,CHEM212/CHEM 212 Module 3 Exam Biochemistry
with Lab | Portage Learning | 26/27 Actual(PDF)
1. Which functional group is present on the alpha carbon of every standard amino acid?
A) A sulfhydryl group
B) A hydroxyl group
C) An amino group and a carboxyl group
D) An amide group and a carbonyl group
Correct Answer: An amino group and a carboxyl group
Rationale: Every standard amino acid contains a central alpha carbon bonded to an amino group (–
NH2), a carboxyl group (–COOH), a hydrogen atom, and a variable R group. Sulfhydryl, hydroxyl, and
amide groups appear only in specific amino acid side chains, not universally. This fundamental
structure defines amino acids as the building blocks of proteins.
2. A peptide bond forms between which two components of adjacent amino acids?
A) The R group of one amino acid and the amino group of another
B) The carboxyl group of one amino acid and the amino group of another
C) The alpha carbon of one amino acid and the R group of another
D) The amino group of one amino acid and the amino group of another
Correct Answer: The carboxyl group of one amino acid and the amino group of another
Rationale: Peptide bond formation is a dehydration condensation reaction between the carboxyl
group of one amino acid and the amino group of the next, releasing water. The R groups and alpha
carbons are not directly involved in the peptide bond. This covalent linkage defines the primary
structure of proteins.
3. Which amino acid is unique because its side chain is connected to the amine group, forming a cyclic
structure?
,A) Glycine
B) Cysteine
C) Proline
D) Histidine
Correct Answer: Proline
Rationale: Proline is the only standard amino acid whose side chain cyclizes back onto the nitrogen of
the amino group, creating a secondary amine. Glycine has only a hydrogen as its side chain, cysteine
contains a sulfhydryl group, and histidine has an imidazole side chain. This structural feature makes
proline a helix-breaker.
4. A protein loses its biological activity after exposure to high temperature. Which structural level is
most directly disrupted?
A) Primary structure
B) Secondary and tertiary structures
C) Quaternary structure only
D) The peptide bond backbone
Correct Answer: Secondary and tertiary structures
Rationale: Denaturation disrupts the non-covalent interactions—hydrogen bonds, hydrophobic
interactions, and ionic bonds—that maintain secondary and tertiary structures. The primary structure
and peptide bonds remain intact because they are covalent. Quaternary structure may also be
affected, but secondary and tertiary changes are the primary consequence.
5. In the alpha helix, hydrogen bonds form between which atoms?
A) Between R groups of amino acids four residues apart
B) Between the carbonyl oxygen of one residue and the amide nitrogen of a residue four positions
ahead
C) Between adjacent amino acid side chains
D) Between the amino group and the R group of the same residue
, Correct Answer: Between the carbonyl oxygen of one residue and the amide nitrogen of a residue four
positions ahead
Rationale: The alpha helix is stabilized by hydrogen bonds between the backbone carbonyl oxygen of
one amino acid and the backbone amide hydrogen of the amino acid four residues away. R groups
project outward and do not participate in this bonding. This regular pattern creates the helical
structure.
6. Which of the following best describes the quaternary structure of hemoglobin?
A) A single polypeptide chain folded into a globular shape
B) Two alpha chains and two beta chains held together by non-covalent interactions
C) A linear sequence of amino acids joined by peptide bonds
D) An alpha helix and beta sheet arrangement within one chain
Correct Answer: Two alpha chains and two beta chains held together by non-covalent interactions
Rationale: Hemoglobin is a tetramer composed of two alpha and two beta globin chains, each
containing a heme group. Quaternary structure refers to the assembly of multiple polypeptide
subunits. A single chain describes tertiary structure, while the linear sequence is primary structure.
