Question 1

Which of the following statements about cystine is true?
A) cystine is two cysteine molecules covalently attached by a disulfide bridge
B) cystine is formed by the reduction of two cysteines
C) cystine is a dipeptide
D) cystine is a tripeptide
E) cystine forms disulfide bridges with methionine

Accepted Answer

Cystine is formed when two cysteine molecules are covalently attached to each other through a disulfide bond. This bond is formed when the sulfhydryl groups (-SH) on cysteine oxidize to become a disulfide bond (-S-S-) between the two cysteine molecules. Therefore, the correct statement is A.

Question 2

The primary sequence of proteins is best described by the statement that proteins
A) contain amino acids.
B) include all the nitrogen compounds of tissues.
C) are salts of amino acids linked with one another.
D) are polymers formed by hydrogen bonding between the amino acids.
E) are polymers formed by amide linkages between the carboxyl and alpha-amino groups of alpha-amino acids.

Accepted Answer

between the carboxyl and alpha-amino groups of alpha-amino acids.
The primary structure of a protein refers to the specific linear sequence of amino acids linked together by peptide bonds, which are a type of amide linkage. This sequence determines the unique characteristics and function of each protein. The peptide bond is formed through a dehydration synthesis reaction between the carboxyl group of one amino acid and the alpha-amino group of another, resulting in the release of a water molecule. This process repeats to form long chains of amino acids, which are the basic structure of proteins.

Question 3

Which of the following mutations would you expect to cause the most drastic alteration in the functional performance of a polypeptide chain? (Assuming the replacement is not at the active site)
A) Replacement of glutamate with lysine.
B) Replacement of valine with alanine.
C) Replacement of aspartic acid with glutamic acid.
D) Replacement of lysine with arginine.

Accepted Answer

Glutamate and lysine have very different properties - glutamate is acidic while lysine is basic. This replacement could potentially disrupt the electrostatic interactions and hydrogen bonding within the polypeptide chain, as well as affect the overall charge distribution of the protein, leading to significant alterations in its function. The other replacements are less drastic because the properties of the amino acids are more similar.

Question 4

Edman degradation refers to
A) a metabolic pathway for catabolism of proteins.
B) a chemical method for determination of the N-terminal amino acid sequence of proteins and polypeptides.
C) an enzymatic method for determination of the C-terminal sequence of proteins and polypeptides.
D) a chemical method for determination of the C-terminal sequence of proteins and polypeptides.
E) None of the above statements correctly refer to the Edman degradation.

Accepted Answer

Edman degradation is a chemical method used to determine the N-terminal amino acid sequence of proteins and polypeptides. It involves the sequential removal of one amino acid from the N-terminus of the protein and its identification by chromatographic or spectroscopic analysis. This method is widely used in protein sequencing and characterization.

Question 5

All of the following statements about the alpha-helix are true EXCEPT:
A) the alpha-helix is right-handed.
B) there are 3.6 amino acids per turn of the alpha-helix.
C) the R groups of the amino acids are externally directed.
D) the alpha-helix is stabilized primarily by hydrophobic interactions.
E) the structure of the alpha-helix was predicted before it was observed in a protein

Accepted Answer

The alpha-helix is primarily stabilized by hydrogen bonding between the carbonyl group of one amino acid and the amino group of another amino acid, not by hydrophobic interactions.

Question 6

The following is the structure of which amino acid? (CH3)2-CH-CH2-CHNH2-COOH
A) isoleucine
B) threonine
C) arginine
D) valine
E) lysine

Accepted Answer

The answer of The following is the structure of which...

Question 7

Which statement below is most correct regarding the alpha helical structure of polypeptide chains in proteins?
A) It is primarily dependent upon hydrogen bonding.
B) It is primarily dependent upon disulfide linkages.
C) It is found in all proteins.
D) It refers to interactions between adjacent chains in fibrous proteins.

Accepted Answer

The alpha helical structure of polypeptide chains is primarily dependent upon hydrogen bonding between the carbonyl group of one amino acid and the amino group of an amino acid four residues down the chain. Disulfide linkages are more involved in stabilizing tertiary and quaternary protein structure, and not specifically the alpha helix. Not all proteins contain alpha helices, and this structure is not specific to fibrous proteins.

