301. A graph plots average net charge of an amino acid on the vertical axis against \(\mathrm{pH}\) on the horizontal axis. The curve decreases as \(\mathrm{pH}\) rises and crosses the zero-charge line at \(\mathrm{pH=5.8}\). The value \(5.8\) represents:
ⓐ. the isoelectric point of the amino acid
ⓑ. the maximum positive charge of the amino acid
ⓒ. the number of ionisable groups in the amino acid
ⓓ. the temperature at which the amino acid denatures
Correct Answer: the isoelectric point of the amino acid
Explanation: The zero crossing marks the condition at which positive and negative charges balance. By definition, this \(\mathrm{pH}\) is the isoelectric point. To the left of the crossing, the amino acid has a more positive average charge. To the right, it has a more negative average charge. The graph represents acid–base behaviour and does not provide a temperature or a direct count of ionisable groups.
302. Three amino acids are placed in a buffer of \(\mathrm{pH=6.0}\).
| Amino acid | \(\mathrm{pI}\) |
|---|
| P | \(3.0\) |
| Q | \(6.0\) |
| R | \(9.0\) |
Their expected behaviour in an electric field is:
ⓐ. P moves to the negative electrode, Q remains nearly stationary, and R moves to the positive electrode
ⓑ. P remains nearly stationary, Q moves to the negative electrode, and R moves to the positive electrode
ⓒ. P moves to the positive electrode, Q remains nearly stationary, and R moves to the negative electrode
ⓓ. P and R remain nearly stationary, while Q moves to the positive electrode
Correct Answer: P moves to the positive electrode, Q remains nearly stationary, and R moves to the negative electrode
Explanation: For P, the buffer \(\mathrm{pH}\) is above its \(\mathrm{pI}\), so P has a negative net charge and moves toward the positive electrode. For Q, \(\mathrm{pH=pI}\), so its average net charge is zero and net migration is minimal. For R, the buffer \(\mathrm{pH}\) is below its \(\mathrm{pI}\), so R is positively charged. It therefore moves toward the negative electrode. The three behaviours follow from comparing each \(\mathrm{pI}\) separately with the common buffer \(\mathrm{pH}\).
303. An amino acid solution is initially maintained at its \(\mathrm{pI}\). Addition of sufficient acid will cause the amino acid to:
ⓐ. become more negatively charged through deprotonation
ⓑ. become more positively charged through protonation
ⓒ. lose both its amino and carboxyl groups
ⓓ. remain at zero net charge regardless of the new \(\mathrm{pH}\)
Correct Answer: become more positively charged through protonation
Explanation: At the isoelectric point, the amino acid has zero average net charge. Addition of acid lowers the \(\mathrm{pH}\) below the \(\mathrm{pI}\). The greater proton concentration favours protonation of ionisable groups. The amino acid therefore develops a positive net charge. The covalent amino-acid skeleton remains intact during this ordinary acid–base change.
304. In the peptide
\[
\mathrm{H_2N-Ala-Gly-Ser-COOH}
\]
the N-terminal and C-terminal residues are, respectively:
ⓐ. Gly and Ser
ⓑ. Ser and Ala
ⓒ. Gly and Ala
ⓓ. Ala and Ser
Correct Answer: Ala and Ser
Explanation: Peptide sequences are written conventionally from the N terminus to the C terminus. The residue with the free amino group at the left end is alanine. The residue with the free carboxyl group at the right end is serine. Glycine lies between them and is an internal residue. Reversing the sequence would describe a different tripeptide with different terminal residues.
305. Alanylglycine and glycylalanine are different compounds mainly because:
ⓐ. their residues occur in opposite N-to-C sequences
ⓑ. one is a dipeptide whereas the other is a free amino-acid mixture
ⓒ. they contain different numbers of amino-acid residues
ⓓ. their peptide bonds belong to different functional-group classes
Correct Answer: their residues occur in opposite N-to-C sequences
Explanation: Alanylglycine has alanine at the N terminus and glycine at the C terminus. Glycylalanine has the reverse sequence. Both are dipeptides, both contain one peptide bond and both yield alanine and glycine on complete hydrolysis. Their overall amino-acid compositions are identical. Sequence direction nevertheless changes the molecular structure, so the two names represent different compounds.