7. Which amino acid is classified as nonpolar and aliphatic?
A) Serine
B) Valine
C) Aspartate
D) Lysine
Correct Answer: Valine
Rationale: Valine has a branched aliphatic side chain, making it nonpolar and hydrophobic. Serine is
polar uncharged, aspartate is acidic, and lysine is basic. The five groupings—nonpolar aliphatic,
nonpolar aromatic, polar neutral, acidic, and basic—classify all 20 standard amino acids.
with Lab | Portage Learning | 26/27 Actual(PDF)
1. Which functional group is present on the alpha carbon of every standard amino acid?
A) A sulfhydryl group
B) A hydroxyl group
C) An amino group and a carboxyl group
D) An amide group and a carbonyl group
Correct Answer: An amino group and a carboxyl group
Rationale: Every standard amino acid contains a central alpha carbon bonded to an amino group (–
NH2), a carboxyl group (–COOH), a hydrogen atom, and a variable R group. Sulfhydryl, hydroxyl, and
amide groups appear only in specific amino acid side chains, not universally. This fundamental
structure defines amino acids as the building blocks of proteins.
2. A peptide bond forms between which two components of adjacent amino acids?
A) The R group of one amino acid and the amino group of another
B) The carboxyl group of one amino acid and the amino group of another
C) The alpha carbon of one amino acid and the R group of another
D) The amino group of one amino acid and the amino group of another
Correct Answer: The carboxyl group of one amino acid and the amino group of another
Rationale: Peptide bond formation is a dehydration condensation reaction between the carboxyl
group of one amino acid and the amino group of the next, releasing water. The R groups and alpha
carbons are not directly involved in the peptide bond. This covalent linkage defines the primary
structure of proteins.
3. Which amino acid is unique because its side chain is connected to the amine group, forming a cyclic
structure?
,A) Glycine
B) Cysteine
C) Proline
D) Histidine
Correct Answer: Proline
Rationale: Proline is the only standard amino acid whose side chain cyclizes back onto the nitrogen of
the amino group, creating a secondary amine. Glycine has only a hydrogen as its side chain, cysteine
contains a sulfhydryl group, and histidine has an imidazole side chain. This structural feature makes
proline a helix-breaker.
4. A protein loses its biological activity after exposure to high temperature. Which structural level is
most directly disrupted?
A) Primary structure
B) Secondary and tertiary structures
C) Quaternary structure only
D) The peptide bond backbone
Correct Answer: Secondary and tertiary structures
Rationale: Denaturation disrupts the non-covalent interactions—hydrogen bonds, hydrophobic
interactions, and ionic bonds—that maintain secondary and tertiary structures. The primary structure
and peptide bonds remain intact because they are covalent. Quaternary structure may also be
affected, but secondary and tertiary changes are the primary consequence.
5. In the alpha helix, hydrogen bonds form between which atoms?
A) Between R groups of amino acids four residues apart
B) Between the carbonyl oxygen of one residue and the amide nitrogen of a residue four positions
ahead
C) Between adjacent amino acid side chains
D) Between the amino group and the R group of the same residue
, Correct Answer: Between the carbonyl oxygen of one residue and the amide nitrogen of a residue four
positions ahead
Rationale: The alpha helix is stabilized by hydrogen bonds between the backbone carbonyl oxygen of
one amino acid and the backbone amide hydrogen of the amino acid four residues away. R groups
project outward and do not participate in this bonding. This regular pattern creates the helical
structure.
6. Which of the following best describes the quaternary structure of hemoglobin?
A) A single polypeptide chain folded into a globular shape
B) Two alpha chains and two beta chains held together by non-covalent interactions
C) A linear sequence of amino acids joined by peptide bonds
D) An alpha helix and beta sheet arrangement within one chain
Correct Answer: Two alpha chains and two beta chains held together by non-covalent interactions
Rationale: Hemoglobin is a tetramer composed of two alpha and two beta globin chains, each
containing a heme group. Quaternary structure refers to the assembly of multiple polypeptide
subunits. A single chain describes tertiary structure, while the linear sequence is primary structure.
7. Which amino acid is classified as nonpolar and aliphatic?
A) Serine
B) Valine
C) Aspartate
D) Lysine
Correct Answer: Valine
Rationale: Valine has a branched aliphatic side chain, making it nonpolar and hydrophobic. Serine is
polar uncharged, aspartate is acidic, and lysine is basic. The five groupings—nonpolar aliphatic,
nonpolar aromatic, polar neutral, acidic, and basic—classify all 20 standard amino acids.