Question 8

When a protein is hydrolyzed there is
A) a decrease in free carboxyl groups.
B) an increase in free amino groups.
C) a large decrease in pH.
D) formation of peptide bonds.
E) a decrease in free amino groups.

Accepted Answer

When a protein is hydrolyzed, peptide bonds are broken, leading to the release of free amino acids, which increases the number of free amino groups.

Question 9

Which of the following is not an example of secondary structure;
A) Beta-pleated sheet
B) collagen triple helix
C) Alpha-helix
D) Beta-bend
E) disulfide bridge

Accepted Answer

Disulfide bridge refers to a covalent bond between two cysteine residues and does not describe a type of secondary structure. The other choices represent common examples of secondary structure in proteins.

Question 10

Hemoglobin A differs from hemoglobin S in having two more anionic groups (both hemoglobins have a molecular weight of approximately 67,000). Which one of the following separation methods is most likely to distinguish between these two
A) Precipitation with ethanol
B) Gel filtration
C) Ultracentrifugation
D) Electrophoresis

Accepted Answer

Electrophoresis separates molecules based on their charge and size differences. As hemoglobin A and hemoglobin S differ in their charge due to the two extra anionic groups in hemoglobin A, electrophoresis is the most likely separation method to distinguish between these two hemoglobins.

Question 11

The formation of a peptide bond between two amino acids
A) creates partial double-bond character among three atoms.
B) permits unrestricted rotations about each chemical bond.
C) places the alpha-carbons in the "centers" of two different planes.
D) retains the chemical properties of the typical ester bond.

Accepted Answer

The formation of a peptide bond involves a condensation reaction between the carboxyl group of one amino acid and the amino group of another. This results in the loss of a water molecule and the formation of a covalent bond between the amino and carboxyl groups. Due to the partial double bond character of the carbonyl carbon - nitrogen bond, the peptide bond is rigid and planar, which restricts rotation around the bond. This phenomenon is known as peptide bond resonance.

Question 12

What leads to "superfolds"?
A) Evolutionary development
B) Ratio of a-helices to b-pleated sheets
C) Similar function
D) Similar primary structure
E) Thermodynamic stability

Accepted Answer

Superfolds are characterized by unique and stable tertiary structures, which are formed through complex interactions of multiple amino acid residues. These stable structures are a result of thermodynamic stability, where the protein's energetically favorable conformation is adopted. Thus, Choice E, thermodynamic stability, is the best answer to what leads to superfolds.

Question 13

The isoelectric point of arginine is approximately
A) 1
B) 3
C) 5
D) 7
E) 11

Accepted Answer

The isoelectric point of an amino acid is the pH at which it has no net charge. Arginine has three positively charged groups (amino, imino, and guanidino) and one negatively charged carboxyl group. At low pH, the amino and imino groups are protonated, giving the molecule an overall positive charge. As pH is increased, these groups gradually lose protons until the molecule reaches its isoelectric point, at which the guanidino group is no longer charged and the molecule has no net charge. For arginine, this occurs at a pH of approximately 11.

Question 14

Which of the following statements about protein structure is correct?
A) Disulfide interactions are required for the stability of globular structures.
B) Hydrophobic sidechains commonly occur clustered in interior areas of globular proteins.
C) The alpha-helix is maintained chiefly by hydrophobic interactions.
D) Covalent linkages between the protomeric chains are a common feature of globular oligomeric proteins such as hemoglobin and lactate dehydrogenase (LDH).
E) Pleated-sheet structures are found only in fibrous proteins.

Accepted Answer

Hydrophobic sidechains are attracted to each other in order to minimize contact with water molecules, and hence, they are commonly found in interior areas of globular proteins to form a hydrophobic core. Disulfide interactions can contribute to protein stability, but they are not required for globular structure. Alpha-helices are chiefly maintained by hydrogen bonding, while covalent linkages between protein subunits are not required for globular structure. Pleated-sheet structures are found in both fibrous and globular proteins.

Question 15

Each of the following statements about the side chains of amino acid residues is true EXCEPT which one?
A) Isoleucine, valine and tryptophan tend to be in the interior of protein molecules.
B) Catalytically functional groups include aspartate, histidine and cysteine.
C) Tyrosine, phenylalanine and threonine participate in hydrogen bond formation.
D) Lysine, arginine and glutamate form ionic bonds with substrates and other protein components.
E) Lysine, cysteine and tyrosine have pKa's higher than 7.0.

