Navigation
Focuses on how enzymes lower activation energy and the mechanisms of metabolic regulation.
Imagine your body is a high-speed factory where chemical reactions that should take years happen in milliseconds. How does a single protein 'handshake' make life possible, and what happens when a molecular 'wrench' is thrown into the gears?
In the past, scientists described enzymes using the 'Lock and Key' model, suggesting a rigid fit. However, we now use the Induced-Fit Model. When a substrate approaches the active site, the enzyme undergoes a conformational change—a physical 'tightening'—to bind the substrate more securely. This 'molecular hug' strains the substrate's chemical bonds, lowering the activation energy () required for the reaction to proceed. This ensures the transition state is stabilized, allowing the reaction to occur at lightning speed under physiological conditions.
Quick Check
How does the 'Induced-Fit' model differ from the older 'Lock and Key' model?
Answer
The Induced-Fit model describes the enzyme as flexible, changing shape to fit the substrate, whereas the Lock and Key model suggests the enzyme is a rigid, unchanging shape.
Cells must turn enzymes 'on' or 'off' to maintain homeostasis. This is achieved through inhibition. Competitive inhibitors are molecular imposters; they mimic the substrate's shape and compete for the active site. If you increase the substrate concentration, you can 'out-compete' these inhibitors. In contrast, non-competitive inhibitors bind to an allosteric site (a different spot on the enzyme). This binding changes the enzyme's overall shape, making the active site non-functional regardless of how much substrate is present.
Imagine an enzyme reaction where a competitive inhibitor is present. 1. Initially, the reaction rate is low because the inhibitor occupies the active sites. 2. You add 10 times more substrate to the solution. 3. Because the inhibitor and substrate compete for the same spot, the sheer number of substrate molecules makes it statistically more likely for a substrate to bind than an inhibitor. 4. The reaction rate increases toward its maximum velocity ().
Quick Check
If adding more substrate does NOT increase the reaction rate in the presence of an inhibitor, which type of inhibition is likely occurring?
Answer
Non-competitive (allosteric) inhibition.
An enzyme system is at with an enzyme concentration of . If you double the substrate concentration , the rate remains . 1. Identify the limiting factor: Since the rate didn't change when doubled, the enzyme is saturated. 2. To double the rate to , you must double the enzyme concentration to . 3. This demonstrates that at saturation, is directly proportional to .
Which of the following best describes the 'Induced-Fit' model?
If a reaction is at , what is the limiting factor?
Competitive inhibition can be overcome by increasing the substrate concentration.
Review Tomorrow
In 24 hours, try to sketch the difference between a competitive and non-competitive inhibitor binding to an enzyme.
Practice Activity
Look up the drug 'aspirin' and determine if it acts as a competitive or non-competitive inhibitor for the COX enzyme.