Protein Mass Spectrometry

🧪 Mass Spectrometry (Proteomics) – Key Notes

🔑 Core Principles

• Mass spectrometry measures the mass-to-charge ratio (m/z), not the mass directly.

• Molecules must be ionised (charged) to be detected.

• The technique operates in the gas phase, meaning samples transition from liquid → gas.

• Larger molecules (like proteins) acquire multiple charges, unlike small molecules.

⚡ Ionisation & Charge States

• Electrospray ionisation adds protons (H⁺) to molecules, creating charged species.

• Small molecules typically have +1 charge, while proteins can have many charges (e.g., +10 to +100).

• A single protein produces multiple peaks, each representing a different charge state, not different proteins.

📊 CALCULATIONS (Most Important Section)

1. 🧮 Fundamental Equation

The relationship between m/z and molecular weight is:

[
m/z = \frac{M + zH}{z}
]

Where:

• M = molecular weight of the protein

• z = number of charges

• H = mass of a proton (~1.0078 Da)

👉 Rearranged to find molecular weight:

[
M = z(m/z) - zH
]

• You must subtract the mass of added protons when calculating true molecular weight.

2. 🔍 Determining Charge State (z)

Method A: Isotope Peak Spacing

• The difference between C¹² and C¹³ peaks helps determine charge.

• Charge is calculated using:

[
\text{Charge (z)} = \frac{1}{\text{peak spacing}}
]

Examples:

• Spacing = 1 → z = 1

• Spacing = 0.5 → z = 2

• Spacing = 0.33 → z = 3

👉 As charge increases, peak spacing decreases.

Method B: Adjacent Peaks (Proteins)

• Adjacent peaks correspond to z and (z+1) charge states.

• Use two m/z values and solve simultaneous equations:

[
m_1 = \frac{M + zH}{z}, \quad m_2 = \frac{M + (z+1)H}{z+1}
]

• Solve to find z, then calculate M.

Method C: Trial-and-Error (Common in Practice)

• Assume a charge (e.g., z = 10).

• Calculate M from one peak.

• Repeat with adjacent peak using z+1.

• If both give the same M, the charge assumption is correct.

👉 This is often faster than solving equations manually.

3. 🧠 Key Calculation Logic

• Multiple peaks = same molecule, different charges.

• Correct calculation should give consistent molecular weight across peaks.

• Always:

  1. Determine charge (z)

  2. Substitute into equation

  3. Calculate molecular weight (M)

4. 📈 Protein Spectra Interpretation

• Proteins show a distribution of peaks due to multiple charge states.

• Charge states differ by ±1 between adjacent peaks.

• Higher charge → lower m/z value.

5. ⚠ Common Pitfalls

• Do not assume multiple peaks = multiple proteins.

• Forgetting to subtract proton mass leads to incorrect molecular weight.

• Misidentifying charge state causes large calculation errors.

🔬 Why These Calculations Matter

• Changes in molecular weight indicate:

  • Mutations (e.g., sickle cell haemoglobin)

  • Post-translational modifications (e.g., phosphorylation adds mass)

  • Drug binding (mass increases when ligand binds)

• Mass spectrometry is used to detect small mass differences that reflect biological changes.

🧩 Extra Insight (High-Yield Understanding)

• Folded proteins have lower charge states (compact structure).

• Unfolded proteins have higher charge states (more exposed surface → more protonation).

• Therefore, charge distribution can indicate protein structure/conformation, not just mass.

Protein Mass Spectrometry Overview

Introduction

• Speaker: Dr. Aneika Leney

• Event ID: Ибуке: 3000, 400, R-A8 10.00

• Equipment mentioned: Eclipse Mass Spectrometer by Thermo Fisher Scientific

Key Learning Outcomes

1. Recognize a protein mass spectrum.

2. Calculate the molecular weight of proteins and predict protein mass spectra.

3. Understand that mass spectrometry provides more information than mass alone.

Definition of Mass Spectrometry

• IUPAC Definition: "Mass spectrometry is the study of matter through the formation of gas-phase ions that are detected and characterized by their mass and charge."

