Quiz Tweek

Preparing your workspace...

Biomolecules Set-4 MCQ Online Practice Test | Class 12 | Quiz Tweek

Biomolecules Set-4

Practice Important MCQs for Biomolecules — Chemistry, Class 12.

Biomolecules is a high-yield chapter because it connects core organic/ionic concepts (acids–bases, functional groups, stereochemistry) to biologically important molecules like amino acids, proteins, enzymes, and nucleic acids. For board and competitive exams, the key scoring areas are pI/pKa reasoning, amino acid ionization states at different pH, peptide/protein electrophoresis logic, and enzyme kinetics/free-energy links—these frequently appear as direct numerical or conceptual MCQs.

Minutes

Questions

1 / -0

Marking

Question Bank Preview

Q1. The pKa values for glycine are $pK_{a1}(\mathrm{COOH})=2.34$ and $pK_{a2}(\mathrm{NH_3^+})=9.60$ . Using $pI=\dfrac{pK_{a1}+pK_{a2}}{2}$ , the isoelectric point ( $pI$ ) of glycine is nearest to:

(A)

4.17

(B)

5.97

(C)

6.80

(D)

7.47

Q2. An enzyme follows Michaelis–Menten kinetics with $K_m=2.0\ \mathrm{mM}$ and $V_{max}=100\ \mu\mathrm{mol\;min^{-1}}$ . After adding a reversible inhibitor, the Lineweaver–Burk plot slope increases threefold while the y‑intercept is unchanged. At $[S]=2.0\ \mathrm{mM}$ the ratio of reaction rates with inhibitor to without inhibitor, $v_{inh}/v$ , is closest to (use $v=\dfrac{V_{max}[S]}{K_m+[S]}$ ):

(A)

0.33

(B)

0.25

(C)

0.67

(D)

0.50

Q3. Lysine has $pK_a(\mathrm{COOH})=2.18$ , $pK_a(\mathrm{NH_3^+})=8.95$ (α‑amino), and $p_{a}(\mathrm{side\ chain\ NH_3^+})=10.53$ . The isoelectric point $pI$ of lysine is closest to:

(A)

9.74

(B)

5.56

(C)

6.36

(D)

8.35

Q4. Consider the pentapeptide NH2–Gly–Lys–Arg–Asp–Glu–COOH. Given $pK_a(\mathrm{N\!-\!term})=9.0$ , $pK_a(\mathrm{C\!-\!term})=2.0$ , $pK_a(\mathrm{Lys})=10.5$ , $pK_a(\mathrm{Arg})=12.5$ , $pK_a(\mathrm{Asp})=3.9$ , $pK_a(\mathrm{Glu})=4.1$ . In an electrophoresis experiment at pH $6.0$ this peptide will most likely:

(A)

Migrate toward the cathode (negative electrode) because it is net positively charged

(B)

Migrate toward the anode (positive electrode) because it is net negatively charged

(C)

Remain near the origin (no net migration) because its net charge ≈ 0

(D)

Migrate slightly toward the cathode due to partial protonation of some residues

Q5. Two proteins X and Y have identical molecular weight ( $50\ \mathrm{kDa}$ ) but different isoelectric points: $pI_X=4.5$ and $pI_Y=9.0$ . Predict their behaviour in native PAGE at $pH=8.0$ and in SDS–PAGE (denaturing):

(A)

In native PAGE both migrate toward the anode (pH > both pI); in SDS–PAGE Y migrates faster because it has more charged side chains

(B)

In native PAGE X (pI 4.5) will be negatively charged and move toward the anode while Y (pI 9.0) will be positively charged and move toward the cathode; in SDS–PAGE both acquire similar negative charge-to-mass ratio and therefore co‑migrate

(C)

In native PAGE both remain at origin (net neutral at pH 8.0); in SDS–PAGE X migrates faster because SDS binds more strongly to acidic proteins

(D)

In native PAGE X moves to cathode and Y to anode; in SDS–PAGE they separate according to intrinsic charge rather than mass

