Enzymes (2.10)
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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 catalyse anabolic reactions where large molecules are synthesised from their component parts.
For example, fatty acids are synthesised in cells by the fatty acid synthetase enzyme complex.
Enzymes catalyse catabolic reactions where large molecules are broken down into smaller molecules. These reactions usually release energy (and the end-products can be used in respiration to produce ATP).
For example, the enzyme amylase catalyses the hydrolysis (breakdown) of starch into maltose.
Without enzymes catalysing catabolic reactions, large molecules in our food would not be broken down or energy released at a rate fast enough to support life.
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.
The lock-and-key hypothesis of enzyme action suggests that a substrate fits into the rigid structure of an enzyme’s active site precisely, in the same way a specific key fits into its corresponding lock.
When an enzyme and its substrate collide, the side chains of amino acids (R groups) within the enzyme’s active site form temporary bonds with the substrate. This results in the formation of an enzyme–substrate complex.
The enzyme catalyses the reaction, and the products are formed in an enzyme–product complex. The enzyme then releases the products and is left unchanged, ready to be reused.
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.
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
