mass of an amino acid is 110 daltons, what would be the mass of one molecule of each possible kind of polypeptide chain 300 amino acids in length? Is this greater than the mass of the universe?
homologous proteins is to search the database using a short signature sequence indicative of the particular protein function. Why is it better to search with a short sequence than with a long sequence? Do you not have more chances 0 for a “hit” in the database with a long sequence?
3–10 The so-called kelch motif consists of a four-stranded β sheet, which forms what is known as a β pro-
Figure Q3–2 Springlike behavior of titin (Problem 3–11). (A) The peller. It is usually found to be repeated four to seven times, structure of an individual Ig domain. (B) Force in piconewtons versus forming a kelch repeat domain in a multidomain protein. extension in nanometers obtained by atomic force microscopy.
a.
Are the data consistent with your hypothesis that titin’s springlike behavior is due to the sequential unfolding of individual Ig domains? Explain your reasoning.
b.
Is the extension for each putative domain-unfolding event the magnitude you would expect? (In an extended polypeptide chain, amino acids are spaced at intervals of 0.34 nm.)
C.
Why is each successive peak in Figure Q3–2B a little higher than the one before?
D.
Why does the force collapse so abruptly after each peak?
3–12 Rous sarcoma virus (RSV) carries an oncogene called Src, which encodes a continuously active protein tyrosine kinase that leads to unchecked cell proliferation. Normally, Src carries an attached fatty acid (myristoylate) group that allows it to bind to the cytoplasmic side of the plasma membrane. A mutant version of Src that does not allow attachment of myristoylate does not bind to the membrane. Infection of cells with RSV encoding either the normal or the mutant form of Src leads to the same high level of protein tyrosine kinase activity, but the mutant Src does not cause cell proliferation.
a.
Assuming that the normal Src is all bound to the plasma membrane and that the mutant Src is distributed throughout the cytoplasm, calculate their relative concentrations in the neighborhood of the plasma membrane. For the purposes of this calculation, assume that the cell is a sphere with a radius (r) of 10 μm and that the mutant Src is distributed throughout the cell, whereas the normal Src is confined to a 4-nm-thick layer immediately beneath the membrane. [For this problem, assume that the membrane has no thickness. The volume of a sphere is (4/3)πr3.] b.
The target (X) for phosphorylation by Src resides in the membrane. Explain why the mutant Src does not cause cell proliferation.