Understanding Km in enzyme kinetics Steps:

• Recall Km definition: substrate concentration at half Vmax in Michaelis-Menten equation.
• Evaluate statement 1: Km indicates enzyme-substrate affinity (lower Km means higher affinity)—correct.
• Evaluate statement 2: Km equals enzyme's turnover number—incorrect, as Km relates to binding, not kcat.
• Evaluate statement 3: Km is always the dissociation constant Kd—incorrect, as Km = (k-1 + kcat)/k1, not just Kd.
• Evaluate statement 4: Km remains constant with enzyme concentration changes—correct, as it's intrinsic to the enzyme-substrate interaction.

Why B is correct:

• B selects statements 1 and 4, matching the accurate definitions of Km as affinity measure and concentration-independent parameter.

Why the others are wrong:

• A includes 2 and 3, which confuse Km with turnover and Kd.
• C omits 4 and includes incorrect 3.
• D includes incorrect 2 and omits 1.

Final answer: B