Understanding the Protein Mass Spectrum

• Intensity and m/z (mass-to-charge ratio) relationship:

  • The notation (M+H)+ relates to the singly charged state of the molecule.

  • Common isotopes such as Carbon 13 are also represented in the spectrum.

• Molecular weight (MW) calculation formula: $ext{MW} = (m/z)_n - nH^+$ Example Calculation:

  • Given: $(m/z)=600$

  • Calculation:
    $ext{MW} = 600 imes 1 - 1 imes 1.008 = 599 ext{ Da}$

  • Additional m/z values: 600, 601, etc.

Electrospray Ionization

• Process overview:

  • Involves spraying analyte molecules into a solvent through a nozzle.

  • Mechanisms include evaporation, Coulomb fission, resulting in charged droplets.

  • Key structures involved: Taylor cone, parent charged droplet, naked charged analyte.

  • The generation of multiple generations of charged progeny droplets from a single parent droplet.

  • Reference: Banerjee S. et al., International Journal of Analytical Chemistry 2012(8):282574.

Charge States in Mass Spectrometry

• Observed trends in charge states:

  • As the molecular size increases, the number of charges also increases:

  • Amino Acids → 1+

  • Peptides → ~1+ to 4+

  • Proteins → 10+ to >40+

  • Protein complexes can exceed 100 charges.

Analyzing the Peptide Mass Spectrum

• Spectrum characterization for small molecules and peptides:

  • Visualization shows a pattern of m/z vs. intensity with respective states: (M+H)+, (M+2H)2+, etc.

• Understanding how intensity relates to the presence of isotopes (C12 and C13) in the spectrum.

Calculating Molecular Weights in Mass Spectrometry

• Examples of weight calculations:

  1. When charge is known:

  • Given 2290 m/z at 8+, calculation yields:
    $ext{MW} = 2290 imes 8 - 8 imes 1.008 = 18311.7 ext{ Da}$

  1. When charge is unknown:

  • General formula application can determine properties from m/z ratios.

  1. Another approach for unknown charge:

  • $ext{MW} = (m/z)_n - nH^+$

  • For 2617 m/z observed, applying 2290 m/z precedents to solve the unknown charge.

  • Example derived equations:
    $ext{(m/z)}<em>n - nH^+ = (m/z)</em>{n+1} - (n+1)H^+$ and resolving for n.

  • Result yields n=7 based on provided data.

Importance of Protein Molecular Weight

• Observations of mutations, such as a single point mutation in hemoglobin leading to disease (Glu6-Ala) with significant mass change implications.

• Hemoglobin Characteristics:

  • Molecular Weight of normal hemoglobin: 64450 Da.

  • Predicted molecular weight with mutation: 64450 - 58 Da = 64398 Da.

  • Data is utilized to compare weights of wild-type (WT) and mutant forms of proteins.

Applications of Mass Spectrometry in Biology

• Key areas of focus:

  1. Protein-ligand binding.

  2. Changes in protein-protein interactions.

  3. Post-translational modifications on proteins.

  4. Mutations in proteins leading to diseases.

  5. Protein-drug binding interactions.

Native Mass Spectrometry

• Definition: Analysis of proteins and protein complexes in their native state, preserving conformational characteristics.

• Significance:

  • Allows the measurement of protein-protein and protein-ligand interactions more accurately.

  • Comparison of native versus denatured states showing differences in charge state profiles.

• Might struggle with processing large sample sizes; suggestions for faster techniques needed.

Disease Monitoring using Mass Spectrometry

• Specific diseases such as Alzheimer's, Parkinson's (example: α-synuclein), and Creutzfeldt-Jakob Disease are monitored for protein conformation changes.

• Mass spectrometry's role in these observations highlights its importance in diagnosing and understanding progression in these conditions.

Summary

• Learning to interpret mass spectra, calculate molecular weights, and annotate various peaks is critical in both understanding and applying mass spectrometry in research and clinical contexts.