A cell has abundant ATP. ATP binds an allosteric site on phosphofructokinase-1 (PFK-1) and shifts it predominantly to the T state. What happens to glycolysis, and why?
AGlycolysis speeds up, because ATP is providing energy to drive PFK-1 catalysis
BGlycolysis slows, because the T state reduces PFK-1's affinity for its substrate and lowers catalytic rate
CGlycolysis is unaffected, because ATP only acts as a substrate for PFK-1, not as a regulator
DGlycolysis speeds up because the T state is the high-activity conformation of allosteric enzymes
ATP is both a substrate for PFK-1 and an allosteric inhibitor. At high concentrations, it binds the allosteric site and stabilizes the T (tense, low-activity) state — slowing glycolysis via feedback inhibition. The cell already has adequate energy; slowing the pathway prevents wasteful overproduction. Option (a) confuses ATP's dual role: its allosteric inhibitory function dominates at high ATP concentrations even though it is also consumed as a substrate. Option (d) has the states backwards — R is relaxed and active; T is tense and inactive.
Question 2 Multiple Choice
How does allosteric enzyme inhibition differ fundamentally from competitive inhibition?
AAllosteric inhibitors always reduce Vmax; competitive inhibitors only increase apparent Km
BAllosteric inhibitors bind with lower affinity than competitive inhibitors and can be outcompeted by substrate
CAllosteric inhibitors bind a site distinct from the active site and change the enzyme's conformation; competitive inhibitors physically occupy the active site and block substrate binding
DAllosteric inhibition is permanent and irreversible; competitive inhibition is always reversible
The defining distinction is the site of binding. Allosteric means 'other site' — the regulator binds a separate allosteric site and transmits information to the active site through a conformational change in the quaternary structure. Competitive inhibitors resemble the substrate and block the active site directly; adding more substrate can outcompete them. Option (a) is often true but is a consequence, not the definition. Option (d) is wrong — many allosteric regulators are also reversible.
Question 3 True / False
The sigmoidal velocity-vs-substrate curve of allosteric enzymes reflects cooperative binding — binding at one subunit increases substrate affinity at neighboring subunits.
TTrue
FFalse
Answer: True
Cooperative binding is the molecular basis of sigmoidal kinetics. At low substrate concentrations, most subunits are in the T state. When substrate binds one subunit, it nudges neighboring subunits toward the R (active) state through conformational changes in quaternary structure. This makes subsequent substrate binding easier — positive cooperativity. The result is a steep sigmoidal rise in activity once a threshold substrate concentration is crossed, giving the enzyme switch-like behavior.
Question 4 True / False
An allosteric activator increases enzyme activity by competing with the natural substrate for the active site, allowing more productive binding.
TTrue
FFalse
Answer: False
Allosteric activators bind the allosteric site — not the active site. They work by stabilizing the R (relaxed, active) conformation of the enzyme, increasing substrate affinity and catalytic rate through conformational change. An allosteric activator does not resemble the substrate and does not compete for the active site. This is the 'allosteric' distinction — the regulatory binding event and the catalytic binding event happen at different locations.
Question 5 Short Answer
Why do cells use allosteric regulation to control metabolic flux rather than simply synthesizing more or less enzyme as needed?
Think about your answer, then reveal below.
Model answer: Allosteric regulation is nearly instantaneous — conformational changes occur in milliseconds. Changing enzyme concentration by altering gene expression requires transcription, translation, and protein turnover, which takes minutes to hours. Allosteric enzymes allow cells to sense their metabolic state in real time (e.g., the ATP/AMP ratio) and adjust pathway speed immediately without changing how much enzyme is present. This reversibility and speed are essential for moment-to-moment metabolic homeostasis.
The PFK-1 example illustrates this elegantly: when ATP is high, the enzyme slows glycolysis within milliseconds. When AMP accumulates, the enzyme speeds glycolysis just as quickly. No new protein synthesis or degradation is needed. Allosteric regulation is the cell's real-time control system; gene expression changes are for longer-term adaptation. Both mechanisms exist in cells, operating on very different timescales.