Q6. Glycine has $pK_a(\mathrm{COOH})=2.34$ and $pK_a(\mathrm{NH_3^+})=9.60$ . Using $pI=\dfrac{pK_a(\mathrm{COOH})+pK_a(\mathrm{NH_3^+})}{2}$ , the isoelectric point $pI$ of glycine is approximately:

(A)

$6.00$

(B)

$5.97$

(C)

$5.45$

(D)

$6.47$

Q7. Consider the tripeptide $\mathrm{H_2N\!-\!Ala\!-\!His\!-\!Glu\!-\!COOH}$ (free N‑ and C‑termini). Given $pK_a(\mathrm{N\!-\!term})=9.5$ , $pK_a(\text{His side chain})=6.0$ , $pK_a(\mathrm{Glu\;side\;chain})=4.3$ , $pK_a(\mathrm{C\!-\!term})=2.1$ . At pH $6.0$ , what is the approximate net charge on the peptide? (Treat His as 50% protonated at its $pK_a$ .)

(A)

$-1.00$

(B)

$+0.50$

(C)

$0.00$

(D)

$-0.50$

Q8. Aspartic acid has $pK_a(\alpha\text{-COOH})=2.10$ , $pK_a(\mathrm{R\text{-}COOH})=3.90$ , $pK_a(\mathrm{NH_3^+})=9.60$ . Using the Henderson–Hasselbalch relation $pH=pK_a+\log\frac{[\mathrm{A^-}]}{[\mathrm{HA}]}$ , estimate the net charge of aspartic acid at pH $2.50$ (two decimal places).

(A)

$+1.00$

(B)

$0.00$

(C)

$+0.25$

(D)

$-0.75$

Q9. Consider the peptide $\mathrm{H_2N\!-\!Lys\!-\!Ser\!-\!Arg\!-\!COOH}$ with free termini. Given $pK_a(\mathrm{N\!-\!term})=9.6$ , $pK_a(\mathrm{Lys})=10.5$ , $pK_a(\mathrm{Arg})=12.5$ , $pK_a(\mathrm{C\!-\!term})=2.1$ :
(a) Calculate the $pI$ of the unmodified peptide.
(b) After phosphorylation of Ser an additional acidic group (with $pK_a=6.0$ ) is introduced (neutral below its $pK_a$ , $-1$ above). What is the approximate $pI$ after phosphorylation?
Choose the correct pair (unmodified $pI$ , phosphorylated $pI$ ).

(A)

$(11.50,\;10.05)$

(B)

$(11.50,\;9.05)$

(C)

$(10.05,\;11.50)$

(D)

$(12.05,\;10.05)$

Q10. Two 20‑base pair DNA duplexes A and B have %GC of $40\%$ and $60\%$ , respectively. Using the simplified melting temperature formula

T_m = 81.5 + 16.6\log[Na^+] + 0.41(\%\mathrm{GC}) - \frac{675}{N},

where $N$ is duplex length and $\log$ is base‑10, calculate the difference $T_m(\text{B})-T_m(\text{A})$ at (i) $[Na^+]=0.05\,$ M and (ii) $[Na^+]=0.20\,$ M. Which option is correct?

(A)

Difference $\approx 4.10^\circ\mathrm{C}$ at 0.05 M and $\approx 8.20^\circ\mathrm{C}$ at 0.20 M

(B)

Difference $\approx 8.20^\circ\mathrm{C}$ at both 0.05 M and 0.20 M

(C)

Difference $\approx 8.20^\circ\mathrm{C}$ at 0.05 M and $\approx 4.10^\circ\mathrm{C}$ at 0.20 M

(D)

Difference $=0^\circ\mathrm{C}$ at both concentrations

Q11. A zwitterionic amino acid X has $pK_a(\mathrm{COOH})=2.34$ and $pK_a(\mathrm{NH_3^+})=9.60$ . At pH $6.00$ , which ionic form of X predominates?

