In an enzyme mediated reaction, substrate concentration was 1000 times Km 1% of substrate is metabolised to form 12 mmol of substrate in 9 minutes. If in the same reaction enzyme concentration is reduced to 1/3rd and substrate concentration is doubled. How much time is needed to produce same amount of product?

Ans. D. 27 minWe can consider this question in two partsFirst part=1000Km 12mmol of substrate in 9 minutesWhen S=Km in a hyperbolic curve it follow 1st order kinetics i.e. is directly proportion to velocity of reaction. But here its =1000Km means its zero order kinetics, i.e. even if increased velocity do not increase.Second partEnzyme concentration (E') is 1/3 of initial E concentrationE is directly proportional to velocity of reactionis=2000Km So this is zero order kinetics hence velocity of reaction depends only EE'=1/3E, hence New Velocity (V') is 1/3 V initialHence time taken is three times more than initial time takenTime=9x3=27min