[Lec 3] Mastering Biology: Structure of Protein

Question 1

Which biological activity does NOT directly involve proteins? a. Sensing light. b. Breaking food polymers into smaller molecules. c. Changing the shape of a cell. d. Defending cells against viruses. e. None of the above; proteins are involved in all of them.

Answer 1

e.

Question 2

The protein shown here has:

a. primary structure.
b. secondary structure.
c. tertiary structure.
d. All of the above.
e. All of the above, plus quaternary structure.

Answer 2

d.

Question 3

This ribbon diagram represents a protein in water. Even without showing the side chains, it’s clear that the protein’s quaternary structure:

a. is stabilized by hydrogen bonds.
b. is stabilized by forces between backbone groups.
c. is weaker than that of entirely helical proteins.
d. consists of helices.
e. None of the above. There’s no quaternary structure.

Answer 3

e.

Question 4

The human myoglobin protein contains 153 amino acids. If you take one guess at the amino acid sequence, what is your chance of being right?

a. One chance in 153.
b. One chance in 153^20.
c. One chance in 20x153.
d. One chance in 18^153.
e. One chance in 20^153.

Answer 4

e.

Question 5

A biochemist modified a protein so the amino acid lysine occurred where the amino acid aspartic acid previously occurred. This change could:

a. (a) alter the protein’s secondary structure without affecting the primary structure.
b. (b) alter the protein’s tertiary structure.
c. (c) affect the protein’s backbone.
d. Both (b) and (c).
e. (a), (b), and (c).

Answer 5

b.

Question 6

Identify the empirical formula of a free amino acid whose side chain is just H.

a. CH6O2N
b. C2H5O2N
c. C2H4O2N
d. C2H5ON
e. None of the above.

Answer 6

b.

Question 7

An amino acid residue in a protein differs from a free amino acid in having …

a. (a) one less H.
b. (b) one less OH.
c. (c) one less H and one less OH.
d. Either (a) or (b).
e. Could be any of the above.

Answer 7

e.

Question 8

A residue in the middle of a polypeptide has −CH3 as its side chain or R group. How many atoms does the residue contain?

a. 7
b. 9
c. 10
d. 12
e. 13

Answer 8

c.

Question 9

Amino acids are called “acids” because they …

a. contain amino groups in the side chain part.
b. contain carboxyl groups in the side chain part.
c. act as acids when they are bound to proteins.
d. contain carboxyl groups in the backbone part.
e. contain amino groups in the backbone part.

Answer 9

d.

Question 10

In a protein, peptide bonds connect …

a. C-R to N-H.
b. N-H to C-H.
c. C=O to N-H.
d. C=O to C-R.
e. All the above.

Answer 10

c.

Question 11

Which statement is true of the side chains that occur in proteins?

a. (a) Some of them contain only C and H.
b. (b) Some of them contain carboxyl groups.
c. (c) None of them join the backbone at more than one point.
d. Both (a) and (b).
e. All the above.

Answer 11

d.

Question 12

A certain amino acid side chain ionizes at low pH but not at very high pH. What else is true of this side chain?

a. It is acidic.
b. It is one of 7 kinds of amino acids that share this property.
c. It contains a carboxyl group.
d. It donates H+ to water at low pH.
e. It contains an amino group.

Answer 12

e.

Question 13

In this diagram a biological polymer is being broken down; the fuzzy yellow lines represent a chemical reaction that is removing a subunit. Which statement is true?

a. (a) The reaction is removing a nucleotide from a DNA molecule.
b. (b) R represents an atom that could belong to one of 20 chemical elements.
c. (c) The arrow is pointing at a peptide bond.
d. (d) The reaction is a decomposition.
e. Both (b) and (d).

Answer 13

c.

Question 14

The helical foldings of proteins are stabilized mainly by bonds between …

a. ionic groups.
b. S and S.
c. side chains.
d. water molecules.
e. CO and NH.

Answer 14

e.

Question 15

Which of the following is true of pleated sheet foldings within a polypeptide?

a. Its loops are held in place mainly by disulfide bridges.
b. The side chains are parallel to the plane of the sheet.
c. They depend on regular occurrence of CO and NH.
d. They are part of the polypeptide’s quaternary structure.
e. All the above.

Answer 15

c.

Question 16

What will probably be the effect on a protein if you replace the amino acid proline with the amino acid glycine (side chain -H) at several points?

a. The altered protein will have fewer hydrogen bonds than before.
b. The altered protein will have longer helices than before.
c. The altered protein will have shorter helices than before.
d. The primary structure of the altered protein will be shorter than before.
e. There will be less rotation around backbone bonds than before.

Answer 16

b.

Question 17

The helical foldings in proteins …

a. are kept folded by hydrogen bonds.
b. are part of the protein’s primary structure.
c. are kept folded by forces between side chains on adjacent turns of the helix.
d. are kept folded by base-pairing.
e. None of the above.

Answer 17

a.

