Specificity, induced fit and non-productive binding

According to the Induced Fit model of specificity the active site of the free enzyme 

Provided that the substrates being compared have sufficient binding energy to compensate for the unfavourable conformational change of the enzyme, then induced fit cannot explain specificity between competing substrates. Compared with the situation where the enzyme is initially in the active conformation, the induced fit mechanism reduces the value of for all substrates by the same fraction and 

If the conformational change occurs upon initial binding to form the Michaelis complex then the observed binding constant, K, will be increased by a factor 1 /K but will be the same as it would be if all the enzyme were in the active conformation. 

Induced fit can therefore only be used to explain the rates of reactions of very poor substrates compared with very good ones, e.g. the rate of phosphoryl transfer to water compared with that to glucose catalysed by hexokinase. Although water can almost certainly bind to the active site it must have insufficient binding energy to induce the necessary conformational change in the enzyme. 

Non-productive binding is sometimes suggested to account for the low reactivity of poor substrates by suggesting that they bind at sites on the enzyme where the catalytic reaction cannot take place. 

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