Biochemistry sits at the point where chemistry stops being abstract and starts describing living systems. Practice covers carbohydrates, proteins and amino acids, lipids, nucleic acids, enzymes and enzyme kinetics, metabolic pathways, and vitamins and coenzymes. Pathway questions include the regulation step in the explanation, because that is usually what the question is really testing rather than the sequence itself.
Which of the following amino acids is NOT classified as a standard protein amino acid?
Answer: A
Selenocysteine is the 21st amino acid and is not among the standard 20 protein amino acids. It contains selenium instead of sulfur.
Q.2Easy
The primary structure of a protein is stabilized by which type of bond?
Answer: B
Peptide bonds between the carboxyl group of one amino acid and the amino group of the next maintain the primary structure.
Q.3Easy
Which enzyme catalyzes the hydrolysis of peptide bonds from the C-terminal end?
Answer: B
Carboxypeptidase is an exopeptidase that removes amino acids sequentially from the C-terminal end of proteins.
Q.4Easy
Isoelectric point (pI) of a protein is defined as the pH at which:
Answer: B
At isoelectric point, the number of positive charges equals negative charges, resulting in zero net charge and minimum solubility.
Q.5Easy
Which of the following is an example of a globular protein?
Answer: C
Hemoglobin is a globular protein with a compact, spherical structure, unlike fibrous proteins like collagen and keratin.
Q.6Medium
Enzyme cofactors are non-protein substances that are essential for enzyme activity. Which of the following is NOT a cofactor?
Answer: D
Serine is an amino acid, not a cofactor. Cofactors include metal ions and coenzymes like NAD⁺, FAD, and CoA.
Q.7Medium
The Michaelis-Menten equation describes enzyme kinetics. What does Km represent?
Answer: B
Km (Michaelis constant) represents the substrate concentration at which enzyme velocity is half of Vmax, indicating substrate affinity.
Q.8Medium
Which type of enzyme inhibition is characterized by a competitive interaction with the active site?
Answer: D
In competitive inhibition, the inhibitor competes with substrate for the active site and can be overcome by increasing substrate concentration.
Q.9Medium
Denaturation of proteins can be caused by all EXCEPT:
Answer: D
Peptide bond formation is part of protein synthesis and structure, not denaturation. Denaturation involves disruption of secondary and tertiary structures.
Q.10Medium
Which amino acid sequence contains a hydrophobic amino acid that is commonly found in the hydrophobic core of proteins?
Answer: B
Leucine, Isoleucine, and Valine are branched-chain hydrophobic amino acids typically buried in the interior of folded proteins.
Q.11Medium
The quaternary structure of hemoglobin is maintained by interactions EXCEPT:
Answer: D
Quaternary structure involves interactions between different polypeptide chains, not covalent peptide bonds. Only non-covalent interactions maintain quaternary structure.
Q.12Medium
An enzyme shows maximum activity at pH 8.0. At pH 3.0, the enzyme loses its activity. This is primarily due to:
Answer: B
Extreme pH causes ionization of amino acid side chains and disruption of protein structure, leading to denaturation and loss of enzymatic activity.
Q.13Medium
Which of the following correctly matches an enzyme with its substrate?
Answer: B
Lipase catalyzes the hydrolysis of ester bonds in lipids. Amylase acts on carbohydrates, protease on proteins, and nuclease on nucleic acids.
Q.14Hard
A protein with multiple subunits shows cooperative binding of substrate. This phenomenon is best explained by:
Answer: B
Cooperative binding (positive cooperativity) occurs when binding of one substrate molecule increases affinity for subsequent molecules, a classic example of allosteric regulation as seen in hemoglobin.
Q.15Hard
During protein synthesis, a missense mutation changes codon GAA to GUA. Which amino acid substitution occurs?
Answer: A
GAA codes for Glutamic acid (Glu), GUA codes for Valine (Val). This is a non-conservative substitution of a charged to hydrophobic residue.
Q.16Hard
In a double displacement enzyme reaction mechanism, the enzyme forms a covalent intermediate. Which protease follows this mechanism?
Answer: B
Serine proteases (like trypsin, chymotrypsin) form an acyl-enzyme intermediate through a nucleophilic attack by the active site serine residue.
Q.17Hard
A competitive inhibitor with Ki = 0.5 mM and Km = 2 mM is added to an enzyme reaction. What is the apparent Km in presence of this inhibitor at [I] = 1 mM?
Answer: C
Apparent Km = Km(1 + [I]/Ki) = 2(1 + 01.5) = 2(1 + 2) = 6 mM. Competitive inhibition increases apparent Km without changing Vmax.
Q.18Hard
Which statement about the relationship between protein structure and function is INCORRECT?
Answer: B
Tertiary structure requires secondary structure elements. Secondary structures (α-helix, β-sheet) fold into tertiary structure; one cannot exist without the other.
Q.19Hard
A protease enzyme shows reduced activity when Ca²⁺ is removed from the reaction mixture. This indicates that Ca²⁺ acts as a:
Answer: C
Ca²⁺ is required for enzymatic activity and is not consumed in the reaction, making it a cofactor. Many proteases require metal ions as structural or catalytic cofactors.
Q.20Medium
Which of the following is a characteristic feature of allosteric enzymes?
Answer: B
Allosteric enzymes have regulatory sites distinct from active sites and show cooperativity, resulting in sigmoidal kinetics rather than hyperbolic kinetics seen in simple enzymes.