Question 18

What do the three main forces that stabilize protein tertiary structure have in common?

a. (a) They involve the side chains.
b. (b) They involve the water around the protein.
c. (c) They are weaker than covalent bonds.
d. Both (a) and (b).
e. Both (a) and (c).

Answer 18

a.

Question 19

Among the forces that stabilize protein tertiary structure, hydrogen bonds are especially important because they are …

a. more numerous than the other forces.
b. less associated with the backbone than the other forces.
c. stronger than the other forces.
d. more associated with side chains than the other forces.
e. more resistant to environmental disturbance than other forces.

Answer 19

a.

Question 20

Which fact results from the presence of both polar and nonpolar side chains in a protein?

a. Proteins ionize when they are placed in water.
b. pH has a strong effect on secondary structure.
c. Water has a strong effect on tertiary structure.
d. A protein’s folding doesn’t depend on the polarity of the environment.
e. Each protein has many functions.

Answer 20

c.

Question 21

The sequence of polar and nonpolar side chains has a strong effect on a protein’s folding mainly because …

a. nonpolar side chains repel water.
b. polar side chains attract one another.
c. water attracts polar but not nonpolar groups.
d. water repels nonpolar side chains.
e. nonpolar side chains attract one another.

Answer 21

c.

Question 22

The graph shows how the rate of action by a certain enzyme protein responds to temperature. The decrease between 43∘C and 60∘C probably results from …

a. breaking peptide bonds.
b. changing primary structure.
c. breaking hydrogen bonds.
d. breaking disulfide bridges.
e. ionizing side chains.

Answer 22

c.

Question 23

When a protein has been unfolded enough to lose its function, the protein has been …

a. metastasized.
b. denatured.
c. hydrolyzed.
d. distempered.
e. None of the above.

Answer 23

b.

Question 24

The amino acid lysine has an amino group in its side chain. In a protein, a scientist replaced every lysine with serine (side chain -CH2OH). The alteration made the protein’s folding …

a. less dependent on hydrogen bonds.
b. less sensitive to pH.
c. more dependent on amino acid sequence.
d. more sensitive to pH.
e. less sensitive to heat.

Answer 24

b.

Question 25

Which factor is most important in determining a protein’s optimum pH?

a. The locations of side-chain carboxyl groups.
b. The number of backbone carboxyl groups.
c. The number of amino groups in the protein’s backbone.
d. The sensitivity of hydrocarbon side chains to pH.
e. The pH of the protein’s environment.

Answer 25

a.

Question 26

The two cysteine residues in this picture are positioned just right to make …

a. (a) the most common kind of bond that stabilizes tertiary structure.
b. (b) an especially strong hydrogen bond.
c. (c) the strongest kind of bond that stabilizes tertiary structure.
d. Both (a) and (b).
e. Both (b) and (c).

Answer 26

c.

Question 27

Why don’t cells rely more on disulfide bridges to stabilize the folding of proteins?

a. Though strong, disulfide bridges put a strain on the backbone.
b. Disulfide bridges are too weak. Proteins can get more stability from ionic forces.
c. They make the protein rigid. Many proteins change their shape as they work.
d. Disulfide bridges can only occur just after proline in the amino acid sequence.
e. There’s no room for more disulfide bridges. Most proteins have many of them.

Answer 27

c.

Question 28

To make a disulfide bridge, it’s necessary to …

a. perform a hydrolysis reaction.
b. remove two OH groups.
c. remove two H atoms.
d. remove an H and an OH.
e. None of the above.

Answer 28

c.

Question 29

A certain protein is not very sensitive to pH. It may have many side chains with ________ groups.

a. -NH2
b. carboxyl
c. -PO3H2
d. -CH2OH
e. amino

Answer 29

d.

Question 30

Which of the following is NOT true of protein quaternary structure?

a. Hydrogen bonds may hold the polypeptides in contact.
b. A quaternary protein cannot have fewer than two carboxyl groups.
c. A single polypeptide may have quaternary structure.

Answer 30

c.

Question 31

Some of the strongest biological structures (e.g., beaks and claws) are made of many molecules of the protein keratin. What else is true of structures made of keratin?

a. (a) Disulfide bridges bind the proteins together.
b. (b) Each protein is a single long alpha helix.
c. (c) Hair is another example.
d. Both (a) and (b).
e. (a), (b), and (c).

Answer 31

e.

Question 32

Proteins are polymers of _____.

a. glycerol
b. CH2O units
c. amino acids
d. hydrocarbons
e. nucleotides

Answer 32

c.

Question 33

What type of bond joins the monomers in a protein’s primary structure?

a. ionic
b. S - S
c. peptide
d. hydrogen
e. hydrophobic

Answer 33

c.

Question 34

The secondary structure of a protein results from _____.

a. hydrophobic interactions
b. peptide bonds
c. hydrogen bonds
d. bonds between sulfur atoms
e. ionic bonds

Answer 34

c.

Question 35

Tertiary structure is NOT directly dependent on _____.

a. ionic bonds
b. hydrogen bonds
c. bonds between sulfur atoms
d. peptide bonds
e. hydrophobic interactions

Answer 35

d.