Lock and Key Model

The hypothesis of Enzyme Action: Lock and Key Model

The role of enzymes is to catalyze biochemical reactions, that is, to convert substrate into the corresponding product required by the body. To perform catalysis, it must first contact the substrate and then catalyze the reaction through a series of actions. 

Enzymes have high specificity and they can accelerate only one type of reaction. In 1890, E. Fisher suggested that this specificity is due to the enzyme molecule's unique shape, which matches the substrate molecule's shape. This hypothesis is called "key and lock", where the key is the substrate, and the lock - is with the enzyme. The hypothesis is that the substrate fits the enzyme like a key fits a lock.

Mechanism/Hypothesis of Enzyme Action:

The interaction of an enzyme with a substrate can be divided into three stages:

  1. Attachment of a substrate to an enzyme macromolecule.
  2. Direct enzymatic reaction.
  3. Separation of the products of the transformation of the substrate from the enzyme.

1. Attachment of a substrate to an enzyme:

  • The first stage, the fastest, is the limiting stage of the catalytic process as a whole. 
  • Its rate depends on the structures of the enzyme and substrate, the nature of the environment in which the enzymatic reaction is carried out, pH, and temperature. 
  • Enzymes are characterized by specificity concerning substrates and high binding energy with them. 
  • This energy is partly used to deform the substrate and reduce the activation energy of the subsequent chemical reaction.

2. Direct enzymatic reaction:

  • The interaction of the enzyme with the substrate is preceded by the approach and orientation of the substrate concerning the enzyme's active center. 
  • Then enzyme-substrate complexes are formed, the real existence of which can be fixed in various ways. 
  • The most obvious and effective method is X-ray diffraction analysis. 
  • An example is the identification of the enzyme-substrate complex of carboxypeptidase A and its substrate glycyl-L-tyrosine. 
  • The method makes it possible not only to establish the very fact of complex formation but also to determine the types of bonds. 
  • A simpler but sufficiently effective method is the spectral analysis of the enzyme and the corresponding enzyme-substrate complex. 
  • Thus, in particular, enzyme-substrate complexes were identified for several flavin enzymes.

3. Separation of the product:

  • The interaction of the enzyme with the substrate causes a local conformational change in some sites of the protein macromolecule of the enzyme, 
  • As a result the complementarity of its active center to the substrate sharply increases and makes it possible to carry out the catalytic process. 
  • A change in the conformation of an enzyme under the action of a substrate was first shown by D. Koshland and is called induced conformity.

Lock and Key Model of Enzyme Action:

  • The lock and key model is used to explain how enzymes interact with substrates to catalyze biochemical reactions.
  • The lock and key hypothesis was proposed by Emil Fischer in 1894 and remains an important concept in biochemistry today. 
  • The 'lock and key hypothesis' mechanism is related to enzyme specificity. 
  • According to this hypothesis, enzymes have a specific three-dimensional structure with an active site that is complementary in shape and chemical properties to a specific substrate molecule, like a lock and key.
  • Where the active site of an enzyme is like a lock, and the substrate is like a key that fits into the lock. 
  • The active site is a three-dimensional region on the enzyme's surface that has a specific shape, charge distribution, and chemical environment that complements the shape and chemical properties of the substrate molecule.
  • When the substrate binds to the active site of the enzyme, it undergoes a conformational change, forming an enzyme-substrate complex. 
  • This complex undergoes chemical reactions to produce a product, which is then released from the active site, and the enzyme returns to its original state, ready to catalyze another reaction.
  • The lock and key hypothesis suggests that enzymes are particular in their catalytic activity and that only substrates with the correct shape and chemical properties can bind to the active site and undergo catalysis. 
  • This specificity is important for the efficient functioning of enzymes in the body, as it allows them to selectively catalyze specific reactions and avoid unwanted side reactions.

Frequently Asked Questions on Lock and Key Hypothesis

1. What model of enzyme-substrate interaction was proposed by Fischer?
Answer: Emil Fischer suggested that the specificity of enzymes is determined by the exact correspondence between the form of the enzyme and the substrate. This assumption is called the key-lock model.

2. What is the role of the active site in enzymatic catalysis?
Answer: The role of the active site in enzymatic catalysis is to "recognize" and directly "interact" with the substrate. This means that the substrate molecule does not interact with all parts of the enzyme molecule, but only with its strictly defined part - the active center.

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