Which one of the following statements regarding allosteric enzymes is correct?

Correct — D, Each subunit of allosteric enzyme also contains a regulatory site in addition to the active site. An allosteric enzyme carries a second binding site, distinct from the catalytic one, at which a modulator binds; that binding changes the shape of the protein and so raises or lowers activity at the active site. This is what separates allosteric control from competitive inhibition, where the inhibitor occupies the active site itself. The other three statements each assert something that is true of ordinary enzymes but specifically false of allosteric ones, so the item is really testing whether a candidate knows that allosteric enzymes are the exception to standard enzyme kinetics.

• (a)Like other enzymes, allosteric enzymes also obey Michaelis-Menten kinetics. — They do not. Michaelis-Menten kinetics assumes independent, non-interacting catalytic sites; allosteric enzymes show cooperativity between subunits, so their behaviour departs from the Michaelis-Menten equation and is described instead by sigmoidal models.
• (b)The velocity versus substrate concentration graph is a hyperbolic graph, like any other enzyme. — The curve for an allosteric enzyme is sigmoidal — S-shaped — not hyperbolic. The flat early portion and steep middle are the visible signature of cooperative binding, which is why a small change in substrate concentration can switch activity sharply.
• (c)They are always one-subunit enzymes. — The opposite is the case. Allosteric enzymes are characteristically multi-subunit, because cooperativity requires one subunit's conformational change to be transmitted to the others.

Most enzymes are regulated at the active site, and their rate rises with substrate concentration along a rectangular hyperbola that Michaelis-Menten kinetics describes. Allosteric enzymes are built differently. They are oligomeric, with several subunits, and they carry regulatory sites away from the catalytic site. Binding of an effector at a regulatory site shifts the whole protein between a low-affinity and a high-affinity conformation, so the subunits influence one another and the velocity curve becomes sigmoidal. Enzymes at the start of a metabolic pathway are commonly allosteric, which is how a pathway's end product can shut its own production down.

Three of the four options say, in different words, that an allosteric enzyme is just like any other enzyme, and only one says it is different. That pattern is itself a clue, because a question would not be set on allosteric enzymes to establish that they behave normally. On the substance, cooperativity is the single fact that generates all three refutations at once: it requires multiple subunits, it makes the curve sigmoidal, and it breaks the Michaelis-Menten assumption of independent sites.

• An allosteric site is a binding site distinct from the active site; the effector that binds it changes enzyme conformation.
• Allosteric enzymes are typically multi-subunit and show cooperative substrate binding.
• Their velocity versus substrate concentration plot is sigmoidal, not hyperbolic.
• Because of cooperativity they do not follow Michaelis-Menten kinetics.
• Feedback inhibition of the first enzyme of a pathway by its own end product is the classic use of allosteric control.

• Assuming every enzyme obeys Michaelis-Menten kinetics — allosteric enzymes are the standard exception.
• Confusing an allosteric inhibitor with a competitive inhibitor; the competitive one binds the active site, the allosteric one does not.
• Expecting a hyperbolic curve because that is the shape drawn in every introductory enzyme diagram.

Set as a single-correct statement item in which the three wrong options are all ordinary-enzyme facts, so it punishes recognition-by-familiarity.

No directly related past PYQ was found.

• practice — not a real PYQ

  The velocity versus substrate concentration curve of a typical allosteric enzyme is

  • (a)linear
  • (b)hyperbolic
  • (c)sigmoidal
  • (d)parabolic

  Answer(c) sigmoidal — the S-shape reflects cooperative binding between subunits.

• practice — not a real PYQ

  In feedback inhibition of a metabolic pathway, the end product usually inhibits the enzyme catalysing the

  • (a)last step
  • (b)first committed step
  • (c)slowest step near the end
  • (d)reverse reaction

  Answer(b) first committed step — shutting the pathway at its entry avoids wasting intermediates.