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Biomolecules Set-6 MCQ Online Practice Test | Class 12 | Quiz Tweek

Biomolecules Set-6

Practice Important MCQs for Biomolecules — Chemistry, Class 12.

Biomolecules is a high-scoring and frequently asked chapter because it links core organic chemistry concepts (functional groups, stereochemistry, and reactions) with biological applications (amino acids, peptides, proteins, sugars, and enzyme kinetics). Board exams focus on fundamental definitions and derivations (like $pI$ , peptide bond formation, and anomeric properties), while competitive exams test conceptual reasoning through conditions such as pH-dependent ionization and inhibitor effects on enzyme rates.

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Q1. The amino acid glycine has $pK_{a1}=2.34$ (–COOH) and $pK_{a2}=9.60$ (–NH $_3^+$ ). Using $pI=\dfrac{pK_{a1}+pK_{a2}}{2}$ , the isoelectric point of glycine is closest to:

(A)

$pI=4.97$

(B)

$pI=6.97$

(C)

$pI=5.97$

(D)

$pI=3.97$

Q2. Pure $\alpha$ -D-glucose and $\beta$ -D-glucose have specific rotations $[α]_{\alpha}=+112^\circ$ and $[α]_{\beta}=+18.7^\circ$ . For a mixture the observed rotation obeys $[α]_{obs}=f_\alpha[α]_{\alpha}+f_\beta[α]_{\beta}$ with $f_\alpha+f_\beta=1$ . If a freshly prepared solution shows $[α]_{obs}=+52.7^\circ$ , the percentage of the $\alpha$ -anomer at equilibrium is approximately:

(A)

$36\%$

(B)

$64\%$

(C)

$50\%$

(D)

$27\%$

Q3. Calculate the net charge of the tripeptide Gly–Lys–Asp at pH $7.0$ . Use $pK_a$ values: N-terminal $\mathrm{NH_3^+}$ $pK_a=9.0$ , C-terminal $\mathrm{COOH}$ $pK_a=2.0$ , Lys side chain $pK_a=10.5$ , Asp side chain $pK_a=3.9$ . (Consider only termini and side chains; assume groups are fully protonated/deprotonated according to pH vs $pK_a$ .)

(A)

$+2$

(B)

$0$

(C)

$-1$

(D)

$+1$

Q4. Proteins A and B have identical molecular masses (~50 kDa). At pH 7.0, A carries many acidic residues (net negative) while B carries many basic residues (net positive). Which statement best describes their migration in SDS–PAGE (denaturing with SDS) versus native PAGE (non-denaturing at pH 7)?

(A)

On SDS–PAGE, A migrates much slower than B because SDS binds preferentially to basic proteins; on native PAGE both move toward the anode but A faster than B.

(B)

On SDS–PAGE, migration depends on net charge; on native PAGE both move toward the anode because proteins are generally negatively charged at pH 7.

(C)

On SDS–PAGE, B migrates faster than A due to its basic residues; on native PAGE both remain near the origin because opposite charges cancel out.

(D)

On SDS–PAGE, both proteins migrate to comparable positions determined primarily by mass (SDS gives an approximately uniform charge/mass ratio); on native PAGE at pH 7, A (net negative) migrates toward the anode while B (net positive) migrates toward the cathode.

Q5. An enzyme follows Michaelis–Menten kinetics with $V_{\max}=100\ \mu\mathrm{mol\;min^{-1}}$ and $K_m=50\ \mu\mathrm{M}$ . Addition of a competitive inhibitor increases the apparent $K_m$ to $150\ \mu\mathrm{M}$ (no change in $V_{\max}$ ). Subsequent addition of a non-competitive inhibitor reduces $V_{\max}$ to $50\ \mu\mathrm{mol\;min^{-1}}$ while leaving the apparent $K_m$ unchanged. At substrate concentration $[S]=150\ \mu\mathrm{M}$ , the initial rate $v$ (use $v=\dfrac{V_{\max}[S]}{K_m+[S]}$ ) is closest to:

(A)

$25\ \mu\mathrm{mol\;min^{-1}}$

(B)

$50\ \mu\mathrm{mol\;min^{-1}}$

(C)

$33.3\ \mu\mathrm{mol\;min^{-1}}$

(D)

$16.7\ \mu\mathrm{mol\;min^{-1}}$

Q6. Glycine has $pK_a(\mathrm{COOH})=2.34$ and $pK_a(\mathrm{NH_3^+})=9.60$ . What is its isoelectric point $pI$ ?

(A)

$2.34$

(B)

$7.00$

(C)

$5.97$

(D)

$9.60$

Q7. Consider the tripeptide with free termini $\mathrm{H\!-\!Gly\!-\!Asp\!-\!Lys\!-\!OH}$ . Given $pK_a(\text{N-term})=9.0$ , $pK_a(\text{C-term})=2.0$ , $pK_a(\text{Asp side chain})=3.9$ and $pK_a(\text{Lys side chain})=10.5$ , what is the net charge of the peptide at $pH=7.0$ ?

(A)

$0$

(B)

$+2$

(C)

$-1$

(D)

$+1$

Q8. Deamination of cytosine in DNA produces uracil. If such a lesion is left unrepaired and the DNA is replicated, which base-pair substitution will become fixed in the genome?

