Chapter 3: Enzymes Flashcards
What are enzymes and where do they catalyze reactions?
Enzymes are globular proteins that catalyze reactions inside cells (intracellular enzymes) or are secreted to catalyze reactions outside cells (extracellular enzymes).
Describe the mode of action of enzymes.
Enzymes work by binding to substrates at their active site to form an enzyme-substrate complex, lowering the activation energy required for the reaction and ensuring enzyme specificity.
What are the two main hypotheses for enzyme-substrate binding?
The lock-and-key hypothesis, where the substrate fits precisely into the enzyme’s active site, and the induced-fit hypothesis, where the enzyme changes shape slightly to accommodate the substrate.
How can you investigate the progress of enzyme-catalyzed reactions?
By measuring the rate of product formation (e.g., using catalase) or the rate of substrate disappearance (e.g., using amylase).
How can a colorimeter be used in enzyme-catalyzed reactions?
A colorimeter measures color changes, allowing tracking of reaction progress in reactions that involve color changes.
What factors affect the rate of enzyme-catalyzed reactions?
Temperature
pH (using buffer solutions)
Enzyme concentration
Substrate concentration
Temperature Effect On Rate Of Enzyme-Catalyzed Reactions
As temperature increases, the kinetic energy of molecules also increases, leading to more frequent and forceful collisions between enzymes and substrates.
This enhances the rate of reaction up to an optimum temperature, where the enzyme activity is at its peak.
Beyond optimum temperature, when the temperature is too high, the hydrogen bonds in the enzyme which is a protein start to break and the enzyme denatures, meaning the active site changes shape and can no longer bind to the substrate effectively, causing decrease in reaction rate.
pH Effect On Rate Of Enzyme-Catalyzed Reactions
Each enzyme has an optimal pH at which its active site is in the best shape to bind with the substrate.
Deviations from this optimal pH affect the ionic bonds and hydrogen bonds that maintain the enzyme’s structure, leading to a distorted active site and reduced enzyme activity.
Extreme pH levels can lead to denaturation, where the enzyme’s structure is permanently altered.
Enzyme Concentration Effect On Rate Of Enzyme-Catalyzed Reactions
Increasing enzyme concentration provides more active sites available for the substrates to bind, increasing the reaction rate.
However, this increase continues only until the substrate becomes the limiting factor.
Once all substrate molecules are occupied by enzymes, adding more enzyme won’t further increase the rate of reaction.
Substrate Concentration Effect On Rate Of Enzyme-Catalyzed Reactions
As substrate concentration increases, the rate of reaction rises because more substrate molecules collide with enzyme active sites.
This increase continues until the enzyme molecules become saturated with substrate, meaning all active sites are occupied.
At this saturation point (Vmax), the reaction rate plateaus, and additional substrate won’t increase the rate.
What is Vmax in enzyme kinetics?
Vmax is the maximum rate of reaction and is used to derive the Michaelis-Menten constant (Km), which compares the affinity of different enzymes for their substrates.
What is Michaelis-Menten constant (Km)
Half of Vmax is the Km. High values of KM correspond to low enzyme affinity for substrate.
What is Affinity
Affinity is the tendency of the enzyme to bind to the substrate, the bonding. A high affinity means it binds readily, at low concentration. A low affinity means it binds reluctantly, only at higher concentration.
What are inhibitors and how do they affect enzyme activity?
Competitive inhibitors: Compete with the substrate for the active site, slowing reaction rate.
Example : Malonate inhibits succinic dehydrogenase.
Non-competitive inhibitors: Bind elsewhere on the enzyme, altering its shape and reducing activity.
Example : Cyanide inhibits cytochrome oxidase.
What is the difference in activity between immobilized and free enzymes?
Immobilized enzymes (e.g., in alginate) tend to have greater stability and reusability than enzymes free in solution.
