Biological Catalysts – Enzymes

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  • Enzymes are biological catalysts that speed up the rate of the majority of chemical reactions that occur in the cell.
  • They do this by lowering the activation energy required for the reaction to proceed.
  • Enzymes are essential, the rate of most reactions would be too slow without them and the cell would fail to keep up with the biochemical demands
  • Enzymes have an important structural feature called the active site which is the location of the chemical reactions
  • Enzymes are highly specific for one reaction or a class of reactions, based on the structure of their active sites.
  • Enzymes allow reactions to take place on their surface via specific binding of their substrate but do not take part in the reaction.

Enzymes describe a class of proteins that are biological catalysts. That is, they accelerate biological reactions without being used up during the reaction. Enzymes are essential for almost all biochemical reactions in living organisms, and they act by lowering the activation energy required for the reaction to proceed. Enzymes are essential because without them, the reaction rates would be so slow that cells could not keep up with the demand of reactions required to keep the organism running. Enzymes work by binding substrate molecules and holding them so that the chemical reactions can take place more easily.

Almost all enzymes are proteins. There are thousands of different types of enzymes that have different structures and functions, allowing them to work during specific reactions. Enzymes generally end in the name ‘-ase’, which can be a helpful way to remember (or guess!) which proteins are enzymes. Enzymes are usually very large proteins, and are highly selective about which substrates they can act upon. This is due to the structure of an important part of the enzyme called the active site.

Enzyme structure

The structure of an enzyme is crucially important for its function. The reaction that an enzyme catalyses occurs on the active site, which is the area of the protein in which the substrate can bind and the chemical reaction can take place. The active site is usually on the surface of the protein so it can be easily accessed, and usually has a highly specific structure that allows it to bind its substrate and carry out its catalytic activity. This structure is determined by the different side chains and modifications present in amino acids in the chain, and the resulting final structure the enzyme adopts. Usually, each enzyme only has one active site that can bind one substrate or class of substrates.

The general way in which enzymes works is as follows: when the substrate binds, a chemical reaction occurs, forming the product. The product is then released, and the enzyme remains unchanged, and able to catalyse more reactions.

The lock and key hypothesis/ the induced fit model

The lock and key hypothesis explains how enzymes can be so specific with their substrates and the reactions they catalyse. It describes how the enzyme’s active site has a very unique shape that complements the shape of a specific substrate. They can therefore fit exactly together.

The lock and key mechanism often means that an enzyme has specificity for one particular substrate, but it can also be compatible with a family of substrates, such as those possessing particular functional groups or modifications.

However, the lock and key hypothesis is now out of date, and scientists have developed a new model to explain how enzymes and substrates fit together. An important feature of enzymes not covered under the lock and key hypothesis, is that the active site changes shape after the substrate has bound. This ensures an even tighter fit and more precise bonding. This has lead to another hypothesis, the induced fit model, which explains that the contact of the substrate with the active site induces the enzyme to change shape. Once the product is generated, it leaves the surface of the enzyme, which turns back to its original shape.

Because enzymes are very specific in their activity, they are also very sensitive to changes in the environment, and require specific conditions to functions. Factors such as temperature, pH, concentration of the enzyme, and the concentration of the substrate all affect the rate of the reaction.

How do enzymes catalyse reactions?

Enzymes work by lowering the energy of the ‘transition state’, which represents an unstable state that products must pass to join the reaction. In other words, some energy is required in order to start the reaction, or get to the top of the ‘hill’ in the graph.

If the actvation energy is low, the reaction will happen quickly, because it is easier to get over the hill. The mechanism by which enzymes lower the activation energy depends on the specific enzyme and reaction being catalysed. The enzyme could physically bring substrates together and hold them in the correct position or create an optimal environment inside the active site for the reaction to work.

Further reading and references:

[1]. Image activation energy

[2]. Image induced fit

[3]. Image lock and key