2.5 Regulation of Enzyme Activity [HY] Flashcards
Feedback Regulation
Enzymes are subject to regulation by products further down a given metabolic pathway
Feed-forward regulation
enzymes may be regulated by intermediates that precede the enzyme in the pathway
Negative feedback
- Feedback inhibition
- helps maintain homeostasis: once have enough product, turns it off
Competitive Inhibition
- involves occupancy of the active site
- Substrates cannot access enzymatic binding sites if there is an inhibitor in the way
- Raises Km b/c more needed to reach half Vmax
How to counteract competitive inhibition?
- can be overcome by adding more substrate so that the substrate-to-inhibitor ratio is higher
- If more molecules of substrate are available than molecules of inhibitor, then the enzyme will be more likely to bind substrate than inhibitor (assuming the enzyme has equal affinity for both molecules).
Does adding competitive inhibitor alter vmax?
Adding a competitive inhibitor does not alter the value of vmax because if enough substrate is added, it will outcompete the inhibitor and be able to run the reaction at maximum velocity
Noncompetitive Inhibition
- bind to an allosteric site instead of the active site, which induces a change in enzyme conformation
- Once the enzyme’s conformation is altered, no amount of extra substrate will be conducive to forming an enzyme–substrate complex
- the two molecules do not compete for the same site, inhibition is considered
noncompetitive
Allosteric Sites
- non-catalytic regions of the enzyme that bind regulators
- Regulators regulate the availability of the active site
Do noncompetitive inhibitors affect vmax?
- Yes, decreases the measured value of vmax because there is less enzyme available to react
Do noncompetitive inhibitors affect Km?
- No, because any copies of the
enzyme that are still active maintain the same affinity for their substrate
Mixed Inhibition
- results when an inhibitor can bind to either the enzyme or
the enzyme–substrate complex, but has different affinity for each - If the inhibitor had the same affinity for both, it would be a noncompetitive
inhibitor. - Bind at allosteric site
Do mixed inhibitors affect Km?
- alters the experimental value of Km depending on the preference of the inhibitor for the enzyme vs. the enzyme–substrate complex
- If prefers enzyme, increases Km
- If prefers enzyme-substrate complex, lowers Km
Do mixed inhibitors affect vmax?
- yes, vmax is decreased
Uncompetitive Inhibition
- bind only to the enzyme–substrate complex and essentially lock the substrate in the enzyme, preventing its release
- Essentially increasing affinity between the enzyme and substrate
- it’s the formation of the enzyme–substrate complex that creates a conformational
change that allows the uncompetitive inhibitor to bind
Do uncompetitive inhibitors affect vmax?
- yes, lower vmax