Biochemistry - MCQ Practice Questions

Selenocysteine is the 21st amino acid and is not among the standard 20 protein amino acids. It contains selenium instead of sulfur.

Peptide bonds between the carboxyl group of one amino acid and the amino group of the next maintain the primary structure.

Carboxypeptidase is an exopeptidase that removes amino acids sequentially from the C-terminal end of proteins.

At isoelectric point, the number of positive charges equals negative charges, resulting in zero net charge and minimum solubility.

Hemoglobin is a globular protein with a compact, spherical structure, unlike fibrous proteins like collagen and keratin.

Trypsin is a serine protease that specifically recognizes and cleaves peptide bonds on the C-terminal side of positively charged amino acids (Lys and Arg).

Enzyme specificity arises from the precise three-dimensional arrangement of amino acid residues in the active site, which determines substrate recognition and binding through complementary fit.

Leucine is a nonpolar, hydrophobic amino acid that tends to cluster in the protein interior away from the aqueous environment, while polar and charged residues prefer the surface.

Amylase is a hydrolase enzyme that catalyzes the hydrolysis of alpha-1,4-glycosidic bonds in starch, converting it into sugars. Both salivary and pancreatic amylases perform this function.

Km (Michaelis constant) is defined as the substrate concentration at which v = Vmax/2. It provides insight into enzyme-substrate affinity; lower Km indicates higher affinity.

Transferases catalyze the transfer of functional groups (e.g., methyl, phosphoryl, amino) from one substrate to another. Examples include kinases, methyltransferases, and transaminases.

Collagen has a characteristic Gly-X-Y tripeptide repeat pattern where glycine appears at every third position. This allows tight packing in the triple helix structure. Hydroxyproline (formed by post-translational modification of proline) stabilizes the helix.

Chymotrypsin is secreted as chymotrypsinogen. Trypsin (activated by enterokinase) cleaves a specific dipeptide from chymotrypsinogen to generate active chymotrypsin, which then undergoes autolytic cleavage for full activation.

Turnover number (kcat = Vmax/[E]total) represents the number of substrate molecules converted to product per enzyme molecule per unit time at maximum velocity. It indicates catalytic efficiency when substrate is saturating.

Peptidyl transferase activity is catalyzed by the 23S rRNA (in prokaryotes) or 28S rRNA (in eukaryotes) component of the ribosome. This ribozyme catalyzes the formation of the peptide bond between the incoming aminoacyl-tRNA and the growing polypeptide chain.

DNA ligase catalyzes the formation of phosphodiester bonds between adjacent DNA strands. In prokaryotes, it uses NAD+ as the energy source, while in eukaryotes, ATP is used. This is essential for DNA replication, repair, and recombination.

Leucine has a nonpolar, hydrophobic isopropyl side chain. Serine and asparagine are polar, while lysine is positively charged.

Hemoglobin has quaternary structure consisting of 2 α-globin and 2 β-globin subunits held together by non-covalent interactions.

Proteases are endopeptidases and exopeptidases that hydrolyze peptide bonds in proteins. Amylase acts on carbohydrates, lipase on fats, and nuclease on nucleic acids.

The isoelectric point is the pH at which the net charge on the protein is zero, resulting in minimum solubility and maximum precipitation.