Enzymes are specialised proteins that act as biological catalysts, accelerating chemical reactions in cells without being used up in the process. They play an important role in numerous cellular processes, including digestion, metabolism, DNA replication and energy production. Enzymes are essential for life, as they enable reactions to occur at rates necessary for sustaining biological functions under mild conditions of temperature and pH.
Enzymes have a unique three-dimensional structure that is essential for their function. They are composed of one or more polypeptide chains folded into a specific shape, which includes an active site. The folding of the enzyme into its functional conformation is determined by the sequence of amino acids in the protein and is stabilised by various interactions, including hydrogen bonds, ionic bonds, hydrophobic interactions and disulphide bridges.
The primary function of enzymes is to lower the activation energy required for a reaction to occur, thereby increasing the reaction rate. This is achieved through the active site, a specific region of the enzyme with a unique shape and chemical environment perfectly suited to bind to the enzyme’s substrate (the reactant molecule).
- Active Site: The active site is a small, specialised region of the enzyme where substrate molecules bind. It consists of a few key amino acids that create a precise environment for the reaction, including binding sites that interact with the substrate through weak, non-covalent interactions.
- Enzyme-Substrate Complex: When the substrate binds to the active site, it forms an enzyme-substrate complex. This binding stabilises the transition state of the substrate, reducing the activation energy needed for the reaction.
- Catalysis and Product Formation: The enzyme catalyses the conversion of the substrate into the product by facilitating bond-breaking and bond-forming processes. Once the reaction is complete, the product is released, and the enzyme is free to bind with another substrate molecule, ready to catalyse subsequent reactions.
Enzyme models
Enzyme function is often described using models that explain how enzymes interact with their substrates. The two primary models are the lock-and-key model and the induced-fit model.
The lock-and-key model, proposed by Emil Fischer in 1894, is the simpler of the two models and was the first to describe enzyme-substrate interactions. According to this model:
- Specificity: The active site of the enzyme is rigid and precisely shaped to fit a specific substrate, much like a key fitting into a specific lock. This model emphasises the high specificity of enzymes, where only substrates with the correct shape can bind to the enzyme’s active site.
- Mechanism: The substrate binds to the active site without causing significant changes in the enzyme’s structure. The enzyme-substrate complex is formed quickly, and the enzyme catalyses the reaction without altering its shape.
The induced-fit model was proposed by Daniel Koshland in 1958. According to this model:
- Flexibility: The active site is not a perfect fit for the substrate initially. Instead, the enzyme is flexible and undergoes a conformational change upon substrate binding. This change helps to optimise the fit between the enzyme and the substrate.
- Mechanism: When the substrate approaches the enzyme, the active site moulds itself around the substrate, enhancing the interaction. This conformational change not only positions the substrate correctly but also stresses specific bonds within the substrate, lowering the activation energy needed for the reaction.