Proteomics Flashcards
Which is capable of identifying a higher proportion of the proteome accurately? Peptide-centric or protein-centric approaches? Why?
• Peptide-centric approaches are capable of identifying a higher proportion of the proteome than protein-centric
o Many more protein identifications and parameters (e.g. peptides) per protein
o Every peptide generated becomes an individual parameter that represents a protein- important for quantitation
There is a lot of statistical power of using peptides to represent their parent proteins
What is the difference between relative and absolute quantitation?
o Relative vs absolute quantitation
Relative quantitation- how much there is in sample 1 vs in sample 2
Absolute quantitation- how many micrograms of each protein are present within a sample
What is the aim of comparative quantitative shotgun proteomics?
• Measures relative abundance across two different samples (one control and one test)
What are 3 methods to perform comparative quantitative shotgun proteomics?
- Method A-metabolic stable isotope labelling
- Method B- isotope tagging by chemical reaction
- Method C- stable-isotope incorporation via enzyme reaction
Describe the process of metabolic stable isotope labelling for quantitative shotgun proteomics and when it can be used
• Method A-metabolic stable isotope labelling
o Cells growing in culture or living tissues
o During the growth of the cell, incorporating a stable isotope
Stable isotopes used to quantify changes in protein abundance e.g. 2H or 13C
o Isotope is added to growth medium and cells take it up and incorporate it when they undergo protein translation
In this approach, an isotope labelled amino acid e.g. (leu or lys- light and heavy) must be incorporated during protein synthesis
• Cell will make proteins with light (or normal) isotope and proteins with heavy isotope
o Isotopes have identical chemical properties
o The proteins are then digested-should get a light version of the peptide and a heavy version of the peptide, which creates a mass difference
o Do chromatography and mass spectrometry-> the heavy and light versions of the peptides will appear as pairs: one from the control and one from the test
o Peptides tagged with differing isotopes will appear adjacent on a mass spectrum
Separated by known mass difference of the light and heavy amino acids as it is incorporated
o Area under each peak equate to relative abundance of the two peptides
Can use spectral intensity of the peaks in the mass spectrometer as a means of measuring the abundance difference (relative abundance)
What is an example of a tag/method used for metabolic stable isotope labelling
- Stable isotope labelling by amino acids in cell culture (SILAC)
What is the process of stable isotope labelling by amino acids in cell culture (SILAC) and what are its requirements?
• Process-
o Labelled peptides-> optional protein purification-> combine control and test samples and digest with trypsin-> quantitation by MS
• Grow cells in presence of a labelled amino acid for which the cell cannot biosynthesise
o Incorporation of SILAC label over time
• No labelling step as label incorporated during synthesis
o E.g. deuterated leucine (Leu)
o 100% Leu labelled, approx 50% peptides
Leucine is the most common amino acid, and lysines are extremely common in tryptic peptides
• Allows differentiation of leucine and isoleucine
• Needs cells grown in culture
Describe an example of proteomics of B cell differentiation using SILAC quantitative proteomics
- Define B cells
- Differentiation of B cells
- Study details
• Example- proteomics of B cell differentiation using SILAC quantitative proteomics
o B cells are immune cells (lymphocytes)
o Differentiate into plasma cells that secrete antibodies against non-self antigens (eg. bacteria)
o Stimulate differentiation by treating with bacterial lipopolysaccharide (LPS)
o 234 protein expression changes, including a cluster involved in antibody production
o Looked at how protein expression changed over time (samples were collected over several days)
o Performed MS-based quantitation based on unlabelled vs 13C6-Leu
Dynamic quantification shown at the single peptide level during B cell differentiation
Control- day0/day0: this should be identical
o Cluster analysis and cellular location of 234 differentially abundant proteins
Enables for formation of hypotheses
What is cluster analysis?
Cluster analysis-task of grouping a set of objects in such a way that objects in the same group are more similar to each other than to those in other clusters
What are the advantages of stable isotope labelling by amino acids in cell culture (SILAC)?
- Very reproducible
- Generally high labelling efficiency
- Multiple peptides per protein- therefore good statistical confidence
- Compatible with PTMs
- Compatible with most cell-based systems
What are the disadvantages of stable isotope labelling by amino acids in cell culture (SILAC)?
- Need cells growing in culture
- Some cells are fussy- poorly incorporate label
- Human based Tissue studies
- Blood/plasma don’t contain metabolism cells
- Also results in 2 ions or more per peptide in MS scans
Describe the SILAC mouse, how it is produced and why it is so useful
• The SILAC mouse
o For tissue studies
o Take a mouse-> give it a lysine free diet supplemented with either a light or heavy version of the amino acid-> make a SILAC mouse (with either light or heavy amino acid) depending on supplied amino acid
Good incorporation over a period of time for many proteins except for haemoglobin (red blood cells)
o SILAC mouse can reveal information about protein turnover
What is a disadvantage of the SILAC mouse and how is it overcome? What is a potential problem with the solution?
Extended labelling times however do not result in 100% labelling efficiency- most likely due to more complex recycling of amino acids
• This can be overcome by feeding mice over several generations
o Newly born mice only ever know the light/heavy diet
o Results in close to 100% incorporation
o However, sometimes the mothers can eat their young so have to be careful
What is the SILAC fly useful for and how is it produced?
