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Chapter 5: Q15P (page 127)
A pentapeptide has the sequence(using one-letter symbols for amino acids). Calculate the mass of this peptide, as determined by mass spectrometry, to three significant figures.
The mass of this peptide, as determined by mass spectrometry, to three significant figures is .
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Explain why a certain protein has an apparent molecular mass of 90kDwhen determined by gel filtration and 60kDwhen determined by SDS-PAGE in the presence or absence of 2-mercaptoethanol. Which molecular mass determination is more accurate?
Electrospray ionization mass spectrometry (ESI-MS) of proteins involves creating positively charged ions of the protein and separating them according to their mass-to-charge ratio (m/z).
(a) What causes the different positive charges on different particles of the protein?
(b) The amino acid composition (in numbers of residues per chain) of hen egg-white lysozyme (HEWL) is as follows:
P 2 Y 3 N 14 H 1
D 7 M 2 L 8 E 2
C 8 R 11 G 12 F 3
A 12 I 6 K 6 V 6
S 10 W 6 T 7 Q 3
What is the maximum positive charge that can be present on a HEWL ion?
Consult Table 5-1 to complete the following: (a) On a plot of absorbance at 280 nm versus elution volume, sketch the results of gel filtration of a mixture containing human cytochrome c and bacteriophage T7 RNA polymerase and identify each peak. (b) Sketch the results of SDS-PAGE of the same protein mixture showing the direction of migration and identifying each band.
(a) The ESI-MS spectrum below was obtained for hen egg-white lysozyme (HEWL). Using peaks 5 and 6, calculate the molecular mass of HEWL (see Sample Calculation 5-1).
[http://www.astbury.leeds.ac.uk/facil/MStut/mstutorial.htm. Published by A.E. Ashcroft, Astbury Centre for Structural Molecular Biology, University of Leeds (2000)]
(b) What is the charge on the ion that makes peak 5?
You are trying to purify a protein that is soluble in a solution of 2 M ammonium sulfate. After centrifugation to remove other proteins that have precipitated at this high salt concentration, you recover the supernatant to assay the target protein’s activity in a cell culture system.
(a) Explain why the cells die when incubated with the supernatant.
(b) What procedure could you perform to correct the problem? (Hint: See Section 2-1D).
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