Govt Exams
Aconitase requires an iron-sulfur cluster [4Fe-4S] for catalyzing the isomerization of citrate to isocitrate. This is essential for its catalytic mechanism in the TCA cycle.
Serine proteases have an extended substrate-binding site composed of multiple subsites (S1, S1', S2, etc.) that recognize and bind extended substrate peptides. The catalytic triad (Ser-His-Asp) performs the actual catalysis.
Turnover number (kcat) is the number of substrate molecules converted to product per enzyme molecule per unit time when the enzyme is fully saturated. It equals Vmax/[E]total.
Valine, with its branched nonpolar side chain, frequently appears in beta-sheets where it can form hydrophobic interactions and van der Waals contacts that stabilize the sheet structure.
Disulfide bonds (S-S) between cysteine residues form cross-links that stabilize the tertiary structure (within a protein) and quaternary structure (between subunits), particularly important in extracellular proteins.
Negative cooperativity occurs when substrate binding to one subunit decreases the affinity of other subunits for substrate, resulting in a hyperbolic (rather than sigmoidal) binding curve.
Heat or chemical denaturants disrupt hydrogen bonds and hydrophobic interactions, affecting secondary (alpha-helix, beta-sheet) and tertiary (3D fold) structures before affecting primary structure (peptide bonds).
Transferases catalyze the transfer of functional groups (e.g., methyl, phosphoryl, amino) from one substrate to another. Examples include kinases, methyltransferases, and transaminases.
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.
Digestive enzymes are synthesized as inactive zymogens (e.g., pepsinogen, trypsinogen) and are activated by proteolytic cleavage in the appropriate compartments (stomach, small intestine).