306. Complete hydrolysis of \(0.150\,mol\) of a linear hexapeptide consumes water. The mass of water required is:
ⓐ. \(5.40\,g\)
ⓑ. \(9.00\,g\)
ⓒ. \(13.5\,g\)
ⓓ. \(16.2\,g\)
Correct Answer: \(13.5\,g\)
Explanation: \(\textbf{Number of residues in one peptide molecule:}\)
\[
n=6
\]
\(\textbf{Peptide bonds in a linear peptide:}\)
\[
n-1=6-1
\]
\[
\text{Peptide bonds}=5
\]
\(\textbf{Hydrolysis requirement:}\)
Each peptide bond consumes one water molecule.
\[
1\,mol\text{ hexapeptide}\rightarrow5\,mol\mathrm{H_2O}
\]
\(\textbf{Amount of hexapeptide:}\)
\[
n_{\text{peptide}}=0.150\,mol
\]
\(\textbf{Moles of water consumed:}\)
\[
n_{\mathrm{H_2O}}=0.150\times5
\]
\[
n_{\mathrm{H_2O}}=0.750\,mol
\]
\(\textbf{Molar mass of water:}\)
\[
M_{\mathrm{H_2O}}=18.0\,g\,mol^{-1}
\]
\(\textbf{Mass calculation:}\)
\[
m_{\mathrm{H_2O}}=nM
\]
\[
m_{\mathrm{H_2O}}
=0.750\,mol\times18.0\,g\,mol^{-1}
\]
\[
m_{\mathrm{H_2O}}=13.5\,g
\]
\(\textbf{Final answer:}\)
Complete hydrolysis consumes \(13.5\,g\) water.
307. Partial hydrolysis of an unknown tripeptide gives the dipeptides Ala–Gly and Gly–Val. Independent end-group analysis shows alanine at the N terminus and valine at the C terminus. The tripeptide sequence is:
ⓐ. Gly–Ala–Val
ⓑ. Val–Gly–Ala
ⓒ. Ala–Val–Gly
ⓓ. Ala–Gly–Val
Correct Answer: Ala–Gly–Val
Explanation: The fragment Ala–Gly shows that alanine is directly followed by glycine. The fragment Gly–Val shows that glycine is directly followed by valine. Overlapping the common glycine residue gives Ala–Gly–Val. The end-group evidence independently confirms alanine as the N-terminal residue and valine as the C-terminal residue. No other listed sequence satisfies both partial-hydrolysis fragments and the terminal analysis.
308. Examine the following statements about peptide hydrolysis.
Statement I: Complete hydrolysis breaks all peptide bonds.
Statement II: A linear peptide with \(n\) residues consumes \(n-1\) water molecules during complete hydrolysis.
Statement III: Complete hydrolysis normally changes every amino-acid side chain into the same group.
The acceptable statements are:
ⓐ. I only
ⓑ. I and II only
ⓒ. II and III only
ⓓ. I, II and III
Correct Answer: I and II only
Explanation: Complete hydrolysis cleaves every peptide linkage, so Statement I is acceptable. A linear chain containing \(n\) residues has \(n-1\) peptide bonds, and each bond consumes one water molecule during cleavage. Statement II is therefore acceptable. Hydrolysis restores amino and carboxyl groups at the cleavage sites but does not normally convert all side chains into one identical group. Statement III is consequently unacceptable.
309. Fibrous proteins are generally characterised by:
ⓐ. elongated shapes and predominantly structural functions
ⓑ. compact shapes and exclusively catalytic functions
ⓒ. nucleotide chains joined by phosphodiester bonds
ⓓ. highly branched glucose chains used for energy storage
Correct Answer: elongated shapes and predominantly structural functions
Explanation: Fibrous proteins commonly contain long, strand-like or sheet-like molecular arrangements. Their structures are suited to mechanical and structural roles. Keratin and collagen are standard examples. They are often less soluble in water than many globular proteins. Globular proteins instead fold into compact forms and frequently perform catalytic, transport or regulatory functions.
310. The most appropriate comparison between keratin and collagen is:
ⓐ. Keratin mainly supports connective tissues, whereas collagen forms the principal structural protein of hair and nails
ⓑ. Keratin is mainly a compact catalytic protein, whereas collagen is an elongated structural protein
ⓒ. Keratin contributes to the structure of hair and nails, whereas collagen provides strength and support in connective tissues
ⓓ. Keratin and collagen are both compact proteins whose principal function is molecular transport
Correct Answer: Keratin contributes to the structure of hair and nails, whereas collagen provides strength and support in connective tissues
Explanation: Keratin and collagen are both fibrous proteins with predominantly structural roles. Keratin is abundant in structures such as hair and nails, where it contributes toughness and protection. Collagen is a major structural component of connective tissues and provides mechanical strength. Neither protein is primarily classified as a compact globular catalyst or transport protein. Their common fibrous character does not mean that they occur in the same tissues or perform identical structural roles.
