Tandem Mass Spectrometry in Protein Analysis

Protein Function and Analysis

Tandem Mass Spectrometry (MS/MS)

Tandem mass spectrometry is a powerful analytical technique used to identify and characterize proteins by fragmenting them and analyzing the resulting ions. It is a gold standard for sequencing proteins and can analyze purified proteins or proteins within mixtures.

• Definition: Tandem MS (MS/MS) uses two mass spectrometers in sequence, allowing for detailed analysis of protein fragments.

• Purpose: Enables filtering of unwanted ions and produces a much cleaner and more informative mass spectrum.

• Applications: Used for sequencing proteins, identifying post-translational modifications, and analyzing complex mixtures.

Steps in Tandem Mass Spectrometry

1. Protein Purification: The protein of interest is first purified, either from a mixture or as a single protein.

2. Fragmentation: Proteins are enzymatically digested (e.g., with trypsin) into smaller peptide fragments.

3. Ionization: Peptide fragments are ionized, often using electrospray ionization (ESI) or matrix-assisted laser desorption/ionization (MALDI).

4. Mass Selection: The first mass spectrometer selects a specific peptide ion for further fragmentation.

5. Collision-Induced Dissociation (CID): The selected ion is fragmented further in a collision cell, typically with an inert gas.

6. Second Mass Analysis: The second mass spectrometer analyzes the resulting fragment ions, producing a spectrum that can be interpreted to deduce the peptide sequence.

Key Points

• Fragmentation Patterns: Peptide fragments break at specific points, often at peptide bonds, generating a series of ions that differ by the mass of individual amino acids.

• Mass-to-Charge Ratio (m/z): The mass spectrometer measures the m/z of each fragment, which is used to deduce the sequence.

• Data Interpretation: The mass differences between consecutive ions correspond to the masses of amino acid residues, allowing for sequence determination.

Example: Tandem Mass Spectrometry of a Purified Protein and a Single Protein in a Mixture

In the provided diagram, a protein is purified, fragmented, and analyzed using tandem mass spectrometry. The resulting spectra allow for identification of the protein and its sequence, even in complex mixtures.

Practice Questions

• Question: Tandem mass spectrometry combines which of the following devices?

  • Mass spectrometer with HPLC

  • Mass spectrometer with chromatography

  • Mass spectrometer with a PMF table

  • Mass spectrometer with a mass spectrometer (Correct)

• Question: In tandem mass spectrometry of a pure protein, you found a fragment with an m/z of 1,168. It passes into the second mass spec and found ions of 1,137 and 1,082. The mass in Daltons for the possible relevant amino acids are provided: Y (163), N (114), D (115), G (57), L (113), M (131). What is the order of the first two amino acid residues in the 1,268 fragment from N-terminal to C-terminal?

  • a) N, M (Correct)

  • b) N, Y

  • c) M, D

  • d) M, L

  • e) D, N

Additional info:

• Equation for Mass-to-Charge Ratio:

$m/z = \frac{M + nH}{n}$

• Where: M = mass of the ion, n = charge state, H = mass of a proton (1 Da).

Summary Table: Devices Combined in Tandem Mass Spectrometry

Device 1	Device 2	Purpose
Mass Spectrometer	Mass Spectrometer	Sequential analysis of precursor and fragment ions
Mass Spectrometer	HPLC	Separation of peptides before MS analysis (Additional info: not tandem MS/MS)