass spectrometry

Because a 114 Da shift can also arise from artifacts or other sequence features

Because ubiquitination only occurs in bacteria

Because trypsin always removes the modification completely

Q11. Which ubiquitin chain linkage is described as primarily targeting proteins for proteasomal degradation?

K63 linkage

K48 linkage

Tyrosine linkage

N-terminal linkage

Q12. Which sequence motif is associated with N-linked glycosylation?

Asn-X-Ser/Thr, where X is any amino acid except proline

Lys-X-Lys

Ser-Pro-Tyr

Gly-Gly-Lys

Q13. Why is large-scale analysis of intact glycopeptides challenging?

Because glycans are too small to detect

Because glycan heterogeneity complicates spectra and sugars make peptide fragmentation difficult

Because glycopeptides cannot be enriched before MS

Because N-linked glycans cannot be removed enzymatically

Q14. What mass change can be used to help identify an N-glycosylation site after PNGase F treatment in heavy water?

A 3 Da increase on asparagine due to deamidation in 18O water

A 114 Da increase on lysine

A 42 Da increase at the N-terminus

A 57 Da increase on cysteine only

Q15. Phosphorylation plays essential roles in biological processes such as proliferation, differentiation, apoptosis, and cellular communication.

True

False

Q16. Phosphorylation is generally a stoichiometric process in which all copies of a given protein are phosphorylated at the same time.

True

False

Q17. SIMAC combines IMAC and TiO2 to help detect both monophosphorylated and multiphosphorylated peptides.

True

False

Q18. Under general CID conditions, phosphopeptides commonly undergo neutral loss of phosphoric acid or metaphosphoric acid.

True

False

Q19. The chapter states that software tools completely eliminate the need for manual validation of phosphorylation site assignments.

True

False

Q20. Ubiquitination is a reversible, ATP-dependent process catalyzed by an E1-E2-E3 enzyme cascade.

True

False

Q21. K63-linked polyubiquitin chains are described primarily as proteasomal degradation signals in the chapter.

True

False

Q22. Antiubiquitin antibodies and His-tagged ubiquitin approaches are both mentioned as ways to enrich ubiquitinated proteins.

True

False

Q23. O-linked glycosylation is typically attached to the hydroxyl group of serine or threonine residues.

True

False

Q24. Lectin affinity is described as one of the most widely used glycosylation enrichment methods because lectins bind specific sugar moieties.

True

False

Q25. CID of intact glycopeptides usually provides more information about peptide backbone sequence than about glycan structure.

True

False

Q26. Lysine acetylation and N-terminal acetylation are both discussed in the acetylation subsection.

True

False

Q27. Acetylation and trimethylation are easy to distinguish without high mass accuracy because their mass shifts are identical.

True

False

Q28. Protein methylation can occur as mono-, di-, and trimethylation, so multiple possibilities should be considered in database searching.

True

False

Q29. S-nitrosylation is described as a labile reversible cysteine modification that is often measured indirectly rather than by direct MS alone.

True

False