Questions & explanations
1. Which type of secondary structure is stabilized by hydrogen bonds between the carbonyl oxygen of one amino acid and the amide hydrogen of an amino acid four residues later?
- A) Alpha-helix
- B) Beta-pleated sheet
- C) Random coil
- D) Disulfide bridge
Answer: A) Alpha-helix
In an alpha-helix, hydrogen bonds form between the carbonyl oxygen of one amino acid and the amide hydrogen of an amino acid four residues ahead. Beta-pleated sheets involve hydrogen bonds between adjacent chains or segments. Random coil is not a regular secondary structure. Disulfide bridges are covalent bonds in tertiary structure.
2. Which of the following statements about amino acids is correct?
- A) There are 25 standard amino acids that serve as building blocks of proteins.
- B) All 20 standard amino acids are essential and must be obtained from diet.
- C) Amino acids are linked by peptide bonds to form proteins.
- D) Amino acids are classified only by their amino group.
Answer: C) Amino acids are linked by peptide bonds to form proteins.
Amino acids are the building blocks of proteins, and they are joined together by peptide bonds to form polypeptide chains. There are exactly 20 standard amino acids, not 25. Not all 20 are essential; some can be synthesized by the body. Amino acids are classified by their R group, not just the amino group.
3. The quaternary structure of a protein involves:
- A) The sequence of amino acids.
- B) The folding of a single polypeptide chain into a 3D structure.
- C) The arrangement of multiple polypeptide subunits.
- D) The formation of alpha-helices and beta-sheets.
Answer: C) The arrangement of multiple polypeptide subunits.
Quaternary structure refers to the association of two or more polypeptide chains (subunits) into a functional protein. Primary structure is the amino acid sequence. Tertiary structure is the 3D folding of a single chain. Secondary structure includes alpha-helices and beta-sheets.
4. Which of the following factors can denature an enzyme by disrupting its three-dimensional structure?
- Optimum temperature
- High temperature
- Optimum pH
- High substrate concentration
Answer: High temperature
High temperature can break the weak bonds maintaining the enzyme's structure, leading to denaturation and loss of activity. Optimum temperature and pH are conditions where the enzyme functions best, and high substrate concentration increases reaction rate until saturation.
5. The specific region of an enzyme where the substrate binds is called the:
- Allosteric site
- Active site
- Regulatory site
- Binding domain
Answer: Active site
The active site is the region on the enzyme where the substrate binds and catalysis occurs. The other options refer to different sites: allosteric sites are for regulatory molecules, regulatory sites are similar, and binding domain is a broader term.
6. Which polysaccharide serves as a structural component in plant cell walls?
- A) Starch
- B) Glycogen
- C) Cellulose
- D) Inulin
Answer: C) Cellulose
Cellulose is a structural polysaccharide with β-1,4-glycosidic bonds, providing rigidity to plant cell walls. Starch and glycogen are storage polysaccharides with α-glycosidic bonds; inulin is a storage polysaccharide in some plants.
7. The primary structure of a protein refers to:
- A) The sequence of amino acids in the polypeptide chain.
- B) The folding of the polypeptide into alpha-helices and beta-sheets.
- C) The three-dimensional arrangement of a single polypeptide.
- D) The arrangement of multiple polypeptide subunits.
Answer: A) The sequence of amino acids in the polypeptide chain.
Primary structure is the linear sequence of amino acids. Secondary structure involves alpha-helices and beta-sheets. Tertiary structure is the 3D folding of a single polypeptide. Quaternary structure involves multiple polypeptides.
8. A competitive inhibitor of an enzyme:
- Binds to the active site and can be overcome by increasing substrate concentration.
- Binds to a site other than the active site and changes the enzyme's shape.
- Binds irreversibly to the enzyme and permanently inactivates it.
- Binds only to the enzyme-substrate complex.
Answer: Binds to the active site and can be overcome by increasing substrate concentration.
Competitive inhibitors bind reversibly to the active site, competing with the substrate. Increasing substrate concentration can overcome this inhibition. Non-competitive inhibitors bind elsewhere and are not overcome by substrate.
9. Which of the following is a primary metabolite in plants?
- A) Morphine
- B) Nicotine
- C) Sucrose
- D) Rubber
Answer: C) Sucrose
Primary metabolites are directly involved in growth, development, and reproduction; sucrose is a primary metabolite. Morphine, nicotine, and rubber are secondary metabolites, which are not essential for basic life processes.
10. In the zwitterionic form of an amino acid, which groups are ionized?
- A) Amino group is protonated (-NH3+) and carboxyl group is deprotonated (-COO-).
- B) Amino group is deprotonated (-NH2) and carboxyl group is protonated (-COOH).
- C) Both amino and carboxyl groups are protonated.
- D) Both amino and carboxyl groups are deprotonated.
Answer: A) Amino group is protonated (-NH3+) and carboxyl group is deprotonated (-COO-).
In the zwitterionic form, the amino group gains a proton to become -NH3+ and the carboxyl group loses a proton to become -COO-, resulting in a neutral molecule with both positive and negative charges.
11. Which of the following polysaccharides is composed of N-acetylglucosamine units?
- A) Inulin
- B) Chitin
- C) Starch
- D) Cellulose
Answer: B) Chitin
Chitin is a polysaccharide made of N-acetylglucosamine units, found in exoskeletons of arthropods and fungal cell walls. Inulin is a polymer of fructose, starch of glucose, and cellulose of glucose.
12. A holoenzyme consists of:
- Only the protein part (apoenzyme).
- Only the cofactor.
- The apoenzyme and the cofactor together.
- Multiple enzyme subunits.
Answer: The apoenzyme and the cofactor together.
A holoenzyme is the catalytically active form consisting of the protein part (apoenzyme) and its non-protein cofactor (prosthetic group, coenzyme, or metal ion). The apoenzyme alone is inactive.