5.2 Enzymes
Enzymes are organic catalysts produced by living organisms that speed up biochemical reactions essential to sustain life. They operate by binding to specific substrates at their active sites to form enzyme-substrate complexes, significantly lowering the activation energy required for reactions to take place.
Learning Objectives
- Define enzymes, substrates, active sites, and enzyme-substrate complexes.
- Explain the naming system of enzymes based on the IUBMB nomenclature and its exceptions.
- State the general characteristics of enzymes.
- Differentiate between intracellular and extracellular enzymes.
- Trace the sequence of events involved in the production and secretion of extracellular enzymes.
- Explain the mechanism of enzyme action using the ‘lock and key’ hypothesis and the concept of activation energy.
- Describe and interpret the effects of temperature, pH, substrate concentration, and enzyme concentration on the rate of enzyme-catalysed reactions.
Pathway of Extracellular Enzyme Synthesis and Secretion
The correct sequence of organelles involved in producing, transporting, modifying, and secreting extracellular enzymes.
Ribosome
The site where proteins are synthesised.
Rough Endoplasmic Reticulum
Synthesised proteins enter the lumen and are transported through it.
Transport Vesicle
Buds off from the RER and carries proteins to the Golgi apparatus.
Golgi Apparatus
Modifies proteins into functional enzymes and packages them into secretory vesicles.
Secretory Vesicle
Buds off from the tips of the Golgi apparatus and moves towards the plasma membrane.
Plasma Membrane
Secretory vesicles fuse with it and release enzymes outside the cell by exocytosis.
Short Explanation
Definition and Basic Concepts
Enzyme — an organic catalyst mostly made up of proteins and produced by living cell organisms. A substrate is the substance needed for an enzyme-catalysed reaction. The active site is a specific site on the enzyme molecule where the substrate binds. When a substrate binds with the active site, a temporary enzyme-substrate complex is formed.
Enzyme Nomenclature
Introduced by the International Union of Biochemistry and Molecular Biology (IUBMB) in the 1960s. Enzymes are usually named by adding the suffix ‘-ase’ to the name of the substrate. For example, lactase catalyses the hydrolysis of lactose into glucose and galactose: \(\text{Lactose} + \text{water} \xrightarrow{\text{Lactase}} \text{Glucose} + \text{galactose}\). Some enzymes discovered before this system keep their traditional names, such as trypsin, pepsin, and renin.
General Characteristics of Enzymes
Enzymes act rapidly, are biological catalysts, are needed only in small quantities, are reusable, are highly specific, are mostly reversible, may require cofactors, and are susceptible to inhibitors.
Intracellular and Extracellular Enzymes
Intracellular enzymes are synthesised within a cell for its own internal use, such as hexokinase in glycolysis. Extracellular enzymes are synthesised in a cell and secreted outside the cell to function, such as trypsin produced by pancreatic cells and secreted into the duodenum to digest polypeptides.
Mechanism of Enzyme Action
In the ‘lock and key’ hypothesis, the enzyme is the lock and the substrate is the key. The substrate binds to the active site to form an enzyme-substrate complex, a catalytic reaction occurs, products detach, and the enzyme remains unchanged. Enzymes work by lowering the activation energy, which is the energy needed to break chemical bonds in substrate molecules before a reaction can begin.
Factors Affecting Enzyme Activity
Temperature, pH, substrate concentration, and enzyme concentration all affect the rate of enzyme-catalysed reactions. Each enzyme has an optimum temperature and pH. At extreme temperatures or pH values, enzymes become denatured and lose their three-dimensional active site conformation.
Biological Lens Note
The bacterium Alcanivorax borkumensis produces hydroxylase enzymes to break down marine oil spills into non-toxic substances.
Try to Answer First
Answer in your mind, then press “Check Answer”.
What is meant by activation energy, and how do enzymes affect it?
Explain why the reaction rate of an enzyme-catalysed reaction plateaus at a high substrate concentration when the enzyme concentration is kept fixed.
Name the organelle that packages and modifies proteins into functional enzymes before secreting them into secretory vesicles.
