Biomolecules — Questions & Answers

Sample questions from Biomolecules

1. Q1. Starch is stored in plants rather than free glucose, even though starch must be hydrolysed before use. The chief advantage is

   • A.A large number of glucose units can be stored as one osmotically almost inactive macromolecule, avoiding the enormous osmotic pressure that the equivalent free glucose would exert✓
   • B.Starch contains more energy per gram of carbon than glucose
   • C.Starch is sweeter and therefore attracts pollinators
   • D.Glucose cannot be transported across cell membranes
   Solution

   Osmotic pressure depends on the number of dissolved particles, so polymerising thousands of glucose molecules into one insoluble granule reduces the particle count by the same factor and the cell avoids taking up water uncontrollably. Energy per glucose unit is essentially unchanged, which is why polymerisation costs the plant almost nothing in fuel value.

2. Q2. Which explanation accounts for why competitive enzyme inhibitors, which structurally resemble the natural substrate and bind reversibly at the active site, can typically be overcome by sufficiently increasing substrate concentration, whereas non-competitive inhibitors (binding at a separate site) generally cannot be overcome this way?

   • A.A competitive inhibitor competes directly with substrate for the same active site, so a sufficiently high substrate concentration can outcompete the inhibitor for binding, restoring near-normal reaction velocity, a non-competitive inhibitor binds at a distinct site and can still exert its inhibitory effect regardless of how much substrate is present, since it does not directly compete with substrate for the active site✓
   • B.Competitive and non-competitive inhibitors are mechanistically identical in every respect
   • C.Increasing substrate concentration can overcome any type of enzyme inhibition equally well
   • D.Non-competitive inhibitors bind at the same site as the substrate, identical to competitive inhibitors
   Solution

   This distinction in how substrate concentration affects the two inhibition types directly reflects their different binding sites and mechanisms, competitive inhibition is a direct competition for the same active site (mass-action kinetics favour substrate at high concentration), while non-competitive inhibition occurs at a separate site and affects enzyme function (often through an induced conformational change) regardless of substrate concentration, this kinetic distinction is a standard diagnostic tool used to classify inhibitor mechanisms experimentally.

3. Q3. Silk fibroin and wool keratin are both fibrous proteins, yet keratin is far more resistant to stretching and to reducing agents than typical silk. The most chemically defensible reason is

   • A.Keratin is rich in cysteine residues and is extensively cross-linked by covalent disulfide bonds between chains, whereas silk fibroin relies mainly on interchain hydrogen bonding in beta-sheets✓
   • B.Keratin contains peptide bonds while silk fibroin does not
   • C.Silk fibroin is a globular protein
   • D.Keratin contains glycosidic linkages that resist stretching
   Solution

   Both are fibrous, water-insoluble structural proteins, but keratin contains a high proportion of cysteine, and oxidation of pairs of -SH groups creates covalent -S-S- cross-links between adjacent chains, these covalent bridges give high tensile strength and are cleaved by reducing agents (the basis of permanent-waving of hair), silk fibroin is instead stabilised largely by hydrogen bonds in antiparallel beta-sheets.

4. Q4. Which explanation best accounts for why the anomeric effect (a stereoelectronic preference influencing the equilibrium ratio of alpha and beta anomers at mutarotation equilibrium) causes alpha-D-glucose to be present in somewhat greater proportion than might be predicted from purely steric (equatorial versus axial substituent) considerations alone?

   • A.The anomeric effect always completely eliminates the beta anomer from the equilibrium mixture
   • B.The anomeric effect has no influence on the alpha/beta ratio at mutarotation equilibrium
   • C.Purely steric considerations alone perfectly predict the observed alpha/beta equilibrium ratio for glucose
   • D.A stabilising orbital interaction (donation from the ring oxygen's lone pair to the antibonding orbital of the C1-O(anomeric) bond) favours an axial orientation of the anomeric substituent under certain electronic circumstances, partially offsetting the simple steric preference for an all-equatorial (beta) arrangement and shifting the observed equilibrium ratio✓
   Solution

   The anomeric effect is a well-documented stereoelectronic phenomenon in carbohydrate chemistry, arising from a favourable orbital interaction that can stabilise an axial orientation at the anomeric centre beyond what simple steric (equatorial-preferring) arguments alone would predict, this is why the observed alpha:beta equilibrium ratio for D-glucose in solution (roughly 36:64) does not correspond to what a purely steric argument (which would predict overwhelming preference for the all-equatorial beta form) alone would suggest.

5. Q5. A protein solution is dialysed against water through a membrane permeable to small ions but not to the protein. After dialysis the protein is found to have lost a bound cofactor and to be inactive, but activity returns when the dialysate is added back. The cofactor is best described as

   • A.A loosely bound coenzyme or activator, since it is small enough to pass the membrane and its removal is reversible✓
   • B.A covalently attached prosthetic group
   • C.A second polypeptide subunit of the enzyme
   • D.An irreversible inhibitor
   Solution

   Passage through the membrane shows that the cofactor is a small molecule or ion, and restoration of activity on adding the dialysate back shows the association is non-covalent and reversible. This is the defining behaviour of a dissociable coenzyme or metal activator; a covalently bound prosthetic group would not dialyse away, and an inhibitor would decrease rather than restore activity.

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