hey guys quick back a mr. basics here let's talk about lock and key model lock and key model is one of the earliest model proposed for the mechanism of enzyme action it was proposed by ml Fisher in the year 1894 it was proposed based on the fact that enzymes are very specific for the substrate this is just like keys are specific for the locks according to this model the active site of enzyme has same conformation instead of substrate this allows binding of substrate in the active site of the enzyme just like key fits in log the enzyme then acts on the substrate and converts it into a product one of the major drawbacks of this proposal is that it fails to explain the transition state that occurs when enzyme binds with its substrate now take a boss big boss and like to understand what does transition state means when lock and key model was proposed it was just a hypothesis scientists were curious to test his hypothesis and wanted to know how the enzyme converts a substrate into a product however enzymes are very fast and they can convert several substrate molecules into product in a fraction of a second if we have a syringe filled with a solution and we purge the solution in the pipe then it takes some time for the solution to reach the end of the Bible let's say the length of the pipe is 10 centimeters and let's assume that it takes 1 second to reach at the end of the pipe this means it will take 1/2 second to reach 5 centimeters of the pipe and one end of the second to reach 1 centimeter the same technique can be used for enzyme and substrate we can have two syringes one filled with a solution of enzyme and the other filled with the solution of substrate when both the syringes are purged in a single pipe the enzyme acts on the substrate and starts the reaction to know what is happening in one tenth of the second we can have an outlet from the pipe and have an inhibitor in the collection tube the same can be done for different time intervals when scientists analyzed this mixture they discovered that the substrate is converted into an intermediate state or a transition state before getting converted into a product when the transition state was discovered the locking key model failed to explain the transition state of the substrate look at the story short the lock and key model remained popular for several decades after its proposal however in 1930s when the transition state was discovered the locking key model failed to explain the transition state of the substrate a new model was required to explain the mechanism of enzyme action this new model was known as the induced fit model