A graph of enzyme velocity (v) vs substrate concentration [S] for an allosteric enzyme shows a sigmoidal (S-shaped) curve rather than a hyperbolic curve. This sigmoidal kinetics indicates:
- A The enzyme undergoes substrate-induced conformational change (induced fit) with each substrate molecule binding
- B Positive cooperativity: binding of one substrate molecule to one subunit increases the affinity of remaining subunits for substrate via conformational transmission ✓
- C The enzyme has two distinct active sites with different Km values, causing a biphasic saturation curve
- D The allosteric inhibitor present in the reaction mixture competes with substrate to produce the sigmoidal appearance
Explanation
Sigmoidal v vs [S] kinetics is the hallmark of positive cooperativity in oligomeric allosteric enzymes. When the first substrate molecule binds one protomer, it induces a conformational change (T-state to R-state in the MWC concerted model, or sequential change in the KNF sequential model) that increases substrate affinity in remaining protomers. This cooperative binding causes the curve to be steeper around the S0.5 (substrate concentration at half-maximal velocity) compared to Michaelis-Menten kinetics, generating the sigmoid. Hemoglobin's oxygen binding is the classic example. Hill coefficient (n) > 1 quantifies cooperativity.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.