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Problem 18.10
Is serine chiral? Draw serine and identify the chiral atom. Explain why serine is chiral.
Problem 18.101
Four of the most abundant amino acids in proteins are leucine, alanine, glycine, and valine. What do these amino acids have in common? Would you expect these amino acids to be found on the interior or on the exterior of the protein?
Problem 18.102a
Globular proteins are water-soluble, whereas fibrous proteins are insoluble in water. Indicate whether you expect the following amino acids to be on the surface of a globular protein or on the surface of a fibrous protein.
a. Ala
Problem 18.105a
A family visits a pediatrician with their sick child. The four-month-old baby is pale, has obvious episodes of pain, and is not thriving. The doctor orders a series of blood tests, including a test for hemoglobin types. The results show that the infant is not only anemic but that the anemia is due to sickle-cell anemia. The family wants to know if their other two children have sickle-cell anemia, sickle-cell trait, or no sickle-cell gene at all.
a. What test will be used?
Problem 18.110a
Which of the following amino acids is most likely to be found on the outside of a soluble protein, and which of them is more likely to be found on the inside? Explain each answer. (Hint: Consider the effect of the amino acid side chain in each case and that the protein is folded up into its globular form.)
a. Valine
Problem 18.112
List the amino acids with side chains that are capable of hydrogen bonding. Draw an example of two of these amino acids hydrogen bonding to one another. For each one, draw a hydrogen bond to water in a separate sketch. Refer to Section 8.2 for help with drawing hydrogen bonds.
Problem 18.15
Valine is an amino acid with a nonpolar side chain, and serine is an amino acid with a polar side chain. Draw the two dipeptides that can be formed by these two amino acids. Identify the peptide bond.
Problem 18.19
There are eight amino acids in vasopressin. How many peptide bonds are in this small protein?
Problem 18.20a
What atoms are present in a planar unit in a protein chain?
Problem 18.20b
How many amino acid units do these atoms come from? Why are these units planar?
Problem 18.21
How many ways can four different amino acids be arranged in a peptide so that each peptide is unique?
Problem 18.23
Examine the α-helix in Figure 18.1 and determine how many backbone C and N atoms are included in the loop between an amide hydrogen atom and the carbonyl oxygen to which it is hydrogen bonded.
Problem 18.24
Consult the β-sheet in Figure 18.2 and (a) name the bonding responsible for the sheet formation and (b) identify the specific atoms responsible for this bonding.
Problem 18.31
Both α-keratin and tropocollagen have helical secondary structure. How do these molecules differ in (a) amino acid composition and (b) three-dimensional structure?
Problem 18.36
Draw the hexapeptide Asp-Gly-Phe-Leu-Glu-Ala in linear form showing all of the atoms, and show (using dotted lines) the hydrogen bonding that stabilizes this structure if it is part of an α-helix.
Problem 18.38a
Cell membranes are studded with proteins. Some of these proteins, involved in the transport of molecules across the membrane into the cell, span the entire membrane and are called transmembrane proteins. The interior of the cell membrane is hydrophobic and nonpolar, whereas both the extracellular and intracellular fluids are water-based.
a. List three amino acids you would expect to find in the part of a transmembrane protein that lies within the cell membrane.
Problem 18.39
Threonine has two chiral centers. Draw l-threonine and indicate which carbon atoms are chiral. Which carbon atom is responsible for d and l configuration?
Problem 18.48
Draw leucine and identify any chiral carbon atoms with arrows.
Problem 18.50
Is phenylalanine hydrophilic or hydrophobic? Explain why.
Problem 18.5a
Indicate whether each of the following molecules is an α-amino acid or not, and explain why.
a. <IMAGE>
Problem 18.58
Proteins are usually least soluble in water at their isoelectric points. Explain.
Problem 18.59
How could you make the zwitterion of aspartic acid more soluble in water?
Problem 18.6
Using Table 18.3, name the α-amino acids that (a) contain an aromatic ring, (b) contain sulfur, (c) are alcohols, and (d) have alkyl-group side chains.
Problem 18.67
What is the sequence of atoms along the 'backbone' of a protein?
Problem 18.68
Bradykinin, a peptide that helps to regulate blood pressure, has the primary structure Arg-Pro-Pro-Gly-Phe-Ser-Pro-Phe-Arg.
b. Bradykinin has a very kinked secondary structure. Why?
Problem 18.68a
Bradykinin, a peptide that helps to regulate blood pressure, has the primary structure Arg-Pro-Pro-Gly-Phe-Ser-Pro-Phe-Arg.
a. Draw the complete structural formula of bradykinin.
Problem 18.7
Valine is an amino acid with a nonpolar side chain and serine is one with a polar side chain. Draw the two amino acids.
a. Why is the side chain for valine nonpolar, whereas the side chain for serine is polar?
Problem 18.74
Give an example of a protein containing primarily alpha-helices. Is this a fibrous or globular protein?
Problem 18.76b
What kind of bond would you expect between the side chains of the following amino acids?
b. Alanine and leucine
Problem 18.76c
What kind of bond would you expect between the side chains of the following amino acids?
c. Aspartic acid and asparagine
