This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Enzymes, Subject name: Biochemistry
True about Km 
Select and explain. 
A. Half the substrate concentrate at which velocity is maximum, B. Substrate concentration at which reaction rate is half the maximum, C. Michaelis constant, D. Dissociation constant of enzyme-substrate complex
B: Substrate concentration at which reaction rate is half the maximum
Answer: B: Substrate concentration at which reaction rate is half the maximum 
Explanation: E + S --------------> E--S ---------------> E + PK2 If concentration of substrate is increased, the forward reaction K1 is increased, and so K3 as well as total velocity is correspondingly enhanced. The three different constants may be made into one equation,Km = K2 + K3 K1Km is called as Michaelis Constant.It is fuher shown thatVelocity (v) = Vmax Km + When concentration of substrate is made equal to Km, i.e.When = KmVelocity (v) = Vmax = Vmax = Vmax + 2 2or v = 1/2 VmaxRef: DM Vasudevan, Page no: 48