• The SILAC fly
o For tissue studies
o Make yeast in culture that incorporates light or heavy version of amino acids and exclusively breed one set to eat heavy or light yeast food
What is the SILAC worm useful for and how is it produced?
• The SILAC worm
o For tissue studies
o Grow SILAC worm on E.Coli.agar plate that either has heavy or light amino acid
Worms eat the E.Coli and either become labelled with heavy or light amino acid
o Compare the proteomes of the worms
What is the procedure for isotope tagging by chemical reaction in comparative shotgun proteomics and when can it be used?
o Can be used on all cell and tissue types as incorporated after protein synthesis
o Addition of a chemical tag specific for a handle in peptide sequences
Use a specific chemical handle to add a mass tag to peptides
Handles most commonly the primary amines of peptide N-termini or Lys residues or the -SH (thiol) group of Cys
Isotopes/chemical tags have identical chemical properties but are different in mass or are isobaric
o Area under each peak equates to relative abundance of the two peptides
What are isobaric tags in isotope tagging by chemical reaction in comparative shotgun proteomics and why are they useful?
• Isobaric- has the same mass
o Enables for quantitation at tandem mass spectrometry level
o Peptides tagged with differing isotopes will appear adjacent on a mass spectrum
What are tags used in isotope tagging by chemical reaction in comparative shotgun proteomics?
- Tags- Isotope coded affinity tags (ICAT)
- Tags- Isobaric affinity tags (iTRAQ-Isobaric tags for relative and absolute quantitation)
- Tags- Tandem Mass tags
Describe the process of isotope coded affinity tags for isotope tagging by chemical reaction in comparative shotgun proteomics and the quantification process
o Process:
Sample-> lysis-> label mix samples and digest-> 2-DLC separation -> MS scan-> selected for MS/MS
o Label proteins via cysteine residues
o Two tags therefore pair-wise comparisons
o Quantification procedure
Mix label sample 1 and label sample B in equal amounts-> combine and proteolyze-> avidin affinity enrichment-> MS1 scan -> get sequence-> protein identification and quantification
1-3 peptides per protein are used to accurately quantify the relative abundance of each protein
• Limited by amounts of cysteines
Non-cys containing peptides discarded > reduces sample complexity, but also loses information such as phosphorylation
Describe the structure of the isotope coded affinity tags for isotope tagging by chemical reaction in comparative shotgun proteomics and the purpose of this structure
o 3 components-
Thiol-specific group enables protein/peptide binding (functional group)
Linker of differing mass to differentiate control and test groups
• Chemically inert
Affinity tag for purification (biotin)
o Light tag (H) or heavy (D) tag used: differ by 8 Da
o Has a biotin molecule for affinity purification
What are the advantages of ICAT (isotope coded affinity tags) for isotope tagging by chemical reaction in comparative shotgun proteomics
- Reduces complexity
- Easy analysis
- Reproducible
What are the disadvantages of ICAT (isotope coded affinity tags) for isotope tagging by chemical reaction in comparative shotgun proteomics
-Not all proteins contain Cys (20-30% cannot be analysed)
Cysteines are the second least common coding amino acid (only makes up 1.1% or 1.3% of all known coding amino acids in the protein database)
Hence, this tag doesn’t bind many peptides (drawback)
-Those that do contain Cys may have only 1 or 2, hence replicates are needed for statistical analysis
-8 Da ICAT tag difference may complicate MS spectra- by doubling the number of ions in the MS scan
Describe the process of using isobaric affinity tags (iTRAO) for isotope tagging by chemical reaction in comparative shotgun proteomics and the process of quantification using these tags
• Tags- Isobaric affinity tags (iTRAQ-Isobaric tags for relative and absolute quantitation)
o Can do many samples (8 or more)
o Process:
Samples-> lysis-> digest-> label-> mix samples and 2-DLC-> MS scan shows several peptides from same protein-> MS/MS fragmentation cleaves reporter allowing quantitative comparison and identification
o Exploits primary amines
o Only does relative quantitation
o Differentiation at mass spectrometry level-
All peptides are tagged-> no affinity purification
Perform multi-dimensional liquid chromatogaphy
The 4 tags have the same mass so only one peak seen in precursor/MS scan
During MS/MS, tag fragments revealing 4 reporter ions
Reporter mass beneath those seen for y-1 ion
• Intensity of each peak is relative to total abundance of the protein
Allows four way comparison
When undertake fragmentation, remove peptide reactive group, get neutral loss at balance region and release reporter ions-
• Allow for relative quantification at MS:MS level and peptide sequence
Describe the components of isobaric affinity tags (iTRAO) for isotope tagging by chemical reaction in comparative shotgun proteomics and their uses
o 3 components to tag- Peptide reactive group • Amine specific (Lys and N terminal) Isobaric tag (total mass= 145) • Balance (mass 31 to 28) o Neutral loss in MS:MS o Balances the mass change of reporter to maintain a total mass of 145 • Reporter (mass 114 to 117) o Charged o Gives strong signature ion in MS:MS o Gives good b- and y-ion series o Maintains charge state o Maintains ionization efficiency of peptide o Signature ion masses lie in quiet low mass region