(A)

Fully protonated form $\mathrm{NH_3^+{-}CH(R){-}COOH}$

(B)

Fully deprotonated form $\mathrm{NH_2{-}CH(R){-}COO^-}$

(C)

Zwitterionic form $\mathrm{NH_3^+{-}CH(R){-}COO^-}$

(D)

Neutral uncharged form $\mathrm{NH_2{-}CH(R){-}COOH}$

Q12. Pure $\alpha$ - and $\beta$ -D-glucopyranose have specific rotations $[\alpha]_D^{20}(\alpha)=+112.0^\circ$ and $[\alpha]_D^{20}(\beta)=+18.7^\circ$ . An aqueous sample shows $[\alpha]_{\rm obs}=+52.7^\circ$ . Using $[\alpha]_{\rm obs}=f_\alpha[\alpha]_\alpha+(1-f_\alpha)[\alpha]_\beta$ , the approximate equilibrium composition is:

(A)

$\alpha\approx36\%,\ \beta\approx64\%$

(B)

$\alpha\approx50\%,\ \beta\approx50\%$

(C)

$\alpha\approx64\%,\ \beta\approx36\%$

(D)

$\alpha\approx10\%,\ \beta\approx90\%$

Q13. A polypeptide contains 4 Lys, 5 Glu and 3 His residues. At pH $7.00$ , take $pK_a(\mathrm{Lys\;side\;chain})\approx10.5$ , $pK_a(\mathrm{Glu\;side\;chain})\approx4.1$ , $pK_a(\mathrm{His\;side\;chain})\approx6.0$ . Assume the N-terminal is protonated ( $+1$ ) and the C-terminal is deprotonated ( $-1$ ). Estimate the net charge of the polypeptide at pH $7.0$ (use fraction protonated of His $=\dfrac{1}{1+10^{\mathrm{pH}-pK_a}}$ ).

(A)

$-4$

(B)

$0$

(C)

$+1$

(D)

Approximately $-0.73$

Q14. An enzyme-catalysed reaction has $K_m=1.0\ \mathrm{mM}$ and $V_{max}=100\ \mu\mathrm{mol\,min^{-1}}$ . In the presence of an inhibitor at $[I]=10\ \mathrm{mM}$ the observed $V_{max}$ is unchanged but $K_m^{\rm app}=3.0\ \mathrm{mM}$ . If the inhibitor is purely competitive, the inhibition constant $K_i$ is (use $K_m^{\rm app}=K_m\big(1+\dfrac{[I]}{K_i}\big)$ ):

(A)

$0.50\ \mathrm{mM}$

(B)

$5.0\ \mathrm{mM}$

(C)

$15\ \mathrm{mM}$

(D)

$20\ \mathrm{mM}$

Q15. An enzyme increases the rate of a reaction by a factor of $10^6$ compared to the uncatalysed rate at $25^\circ\mathrm{C}$ ( $T=298\ \mathrm{K}$ ). Estimate how much the activation free energy is lowered by the enzyme, $\Delta\Delta G^\ddagger = \Delta G^\ddagger_{\rm uncat}-\Delta G^\ddagger_{\rm cat}$ (use $R=8.314\ \mathrm{J\,mol^{-1}K^{-1}}$ and $\Delta\Delta G^\ddagger = RT\ln\!\frac{k_{\rm cat}}{k_{\rm uncat}}$ ).

(A)

$\approx 34\ \mathrm{kJ\,mol^{-1}}$

(B)

$\approx 20\ \mathrm{kJ\,mol^{-1}}$

(C)

$\approx 55\ \mathrm{kJ\,mol^{-1}}$

(D)

$\approx 8\ \mathrm{kJ\,mol^{-1}}$

More Quizzes for Class 12

Explore other chapters and subjects, or view all quizzes for this class.

Quiz Tweek

Biomolecules Set-4

Question Bank Preview

More Quizzes for Class 12

Chemistry

Mathematics

Physics