311. The primary structure of a protein is defined mainly by:
ⓐ. the overall shape formed by several subunits
ⓑ. the pattern of hydrogen bonding in a local backbone region
ⓒ. the exact amino-acid sequence of its polypeptide chain
ⓓ. the arrangement of hydrophobic groups in the protein interior
Correct Answer: the exact amino-acid sequence of its polypeptide chain
Explanation: Primary structure records the order of amino-acid residues from the N terminus to the C terminus. This sequence is maintained by covalent peptide bonds. A complete structural description may also specify the positions of covalent connections such as disulphide links where relevant. Hydrogen-bonded helices and sheets belong to secondary structure. Overall three-dimensional folding and subunit association belong to higher structural levels.
312. Examine the following statements about protein primary structure.
Statement I: Two polypeptides with the same amino-acid composition can have different primary structures.
Statement II: Primary structure influences the higher folding and function of a protein.
Statement III: Primary structure is determined only by the total number of peptide bonds.
The acceptable statements are:
ⓐ. I and II only
ⓑ. I and III only
ⓒ. II and III only
ⓓ. I, II and III
Correct Answer: I and II only
Explanation: Amino-acid composition gives the numbers and types of residues but not their order. Different residue sequences therefore produce different primary structures even when the compositions match. The sequence controls which side chains can interact during folding and strongly influences biological function. Peptide-bond count reveals chain length but cannot identify the order of residues. Statement III is therefore insufficient as a definition of primary structure.
313. A protein variant has the same chain length as the normal protein, but one amino-acid residue has been replaced by another. The variant folds differently and shows much lower biological activity. The observation most directly demonstrates that:
ⓐ. chain length alone determines protein function
ⓑ. peptide bonds are absent from the variant
ⓒ. secondary structure is unrelated to amino-acid sequence
ⓓ. a single residue change can alter folding and activity
Correct Answer: a single residue change can alter folding and activity
Explanation: Replacement of one residue changes the primary sequence without changing the number of residues. The new side chain may differ in charge, size, polarity or bonding ability. These changes can alter local interactions and the final three-dimensional fold. A distorted fold may modify an active site, binding site or structural role. The result shows why sequence information is more important than chain length alone.
314. The most suitable order for describing the development of protein structure is:
ⓐ. quaternary association \(\rightarrow\) primary sequence \(\rightarrow\) local folding \(\rightarrow\) tertiary folding
ⓑ. primary sequence \(\rightarrow\) secondary folding \(\rightarrow\) tertiary folding \(\rightarrow\) possible subunit association
ⓒ. tertiary folding \(\rightarrow\) peptide-bond formation \(\rightarrow\) primary sequence \(\rightarrow\) secondary folding
ⓓ. secondary folding \(\rightarrow\) amino-acid synthesis \(\rightarrow\) quaternary association \(\rightarrow\) primary sequence
Correct Answer: primary sequence \(\rightarrow\) secondary folding \(\rightarrow\) tertiary folding \(\rightarrow\) possible subunit association
Explanation: The amino-acid sequence forms the primary structural foundation. Local regions of the backbone may then adopt secondary structures such as \(\alpha\)-helices or \(\beta\)-sheets. Further folding of one complete chain produces its tertiary structure. If several folded chains associate, quaternary structure is formed. Quaternary structure is not compulsory because many proteins function as single polypeptide chains.
315. Four distinct amino acids—Ala, Gly, Ser and Val—are used exactly once each to form linear tetrapeptides. Ignoring stereochemical variations, the number of possible primary sequences is:
ⓐ. \(8\)
ⓑ. \(12\)
ⓒ. \(24\)
ⓓ. \(16\)
Correct Answer: \(24\)
Explanation: \(\textbf{Residues available:}\)
There are \(4\) distinct amino-acid residues.
\(\textbf{Sequence condition:}\)
Each residue is used exactly once.
\(\textbf{Choice for the first position:}\)
\[
4
\]
\(\textbf{Choice for the second position:}\)
\[
3
\]
\(\textbf{Choice for the third position:}\)
\[
2
\]
\(\textbf{Choice for the fourth position:}\)
\[
1
\]
\(\textbf{Permutation relation:}\)
\[
N=4!