(A)

$G\!:\!C \rightarrow G\!:\!C$

(B)

$G\!:\!C \rightarrow C\!:\!G$

(C)

$G\!:\!C \rightarrow A\!:\!C$

(D)

$G\!:\!C \rightarrow A\!:\!T$

Q9. Assertion (A): The peptide bond ( $-\mathrm{CO}\!-\!\mathrm{NH}-$ ) in polypeptides has partial double-bond character, making the $C$ – $N$ unit planar and restricting rotation.
Reason (R): Peptide bond formation is a condensation (dehydration) reaction that links the $\alpha$ -carboxyl group of one amino acid to the $\alpha$ -amino group of another with elimination of water.

(A)

Both A and R are true and R explains A.

(B)

Both A and R are true but R does not explain A.

(C)

A is true but R is false.

(D)

A is false but R is true.

Q10. An enzyme follows Michaelis–Menten kinetics with $K_m=2.0\ \mathrm{mM}$ and $V_{max}=100\ \mu\mathrm{mol\,min^{-1}}$ . A competitive inhibitor is present at $[I]=2.0\ \mathrm{mM}$ with $K_i=1.0\ \mathrm{mM}$ . Calculate the apparent $K_m$ in presence of inhibitor and the initial rate $v_0$ at $[S]=2.0\ \mathrm{mM}$ .

(A)

$K_m^{\text{app}}=6.0\ \mathrm{mM};\; v_0=50\ \mu\mathrm{mol\,min^{-1}}$

(B)

$K_m^{\text{app}}=3.0\ \mathrm{mM};\; v_0=40\ \mu\mathrm{mol\,min^{-1}}$

(C)

$K_m^{\text{app}}=6.0\ \mathrm{mM};\; v_0=25\ \mu\mathrm{mol\,min^{-1}}$

(D)

$K_m^{\text{app}}=2.0\ \mathrm{mM};\; v_0=50\ \mu\mathrm{mol\,min^{-1}}$

Q11. A polypeptide consists of $150$ amino acid residues linked by peptide bonds. How many peptide bonds are present and how many molecules of $H_2O$ are produced during its biosynthesis by successive condensation reactions?

(A)

$150$ peptide bonds; $150\ H_2O$ molecules

(B)

$149$ peptide bonds; $150\ H_2O$ molecules

(C)

$149$ peptide bonds; $149\ H_2O$ molecules

(D)

$150$ peptide bonds; $149\ H_2O$ molecules

Q12. Consider the dipeptide Lys–Glu (Lys at N‑terminus, Glu at C‑terminus). Given $pK_a(\alpha\text{-COOH})=2.0$ , $pK_a(\alpha\text{-NH}_3^+)=9.0$ , $pK_a(\text{Glu side chain})=4.1$ and $pK_a(\text{Lys side chain})=10.5$ . What is the net charge of the neutral form of this dipeptide at $pH\;7.0$ ?

(A)

$+1$

(B)

$0$

(C)

$-1$

(D)

$+2$

Q13. A protein shows a single band at $40\ \text{kDa}$ on SDS–PAGE when run without a reducing agent, but upon treatment with a reducing agent (which breaks disulfide bonds) it yields two bands at $25\ \text{kDa}$ and $15\ \text{kDa}$ . Which of the following structures best explains these observations?

(A)

A single polypeptide chain of $40\ \text{kDa}$ with no disulfide bonds

(B)

A homodimer of two identical $20\ \text{kDa}$ subunits held by noncovalent interactions

(C)

A $25\ \text{kDa}$ polypeptide contaminated with an unrelated $15\ \text{kDa}$ protein

(D)

A heterodimer composed of $25\ \text{kDa}$ and $15\ \text{kDa}$ subunits linked by one or more disulfide bond(s)

Q14. An enzyme follows Michaelis–Menten kinetics with $V_{\max}=120\ \mu\text{mol min}^{-1}$ and $K_m=3\ \text{mM}$ . In the presence of a certain inhibitor the measured initial rates are $30\ \mu\text{mol min}^{-1}$ at $[S]=1\ \text{mM}$ and $60\ \mu\text{mol min}^{-1}$ at $[S]=10\ \text{mM}$ . Which type of inhibition is most consistent with these observations?

(A)

Uncompetitive inhibition

(B)

Competitive inhibition

(C)

Pure noncompetitive inhibition

(D)

Mixed-type inhibition

Q15. A protein has molecular mass $60.00\ \text{kDa}$ and at $pH\;8.5$ has net charge $-10e$ . It contains $20$ lysine residues that are positively charged at $pH\;8.5$ . If all lysines are acetylated, each acetylation neutralizes the lysine positive charge and increases the mass by $42\ \text{Da}$ per lysine. Assuming electrophoretic mobility on a native gel is approximately proportional to the charge/mass ratio, what is the expected change in the protein's mobility toward the anode at $pH\;8.5$ after complete acetylation?

(A)

Mobility decreases (moves more slowly toward the anode)

(B)

Mobility increases (moves faster toward the anode)

(C)

Mobility remains essentially unchanged

(D)

Protein will reverse and migrate toward the cathode

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