Catalysis of Biological Reactions

Numerous chemical reactions within cells would proceed at unfeasibly slow rates under the conditions in which biological organisms exist. This sluggishness arises from the fundamental nature of chemical reactions, entailing breaking and forming bonds, which requires reactant molecules to convert to unstable transition states. The energy that needs to be absorbed from the environment to reach the transition state is termed activation energy.

Activation energy of a reaction with and without enzyme Image source: Microbialmatt - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=126786827

Catalysts are substances that accelerate reactions without being consumed and without altering the reaction equilibrium. In biological systems, these catalysts are primarily proteins known as enzymes. Enzymes facilitate chemical reactions by lowering the activation energy required for the reaction to occur.

In enzyme-catalyzed reactions, the molecules undergoing transformation are referred to as substrates. The substrate binds to a specific region on the enzyme known as the active site, forming an enzyme-substrate complex. The active site's shape and physical properties are tailored to accommodate only particular substrates, which explains the specificity of enzymes to their substrates.

The induced fit model. Substrate binding makes the enzyme undergo a conformational change

During catalysis, the substrate remains bound to the active site, undergoing conversion into products. This interaction is characterised by an induced fit model, where the active site's shape is not rigid but rather adjusts upon substrate binding. Weak interactions between chemical groups on the enzyme and substrate cause the active site to conform around the substrate.

Enzymes employ diverse mechanisms to reduce activation energy in chemical reactions. One approach involves properly orienting substrate molecules within the active site, facilitating bond formation. Alternatively, enzymes may distort substrate molecules, promoting bond-breaking. Some enzymes create a microenvironment within their active sites that favours specific reactions. For instance, the presence of acidic amino acid residues can lower pH, increasing the likelihood of hydrogen ion transfer to the substrate. Additionally, certain enzymes utilise amino acid residues in their active sites to form transient covalent bonds with substrates as part of the reaction process.