Summary of last week’s lecture:
Where will we go today?
The Mass Spec. Process Tandem Mass Spectrometry Controlled FragmentaAon
• Mass-‐to-‐charge raAo
• Importance of vacuum
• Ion focusing and detecAon
• SeparaAng ionizaAon from fragmentaAon
• the use of many stages of mass analysis
• CID, ECD, ETD, SORI, IRMPD
• Allows for product ion scans, precursor ion scans, neutral loss scans, SRM/
MRM
• PepAde fragmentaAon
1
sthalf 2
ndhalf
• A few more examples of pepAde fragmentaAon and sequencing
• Ion generaAon: DesorpAon techniques
A surprise !!!
Peptide fragmentation and sequencing
MS/MS of angiotensin III, [M + H]+ = m/z 932
Iden;fica;on and sequencing of proteins
IntroducAon
• Proteins play a crucial role in most biological processes:
• They funcAon as enzymes, receptors, anAbodies, hormones, for control of gene expression
• They are major components of body Assues (muscles) and fluids
• They transport and store small molecules
• A shorter chain of amino acids (30 – 40) is termed a pepAde, a longer chain a polypepAde.
Proteins are composed of discrete polypepAde chains linked together
• The human body contains > 100 000 proteins (bonded together by amide linkage of 20 amino acids)
• They are organized on four structure levels ( primary to quaternary structure)
• Here only the primary structure is discussed = the sequence of the amino acids (directly encoded by the organism’s genome)
• Proteins are synthesized in the cell body via several steps, which start with the transcrip)on of the geneAc informaAon from the pepAde gene to the mRNA, followed by transla)on of this informaAon into protein synthesis. These nascent proteins are transported to various
intracellular and extracellular locaAons.
• During this transport they undergo a variety of pos,rans-‐la)onal modifica)ons, e. g. acylaAon, carboxylaAon, glycosylaAon, phosphorylaAon and sulfataAon
Mass spectrometry plays an important rule in
determining the primary structure of proteins!!
We need a gentle way to transfer the ‘delicate’ protein into the gas phase and ionize it: 2 main opAons
Matrix-‐Assisted Laser DesorpAon/IonizaAon (MALDI):
• Large proteins may be mulAply charged (doubly or triply charged): [M + H]
+, [ M + 2H]
2+, [M + 3H]
3+• Smaller proteins and pepAdes are always singly charged
Electrospray IonizaAon (ESI):
• Proteins are always mulAply charged
• PepAdes formed by trypAc cleavage are typically doubly are triply charged
Iden;fica;on and sequencing of proteins
Calculating the mass of a multiply charged, INTACT protein:
M = mass of the unknown protein
n = number of charges (corresponding to addition of n protons)
m1 and m2 two adjacent multiply charged ions:
m1 = (M + n)/n (1) m2 = (M + n + 1)/(n + 1) (2)
Neurotensin
Iden;fica;on and sequencing of proteins
General protocol for determinaAon of primary structure of proteins
• Homogenize proteins e.g. from cell culture
• Separate by one or two dimensional gel electrophoresis using e.g. sodium dodecyl sulfate (SDS) – polyacrylamide gel electrophorese (PAGE)
• Measure mass of intact protein by MALDI or ESI
• Reduce disulfide bonds by 2-‐mercaptoethanol to form free sulfydryl groups and derivaAze with iodo aceAc acid (cysteine ogen prone to autooxidaAon
• Cleavage of the reduced and alkylated protein into smaller pepAdes by – Chemical degradaAon ( e.g. with cyanogen bromide (CNBr))
( This cleaves specifically the C – terminal side of methionine) – DigesAon with a protease, Trypsin most ogen used:
Trypsin, cleaves spefically lysine and arginine residues at the C-‐terminal side,
leads to medium sized pepAdes of 1000 – 2000 Da, which can be managed well by MS
• Mass spectrometric analysis
– Either directly of the pepAde mixture – Analysis ager LC (LC/MS)
• MS/MS of each pepAde of the mixture for sequence determinaAon
• IdenAficaAon of the pepAde by comparison with database
Iden;fica;on and sequencing of proteins
MALDI TOF spectrum of a trypAc digest of a 70 kDa heat shock protein
Many pepAde fragments formed
hkp://www.matrixscience.com/cgi/search_form.pl?FORMVER=2&SEARCH=MIS
You can upload your mass spectrum to websites such as this one:
Obtain a list of possible matches
(note: that this example does not correspond to the above spectrum)
Also get a list of matched peaks and the corresponding sequence
… and can see which parts of the library protein match peaks in the input spectrum
In this example, only 49% of the sequence matches
Down-‐sides to the trypAc digest approach to fragmenAng pepAdes/proteins:
• Missed cleavages
• Only parAal sequence coverage
• PoskranslaAonal modificaAons (PTMs) such as S-‐S bridges, phosphorylaAon, glycosylaAon, acetylaAon…
• Peaks from trypsin
Newer methods for fragmen;ng proteins
Electron Capture DissociaAon (ECD)
slow electrons are captured by a trapped ion, resulAng in extensive fragmentaAon along the backbone
Electron Transfer DissociaAon (ETD)
electron is transferred from one ion to another, resulAng in
fragmentaAon