Enzymes as proteins (3.1.4.2)
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Enzymes are a type of globular protein. They have a specific 3-dimensional shape as a result of their primary structure (sequence of amino acids).
Activation energy is the energy needed to start a reaction.
Enzymes act as biological catalysts, lowering the activation energy required for a reaction to occur by forming weak interactions with the substrate. These interactions can strain the bonds in the substrate, making them more likely to break or form.
Enzymes are not altered or used up during a reaction; they can be reused.
In cells, many metabolic reactions have high activation energies and would occur too slowly at normal biological temperatures. Enzymes lower the activation energy, allowing reactions to proceed quickly and efficiently, supporting life without raising the cell’s temperature.
Enzymes play a crucial role in regulating metabolism, determining structures and functions from the cellular to the whole organism level.
Enzymes are essential for various biochemical processes and can function both inside cells (intracellular) and outside cells (extracellular).
Intracellular enzymes function within cells, catalysing numerous metabolic processes essential for cell survival and function.
Catalase is an example of an intracellular enzyme and is found in nearly all living organisms that respire aerobically. It decomposes hydrogen peroxide, a harmful byproduct of aerobic metabolism, into water and oxygen:
Extracellular enzymes are secreted by cells to function outside of the cell. They play crucial roles in digestion, where large nutrient molecules need to be broken down or modified outside the cell.
For example, amylase is an enzyme produced by cells in the salivary glands and pancreas. When released into saliva, it catalyses the hydrolysis of starch (a polysaccharide) into maltose (a disaccharide).
Enzymes are globular proteins with a specific tertiary (3-dimensional) structure. An area within this structure, called the enzyme’s active site, is where substrates bind, forming an enzyme–substrate complex.
The shape of an enzyme’s active site is complementary to the substrate(s) involved in the specific reaction it catalyses.
Owing to enzyme specificity (a substrate only fits the active site of one type of enzyme), enzyme action was previously modelled on a ‘lock and key’. This hypothesis described the shape of an enzyme’s active site as rigid and exactly matching the shape of the substrate, like a key fitting into its corresponding lock.
Scientists observed that other molecules could bind to enzymes outside of their active site and alter the enzyme’s activity by altering its shape. This showed that enzymes are not rigid structures, so an amended hypothesis was needed: the induced fit model.
The induced fit model of enzyme action suggests that enzymes are flexible and can change shape in the presence of a substrate. These changes in shape are known as conformational changes.
As an enzyme and substrate collide, the side chains of amino acids (R groups) within the enzyme’s active site form temporary bonds with the substrate. These weak interactions induce a change in the enzyme’s tertiary structure, strengthening the binding in the enzyme–substrate complex. This strains the bonds in the substrate making them easier to break (lowering the activation energy required for the reaction).
The products are formed within an enzyme–product complex and are then released. The enzyme returns to its original shape, ready to be reused.
The progress of enzyme-catalysed reactions can be investigated by:
- Measuring the rate of product formation
- Measuring the rate of substrate disappearance.
The rate of an enzyme-catalysed reaction can be measured from a graph of the amount of product formed or substrate used through time.
The gradient of the curve at any given point is equal to the rate of the reaction (units are always ‘per unit time’). This changes over time if the relationship is curved.
Enzymes work best at their optimum pH (concentration of ions). Reactions will slow as the pH is increased or decreased away from the optimum.
ions interact with the hydrogen and ionic bonds forming an enzyme’s tertiary structure. Outside the enzyme’s optimal pH ([] is too high or too low), they can disrupt this bonding, altering the shape of the enzyme and its active site. When the shape of the active site is no longer complementary to the substrate, the enzyme is denatured, and it cannot function.
Temperature affects the speed at which molecules move and, therefore, the rate at which enzyme and substrate molecules collide.
Enzymes have an optimum temperature, where they catalyse reactions fastest. This is the temperature at which the maximum number of enzyme–substrate complexes form.
At low temperatures, molecules have less kinetic energy and move more slowly. This leads to fewer successful collisions between enzymes and substrates. As a result, fewer enzyme–substrate complexes form and the reaction rate is slower.
Higher temperatures provide more kinetic energy. This results in more frequent and successful collisions between enzymes and substrates, accelerating the reaction.
Beyond the optimum temperature, too much kinetic energy breaks the hydrogen and ionic bonds that maintain the enzyme’s tertiary structure and shape the active site. The enzyme is said to be denatured. If the enzyme cannot bind the substrate, it cannot catalyse the reaction and the reaction rate drops sharply.
Denaturation is often irreversible.
Initially, increasing substrate concentration increases the rate of reaction. With more substrate molecules present, the chance of forming enzyme–substrate complexes increases.
The rate of reaction subsequently plateaus as all active sites become saturated. The rate of reaction reaches its maximum . Beyond this point, further increase in substrate concentration will not increase the reaction rate, as there are no free active sites for additional substrate molecules to bind to. The enzyme concentration becomes the limiting factor in the reaction.
Initially, increasing enzyme concentration increases the rate of reaction because there are more active sites available to form enzyme–substrate complexes. With ample substrate available, the initial reaction rate increases linearly with enzyme concentration.
If the substrate is limited, increasing the enzyme concentration further will not increase the reaction rate, because substrate availability becomes the limiting factor. The reaction rate will have reached
Inhibitors are molecules which stop enzymes from functioning or slow them down. They can bind to enzymes reversibly or irreversibly. There are two types of inhibitors:
- Competitive inhibitors have a similar shape to the substrate and compete to bind to the enzyme’s active site. By forming enzyme–inhibitor complexes, competitive inhibitors reduce the formation of enzyme–substrate complexes.
- Non-competitive inhibitors bind to the enzyme at an alternative site (or allosteric site), which alters the shape of the active site. This reduces the activity of the enzyme or prevents the substrate from binding.
Competitive and non-competitive inhibitors decrease the reaction rate. At sufficiently high inhibitor concentrations, the reaction can cease entirely.
Increasing substrate concentration can counteract the effect of a competitive inhibitor, as more substrate molecules are available to collide with enzymes and form enzyme–substrate complexes.
Increasing substrate concentration does not restore the reaction rate in the presence of a non-competitive inhibitor, since the inhibitor changes the enzyme’s active site, preventing enzyme–substrate complex formation.
