Lock and Key Theory vs Induced Fit Model

[Sjs1234]

I am reading EK Bio and this confused me.

In the text it says for the lock and key theory:
"In this theory, the active site of the enzyme has a specific shape like a lock that only fits a specific substrate, the key. The lock and key model explains some but not all enzymes"

For induced fit it says:
"In a second theory called the induced fit model, the shape of both the enzyme and the substrate are altered upon binding"

Now both of these makes sense, but I thought these two went hand in hand. In that, a substrate must have the right shape to fit in the active site, and that once it fits into the active site it is changed by the enzyme in someway to help quicken the reaction. This makes it seem like these are two separate theories.

Also, how does the lock and key model not explain all enzymes, how can the substrate and enzyme NOT fit together, if they do not have complementary shapes that fit.

thanks

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[PiBond]

I am reading EK Bio and this confused me.

In the text it says for the lock and key theory:
"In this theory, the active site of the enzyme has a specific shape like a lock that only fits a specific substrate, the key. The lock and key model explains some but not all enzymes"

For induced fit it says:
"In a second theory called the induced fit model, the shape of both the enzyme and the substrate are altered upon binding"

Now both of these makes sense, but I thought these two went hand in hand. In that, a substrate must have the right shape to fit in the active site, and that once it fits into the active site it is changed by the enzyme in someway to help quicken the reaction. This makes it seem like these are two separate theories.

Also, how does the lock and key model not explain all enzymes, how can the substrate and enzyme NOT fit together, if they do not have complementary shapes that fit.

thanks

The reason the lock and key theory cannot explain all enzymes is that it does not explain how an active site that perfectly accommodates substrates could also accommodate products before they are released from the enzyme.

Lock and key and induced fit theories are separate ideas that help to explain active site specificity (which is the real take-home message for the MCAT).

 

[loveoforganic]

Not exactly. Proteins work by providing stabilizing binding sites (either through covalent, electrostatic, or hydrogen bonds) for a certain arrangement of atoms. Lock and key theory states that this arrangement of atoms matches the reactants of the reaction. However, if that were the case, binding of the reactants to the protein would cause a decrease in dG with no progress forward - in fact, it would raise activation energy. Induced fit theory, on the other hand, states that the arrangement of atoms matches the transition state of the reaction. Thus, binding of the reactants to the protein do not result in a significant drop in dG. The activation energy to reach the transition state is significantly lowered however, allowing those reactants to more easily progress to product.

 

[PiBond]

Not exactly. Proteins work by providing stabilizing binding sites (either through covalent, electrostatic, or hydrogen bonds) for a certain arrangement of atoms. Lock and key theory states that this arrangement of atoms matches the reactants of the reaction. However, if that were the case, binding of the reactants to the protein would cause a decrease in dG with no progress forward - in fact, it would raise activation energy. Induced fit theory, on the other hand, states that the arrangement of atoms matches the transition state of the reaction. Thus, binding of the reactants to the protein do not result in a significant drop in dG. The activation energy to reach the transition state is significantly lowered however, allowing those reactants to more easily progress to product.

True. I was just answering the OP's last question regarding why lock-and-key cannot explain all enzymes. Essential Biochemistry by Pratt and Cornely confirm both of our answers.

 

[RogueUnicorn]

great answer, loveofallthatisunholy