Common Mistakes
- Stating that all enzymes are proteins without qualification. The textbook explicitly highlights that while enzymes are mostly made up of proteins, not all enzymes are synthesised from proteins.
- Confusing the ‘lock’ and the ‘key’. In the ‘lock and key’ hypothesis, the enzyme is the lock and the substrate is the key.
- Misidentifying the limiting factor at plateau. When substrate concentration is excessive and the rate plateaus, enzyme concentration is the limiting factor.
- Assuming all human digestive enzymes have the same optimum pH. Stomach pepsin requires a strongly acidic medium (pH 1.5–2.5) while duodenal trypsin requires an alkaline medium (around pH 8.5).
Concept Misunderstandings
Increasing temperature above \(40^\circ\text{C}\) continues to increase reaction rate indefinitely because kinetic energy keeps rising.
Although kinetic energy rises, extreme heat breaks the chemical bonds that maintain the enzyme's three-dimensional conformation. Once the active site is distorted (denatured), substrates can no longer bind, causing the reaction rate to fall rapidly and halt at \(60^\circ\text{C}\).
Denaturation means the enzyme is completely dissolved or consumed in the reaction.
Denaturation is the loss of the enzyme's specific three-dimensional conformation due to broken chemical bonds. The protein molecule is not consumed; its active site simply loses its shape and can no longer bind the substrate.
Any change in pH causes permanent denaturation.
Slight deviations in pH alter the electric charges (\(\text{H}^+\) ions) of the active site and substrate surface reversibly. When pH returns to optimum, normal charges are restored. Only extreme pH changes break structural bonds and cause permanent denaturation.
Summary
- Enzymes are organic catalysts (mostly proteins) that accelerate chemical reactions by lowering activation energy.
- Most enzymes are named by adding ‘-ase’ to their substrate (e.g., lactase), with historical exceptions like pepsin and trypsin.
- Enzymes are specific, reusable, require small quantities, remain unchanged, and many are reversible or need cofactors.
- Extracellular enzymes follow a synthesis-transport pathway: Ribosome \(\rightarrow\) Rough Endoplasmic Reticulum \(\rightarrow\) Transport vesicle \(\rightarrow\) Golgi apparatus \(\rightarrow\) Secretory vesicle \(\rightarrow\) Plasma membrane.
- In the ‘lock and key’ hypothesis, the enzyme is the lock and the substrate is the key.
- Enzyme activity is regulated by temperature (optimum \(\approx 37^\circ\text{C}\), denatures at \(60^\circ\text{C}\)), pH (pepsin pH 1.5–2.5, salivary amylase pH 6.8, trypsin pH 8.5), substrate concentration, and enzyme concentration.
Short Activity
Objective: test your understanding of enzyme production, mechanism, and factors affecting activity.
A. Objective Quiz
Which organelle is responsible for packaging modified proteins into secretory vesicles during the production of extracellular enzymes?
What occurs to an enzyme when it is exposed to an extreme temperature of \(60^\circ\text{C}\)?
B. Fill in the Blanks
In the ‘lock and key’ hypothesis, the enzyme is represented by the lock, while the substrate is represented by the .
The optimum pH range for the action of pepsin in the stomach is between 1.5 and .
C. Matching / Drag and Drop
Match each enzyme or cellular structure to its correct description or characteristic according to the textbook. Drag each choice to its correct description. On a phone, tap a choice first, then tap the matching box. Tap the Answer Choices area to return a selected item.
C. Match the Biological Component to Its Function or Characteristic
Write the correct biological component next to each description.
| Function or Characteristic | Biological Component |
|---|---|
| 1. An extracellular enzyme that works optimally in an alkaline medium of around pH 8.5 | |
| 2. An intracellular enzyme used in the glycolysis process during cellular respiration | |
| 3. Buds off from the rough endoplasmic reticulum to carry synthesised proteins to the Golgi apparatus | |
| 4. The cellular structure that serves as the site of protein synthesis |
Answer choices: Trypsin • Hexokinase • Transport vesicle • Ribosome
