Enzymes are biological catalysts that speed up chemical reactions in living organisms. They play a vital role in nearly all physiological processes, such as digestion, metabolism, and cellular signaling. Enzymes are proteins, though some RNA molecules also have catalytic activity. The concept of enzymes is very essential for MDCAT students in the subject areas of biochemistry and physiology.
Structure of Enzymes:
Enzymes are usually large proteins, consisting of amino acid chains folded into specific three-dimensional shapes. The active site is the region of the enzyme that binds to the substrate, which is the molecule upon which the enzyme acts. The structure of the enzyme is highly specific to the substrate, and this specificity is important for the proper functioning of the enzyme. Activity of the enzyme may be influenced by temperature, pH, and the presence of inhibitors or activators.
Examples:
The enzyme amylase has an active site that specifically binds to the starch molecules, helping in their breakdown to simpler sugars.
Mechanism of Action:
Enzymes catalyze reactions by reducing the activation energy required for the reaction to take place. They form an enzyme-substrate complex, wherein the enzyme temporarily binds to the substrate, and this facilitates the conversion of the substrate into products. There exist two major models to explain enzyme action: the lock and key model and the induced fit model. In the lock and key model, the enzyme’s active site perfectly fits the substrate just like a key fitting into a lock. In the induced fit model, the enzyme’s active site undergoes a slight conformational change upon substrate binding.
Example:
With the induced fit model, when hexokinase binds to glucose, it slightly changes shape to fit the substrate better, which allows for the phosphorylation of glucose.
Enzyme Classification:
Enzymes can be grouped in respect to the reaction they catalyze. The main classification of the International Union of Biochemistry and Molecular Biology, IUBMB, categorizes the enzymes into six main categories.
Oxidoreductases: Involved in oxidation-reduction reactions.
Transferases: Transfer functional groups (for example, methyl or phosphate groups).
Hydrolases: Catalyze hydrolysis reactions (e.g., breaking bonds with water).
Lyases: Catalyze addition or removal of groups to form double bonds.
Isomerases: Catalyze rearrangement reactions between molecules.
Ligases: Catalyze the joining of two molecules using ATP.
Example:
DNA ligase is a ligase that seals DNA strands together during DNA replication and repair.
Factors Affecting Enzyme Activity:
Several factors can affect the activity of enzymes:
Temperature: Generally, enzyme activity increases with temperature up to a certain optimum, above which the enzyme may denature.
pH: Each enzyme has a specific optimum pH at which it is most active. A change in pH can change the shape of the enzyme and reduce activity.
Substrate Concentration: As substrate concentration increases, enzyme activity increases until all enzyme active sites are occupied (saturation).
Inhibitors: Inhibitors may decrease the activity of an enzyme. The competitive inhibitors bind to the active site and thus prevent the substrate from binding. Non-competitive inhibitors bind elsewhere on the enzyme, altering its shape and function.
Cofactors and Coenzymes: Some enzymes require non-protein molecules called cofactors (metal ions) or coenzymes (organic molecules) to function. Vitamins often act as coenzymes.
Example:
Zinc is a cofactor for the enzyme carbonic anhydrase, which participates in the regulation of blood pH.
Enzyme Kinetics:
Enzyme kinetics is the study of the rates of enzyme-catalysed reactions. The Michaelis-Menten equation relates the rate of the reaction with the concentration of the substrate. The Km value, or Michaelis constant, is a substrate concentration when the rate of reaction reaches half of its maximal value (Vmax). Lower Km values indicate higher affinity between the enzyme and substrate.
Example:
The rate of the reaction catalyzed by lactase, which breaks down lactose, is faster at binding to lactose if its Km is lower, and thus, by simple extension, if the enzyme’s Km is low.
Quiz: Test Your Knowledge of Enzymes
Our MDCAT Quiz on enzymes will help you assess your understanding of enzyme structure, function, and the factors affecting enzyme activity. This quiz is designed to help you prepare for MDCAT questions related to biochemistry and physiology.
Free Flashcard: Key Insights on Enzymes
The free Flashcard set on enzymes covers important aspects, including structure, classification, and kinetics. Use these flashcards to better prepare and be confident in dealing with questions related to the topic during the MDCAT exam.
An understanding of enzyme functions, their mechanisms, and how they are influenced is a fundamental necessity for great performance in both the MDCAT exam and for learning the cellular processes within biology.
Get Your Username and Password for MDCAT Tests
Sign Up Now
Experience the real exam environment with our expertly designed collection of over 25,000 MCQs MDCAT Mock Tests.
