Revise Mode of action of enzymes for Cambridge 9700 (3.1)

Enzymes are globular proteins that act as biological catalysts. Their specific three-dimensional tertiary structure, maintained by bonds like hydrogen bonds and ionic bonds, is crucial for their function. The exterior of the enzyme typically features hydrophilic R-groups, making it soluble in the aqueous environment of the cell. Their primary role is to increase the rate of specific biochemical reactions without being consumed or permanently altered. They achieve this by providing an alternative reaction pathway with a significantly lower activation energy. Activation energy is the minimum energy required to initiate a chemical reaction. By reducing this energy barrier, enzymes enable reactions to proceed much faster at physiological temperatures, conditions under which these reactions would otherwise be too slow to sustain life.

Every enzyme possesses a unique three-dimensional region called the active site. This is a specially shaped pocket or groove on the enzyme's surface, formed by the precise folding of its polypeptide chains which brings a small number of specific amino acid residues into close proximity. The active site's specific shape and chemical properties (e.g., charge distribution, hydrophobicity) are determined by these residues and are complementary to those of its particular reactant molecule, known as the substrate. This high degree of specificity ensures that each enzyme typically catalyses only one type of reaction, preventing unwanted side reactions and ensuring precise control over metabolic pathways.

For an enzyme to catalyse a reaction, the substrate must first bind to its active site, forming a temporary structure called the enzyme-substrate complex (ESC). This binding is typically weak and reversible, involving non-covalent interactions such as hydrogen bonds, ionic bonds, and van der Waals forces. Once the substrate is bound within the active site, the enzyme can induce slight conformational changes, optimising the fit and placing strain on the substrate's bonds, or bringing multiple reactants into close proximity and correct orientation. This stress on the substrate facilitates its conversion into products.

E + S ⇌ ES → E + P

Here, E represents the enzyme, S the substrate, ES the enzyme-substrate complex, and P the products. The double arrow (⇌) indicates reversible binding of the substrate to the enzyme, while the single arrow (→) shows the irreversible conversion of the substrate into products, followed by their release.

The formation of the enzyme-substrate complex is fundamental to the enzyme's ability to lower activation energy. Enzymes achieve this in several effective ways:

• Proximity and Orientation: By binding multiple substrates simultaneously, enzymes hold them in the correct orientation, significantly increasing the probability of a successful reaction.
• Straining Substrate Bonds: The binding process can induce strain in the substrate's existing chemical bonds, making them weaker and easier to break. This brings the substrate closer to its high-energy transition state.
• Providing a Favourable Microenvironment: The active site can create an optimal chemical environment, for instance, by excluding water (creating a non-polar environment) or by having charged R-groups that adjust the local pH, which is more conducive to the reaction than the bulk solution.
• Direct Participation: In some cases, specific amino acid residues within the active site can temporarily form covalent bonds with the substrate, acting as acid-base catalysts to assist in the formation of the reaction's transition state.

Following the reaction, the products are released from the active site, and the enzyme is then free to bind to another substrate molecule and catalyse the reaction again, perfectly illustrating its role as a true catalyst.

The Lock-and-Key Hypothesis (Emil Fischer, 1894):

This early model proposed that the active site of an enzyme has a rigid, fixed shape that is perfectly complementary to the shape of its specific substrate, much like a unique key fits into a unique lock. Binding is considered precise and pre-determined, with no significant alteration to the enzyme's structure upon substrate binding. While it effectively explains enzyme specificity, it struggles to account for the dynamic nature of enzyme-catalysed reactions or how enzymes can catalyse reactions involving multiple substrates.

The Induced-Fit Hypothesis (Daniel Koshland, 1958):

This is a more widely accepted and refined model. It suggests that the active site is not rigid but possesses a degree of flexibility. Upon the initial, relatively weak binding with the substrate, the enzyme undergoes slight conformational changes. This 'induced fit' moulds the active site more precisely around the substrate, optimising the fit and placing strain on the substrate's bonds, facilitating the chemical reaction. This dynamic interaction also helps ensure that only the correct substrate can trigger this optimal binding, further contributing to specificity. The induced-fit model better explains the broad range of enzyme specificities, the formation of transition states, and how enzymes can accommodate a range of related molecules or multiple substrates.

The efficiency of an enzyme is often quantified by its turnover number (k_cat). This is defined as the maximum number of substrate molecules that a single enzyme molecule can convert into product per unit time (usually per second) when the enzyme is fully saturated with substrate. It is a measure of the intrinsic catalytic rate of the enzyme. A high turnover number indicates a very efficient enzyme. For example, catalase, which breaks down hydrogen peroxide, has a very high turnover number, reflecting its role in rapidly detoxifying a harmful metabolic by-product.