Accepted Answer

While most amino acids have side chains that can participate in hydrogen bonding, threonine, tyrosine and phenylalanine are not the only ones. Therefore, statement C is the incorrect statement.

Question 16

At the isoionic point
A) proteins are least soluble.
B) the net ionic charge is zero.
C) repulsive forces between proteins are at a minimum.
D) the pH of the solution may vary widely from protein to protein.
E) all of the above

Accepted Answer

At the isoionic point, proteins are least soluble due to the net ionic charge being zero, which minimizes repulsive forces between protein molecules. This point varies for different proteins, leading to a wide variation in pH levels at which it occurs for different proteins.

Question 17

Which of the following is a kind of physical/chemical interaction of major quantitative importance in the overall structure of most globular proteins and occurs chiefly in the interior areas of the molecule?
A) Disulfide bridges.
B) Hydrophobic bonds.
C) Amide linkages between amino acid sidechains.
D) Ester linkages between sidechains of acidic and alcoholic amino acids.
E) Hydrogen bonds to water molecules.

Accepted Answer

Hydrophobic bonds are a major type of physical/chemical interaction found in the interior of most globular proteins. These interactions occur between nonpolar amino acid sidechains, as these hydrophobic groups tend to cluster together away from the surrounding aqueous environment. This is a major factor in the overall folding and stability of the protein structure.

Question 18

amino acids are classified according to the nature of their side chain. All of the following amino acids are correctly classified EXCEPT
A) Serine : polar, uncharged.
B) Glutamic : polar, acidic.
C) Arginine : polar, basic.
D) Histidine : nonpolar, uncharged.
E) Phenylalanine : nonpolar, uncharged.

Accepted Answer

Histidine is actually polar and positively charged at physiological pH, as it has a basic imidazole side chain. The other amino acids are correctly classified according to their side chain properties.

Question 19

At a pH more acidic than its isoelectric point, an amino acid will carry
A) a net negative charge.
B) a net positive charge.
C) a positive charge equal to the negative charge.
D) no ionic charge.
E) an unknown charge, since its concentration is not specified.

Accepted Answer

At a pH more acidic than its isoelectric point, an amino acid will have a higher concentration of H+ ions in solution, which will protonate the amino and carboxyl groups of the amino acid. This results in the amino acid carrying a net positive charge.

Question 20

The alpha helix structure of a protein can be considered an example of:
A) quaternary structure
B) tertiary structure
C) secondary structure
D) primary structure
E) none of the above

Accepted Answer

The alpha helix structure of a protein is a type of secondary structure, which refers to the local folding patterns that occur in specific regions of the polypeptide chain. Secondary structures are stabilized by hydrogen bonding between nearby amino acid residues, such as between the carbonyl group of one amino acid and the amino group of another. The alpha helix is a specific type of secondary structure in which the polypeptide chain is coiled tightly in a right-handed spiral.

Quiz 3: Protein I: Composition and Structure

Which of the following statements about cystine is true?

The primary sequence of proteins is best described by the statement that proteins

Which of the following mutations would you expect to cause the most drastic alteration in the functional performance of a polypeptide chain? (Assuming the replacement is not at the active site)

Edman degradation refers to

All of the following statements about the alpha-helix are true EXCEPT:

The following is the structure of which amino acid? (CH3) 2-CH-CH2-CHNH2-COOH

Which statement below is most correct regarding the alpha helical structure of polypeptide chains in proteins?

When a protein is hydrolyzed there is

Which of the following is not an example of secondary structure;

Hemoglobin A differs from hemoglobin S in having two more anionic groups (both hemoglobins have a molecular weight of approximately 67,000) . Which one of the following separation methods is most likely to distinguish between these two

The formation of a peptide bond between two amino acids

What leads to "superfolds"?

The isoelectric point of arginine is approximately

Which of the following statements about protein structure is correct?

Each of the following statements about the side chains of amino acid residues is true EXCEPT which one?

At the isoionic point

Which of the following is a kind of physical/chemical interaction of major quantitative importance in the overall structure of most globular proteins and occurs chiefly in the interior areas of the molecule?

amino acids are classified according to the nature of their side chain. All of the following amino acids are correctly classified EXCEPT

At a pH more acidic than its isoelectric point, an amino acid will carry

The alpha helix structure of a protein can be considered an example of: