18. Amino Acids and Proteins

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.

Indicate whether each of the following molecules is an α-amino acid or not, and explain why.  

a.

What makes every standard amino acid unique?

Proteins are usually least soluble in water at their isoelectric points. Explain.

How could you make the zwitterion of aspartic acid more soluble in water?

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?

Is serine chiral? Draw serine and identify the chiral atom. Explain why serine is chiral.

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?

Draw leucine and identify any chiral carbon atoms with arrows.

Identify some differences between the following pairs:

b. complete and incomplete proteins

Match each protein in column A with its function in column B:

Glycine has the zwitterion structure shown. Draw the structure and give the net charge of glycine that will predominate at the indicated pH values (pI = 6.0).

a. pH 1.5

Two of the 20 common amino acids have two chiral carbon atoms in their structures. Identify these amino acids and their chiral carbon atoms.

Which would you expect to be more soluble in water, a peptide containing mostly alanine and leucine or a peptide containing mostly lysine and aspartic acid? Explain. (Hint: Consider side-chain interactions with water.)

What kind of biological function would each of the following proteins perform?

c. Protease