\]
\(\textbf{Calculation:}\)
\[
4!=4\times3\times2\times1
\]
\[
4!=24
\]
\(\textbf{Structural interpretation:}\)
Reversing or rearranging the residues changes the N-to-C sequence and therefore gives a different primary structure.
\(\textbf{Final answer:}\)
The four amino acids can form \(24\) distinct primary sequences.
316. Use the arrangement described below.
A polypeptide backbone coils regularly. Hydrogen bonds form within the same chain between peptide carbonyl and amide groups, while the side chains project outward.
The arrangement is:
ⓐ. a \(\beta\)-pleated sheet
ⓑ. an \(\alpha\)-helix
ⓒ. a quaternary assembly
ⓓ. an unfolded primary chain
Correct Answer: an \(\alpha\)-helix
Explanation: An \(\alpha\)-helix is a regular coiled secondary structure of a polypeptide backbone. It is stabilised mainly by hydrogen bonds between backbone carbonyl and amide groups within the chain. The side chains extend outward from the helical backbone. This arrangement reduces crowding within the centre of the helix. A \(\beta\)-sheet instead contains extended segments lying side by side.
317. Assertion: Hydrogen bonding is important for stabilising an \(\alpha\)-helix.
Reason: The hydrogen bonds form mainly between peptide-backbone carbonyl and amide groups within the same helical segment.
ⓐ. Both Assertion and Reason are true, but Reason does not explain Assertion
ⓑ. Assertion is true, but Reason is false
ⓒ. Assertion is false, but Reason is true
ⓓ. Both Assertion and Reason are true, and Reason explains Assertion
Correct Answer: Both Assertion and Reason are true, and Reason explains Assertion
Explanation: The peptide backbone contains carbonyl oxygen atoms that can accept hydrogen bonds. It also contains amide hydrogen atoms that can participate as donors. Repeated intrachain hydrogen bonding supports the regular coiled geometry of the \(\alpha\)-helix. These interactions involve the backbone rather than requiring identical side chains. The Reason therefore identifies the stabilising interaction described in the Assertion.
318. In a typical \(\alpha\)-helix, most amino-acid side chains:
ⓐ. project outward from the helical backbone
ⓑ. form the peptide bonds along the central axis
ⓒ. remain trapped inside the helix as a continuous core
ⓓ. are removed during hydrogen-bond formation
Correct Answer: project outward from the helical backbone
Explanation: The repeating peptide backbone forms the coiled framework of the helix. Side chains are attached to the \(\alpha\)-carbons and extend outward from this framework. Their outward orientation allows different side chains to interact with the surrounding environment or other parts of the protein. Peptide bonds belong to the backbone and are not formed by side-chain projection. Hydrogen bonding changes neither the identity nor the number of the side chains.
319. The row that correctly distinguishes primary structure from an \(\alpha\)-helical secondary structure is:
| Option | Primary structure | \(\alpha\)-Helical secondary structure |
|---|
| A | Maintained mainly by hydrogen bonds | Maintained only by peptide-bond hydrolysis |
| B | Association of several subunits | Exact amino-acid sequence |
| C | Residue sequence joined by peptide bonds | Regular backbone coil stabilised by hydrogen bonds |
| D | Three-dimensional folding of one chain | Arrangement of several folded chains |
ⓐ. Row A
ⓑ. Row B
ⓒ. Row C
ⓓ. Row D
Correct Answer: Row C
Explanation: Primary structure is the covalent sequence of amino-acid residues linked by peptide bonds. An \(\alpha\)-helix is a local secondary structure formed by regular coiling of the backbone. Hydrogen bonds between backbone groups stabilise the helix. Association of multiple subunits is quaternary structure, while overall folding of one chain is tertiary structure. Row C is the only row that assigns both levels appropriately.
320. A claim states, “When a polypeptide becomes helical, its peptide bonds are broken and replaced by hydrogen bonds.” The most accurate correction is:
ⓐ. peptide bonds remain; hydrogen bonds stabilise the helix
ⓑ. peptide bonds remain, but glycosidic bonds stabilise the helix
ⓒ. hydrogen bonds replace only the peptide bonds inside each turn
ⓓ. new covalent bonds form between every third backbone residue
Correct Answer: peptide bonds remain; hydrogen bonds stabilise the helix
Explanation: Peptide bonds maintain the continuous covalent backbone and define the primary structure. Formation of an \(\alpha\)-helix does not require their cleavage. Instead, suitable backbone carbonyl and amide groups form non-covalent hydrogen bonds. These interactions organise the existing chain into a regular secondary structure. Breaking peptide bonds would fragment the chain rather than simply fold it.
