18. Amino Acids and Proteins

What level of protein structure is determined by the following:

a. Peptide bonds between amino acids?

What level of protein structure is determined by the following:

b. Hydrogen bonds between backbone carbonyl oxygen atoms and hydrogen atoms attached to backbone nitrogen atoms?

How do the following noncovalent interactions help to stabilize the tertiary and quaternary structure of a protein? Give an example of a pair of amino acids that could give rise to each interaction.

a. Hydrophobic interactions

Determine which of the following statements describes a tertiary structure of a protein.

Indicate whether each of the following statements describes a primary, secondary, tertiary, or quaternary protein structure:

______ a) Hydrophobic R groups seeking a nonpolar environment move toward the inside of the folded protein.  

______ b) Protein chains of collagen form a polypeptide chain composed of 3 alpha helices. 

______ c) An active protein contains 4 tertiary subunits. 

______ d) Two polypeptide chains held together by disulfide bridges. 

How do the following interactions help to stabilize the tertiary and quaternary structure of a protein? Give an example of a pair of amino acids that could give rise to each interaction.

b. Disulfide bonds

A portion of a polypeptide chain contains the following sequence of amino acids: —Leu—Val—Cys—Asp— 

a. Which amino acids are likely to be found on the inside of the protein structure? Why?

A portion of a polypeptide chain contains the following sequence of amino acids:

—Leu—Val—Cys—Asp—

c. How does the primary structure of a protein affect its tertiary structure?

Indicate whether each of the following statements describes primary, secondary, tertiary, or quaternary protein structure: 

c. Several polypeptides in a beta-pleated sheet are held together by hydrogen bonds between adjacent chains.

Indicate whether each of the following statements describes primary, secondary, tertiary, or quaternary protein structure:

b. Protein chains of collagen form a triple helix.

Indicate the changes in secondary and tertiary structural levels of proteins for each of the following:

b. Prior to giving an injection, the skin is wiped with an alcohol swab.

Indicate the changes in secondary and tertiary structural levels of proteins for each of the following:

c. To avoid spoilage, seeds are treated with a solution of HgCl₂.

b. Would you expect to find this segment at the center or at the surface of a protein? Why?

Consider the amino acids lysine, valine, and aspartate in an enzyme. State which of these amino acids have R groups that would: (16.1, 16.3)

b. be found in hydrophilic regions

Consider the amino acids lysine, valine, and aspartate in an enzyme. State which of these amino acids have R groups that would: (16.1, 16.3)

c. form hydrogen bonds

Consider the amino acids lysine, valine, and aspartate in an enzyme. State which of these amino acids have R groups that would: (16.1, 16.3)

d. form salt bridges

Classify each of the following proteins according to its function:

a. hemoglobin, oxygen carrier in the blood

Classify each of the following proteins according to its function:

c. keratin, a protein found in hair

Classify each of the following proteins according to its function:

a. insulin, a protein needed for glucose utilization

Classify each of the following proteins according to its function:

c. casein, milk protein

List the type of attractive force disrupted and the level of protein structure changed by the following denaturing treatments:

a. adding salt to soy milk to make tofu

Identify each of the following statements as characteristic of protein denaturation or protein hydrolysis. 

a. Milk curdles when lemon juice is added to it.

Identify each of the following statements as characteristic of protein denaturation or protein hydrolysis. 

b. A protein breaks up into amino acid fragments.

Identify the level of protein structure associated with each of the following:

b. hydrogen bonding between backbone atoms

Identify the level of protein structure associated with each of the following:

d. salt bridges between polypeptides

Indicate what type(s) of intermolecular forces are disrupted and what level of protein structure is changed by the following denaturing treatments:

a. an egg placed in water at 100 °C and boiled for 10 minutes

Indicate what type(s) of intermolecular forces are disrupted and what level of protein structure is changed by the following denaturing treatments:

c. egg whites whipped in a mixing bowl to make meringue

Describe the changes that occur in the primary structure when a protein is denatured versus when a protein is hydrolyzed.

Identify the level of protein structure associated with each of the following:

d. intermolecular forces between R groups

Identify the level of protein structure associated with each of the following:

b. disulfide bridge

Determine whether each of the following statements describes the primary, secondary, tertiary, or quaternary structure of a protein.

b. Peptide bonds join amino acids in a polypeptide chain.

Determine whether each of the following statements describes the primary, secondary, tertiary, or quaternary structure of a protein.

d. Hydrogen bonding between amino acids in the same polypeptide gives a coiled shape to the protein.

Determine whether each of the following statements describes the primary, secondary, tertiary, or quaternary structure of a protein.

b. Hydrogen bonds form between adjacent segments of the backbone of the same protein to form a “folded-fan” structure.

Determine whether each of the following statements describes the primary, secondary, tertiary, or quaternary structure of a protein.

d. Amino acids react in a condensation reaction to form a peptide bond.

Insulin is a protein hormone that functions as two polypeptide chains whose amino acid sequences are as follows:

A chain: GIVEQCCTSICSLTQLENYCN

B chain: FVNQHLCGDHLVEALYLV CGERGFFYTPKT

b. Considering the amino acid sequences, suggest how these two polypeptide chains might be held together in an active insulin molecule.