Aspartate transcarbamoylase (ATCase) is an example of an allosteric enzyme. Which of the following is TRUE of ATCase?

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The question asks which statement about Aspartate transcarbamoylase (ATCase) is TRUE, given that ATCase is an allosteric enzyme. We will evaluate each option in turn.
Option 1: 'Upon binding GTP, a conformation change occurs in the enzyme.' This statement reflects the general concept that allosteric effectors (like GTP) bind to ATCase’s regulatory sites and induce a conformational shift. While this is true in the broad sense—that GTP acts as an allosteric activator and alters conformation—the statement does not capture the most defining feature of ATCase’s structure, nor does it contrast with the inhibitory role of other effectors (e.g., CTP). It is accurate but somewhat incomplete as a standalone description of ATCase’s allosteric regulation.
Option 2: 'It can exist in a T state, which has tight affinity for substrate.' This is misleading. In classic allosteric models, the T (tense) state typically corresponds to a lower affinity for substrate, not tight affinity. Saying the T state has tight affinity contradicts the standard understanding of the T vs R (relaxed) state relationship in allosteric enzymes like ATCase.
Option 3: 'It contains both catalytic and regulatory subunits.' This statement is a structural description that correctly characterizes ATCase. The holoenzyme comprises catalytic subunits (which perform the catalytic reaction) and regulatory subunits (which bind allosteric effectors such as CTP and GTP to modulate activity). This structural feature is a fundamental, defining property of ATCase and aligns with canonical biochemical knowledge of its allosteric regulation architecture.
Option 4: 'CTP, the ultimate product of the pathway controlled by ATCase, is an allosteric activator of ATCase.' This is incorrect. CTP acts as an allosteric inhibitor of ATCase, providing feedback inhibition to slow pyrimidine biosynthesis when product levels are high. Therefore this statement misrepresents the regulatory role of CTP.
In summary, Option 3 accurately states a core structural feature of ATCase and is consistent with its allosteric regulation mechanism. Option 1 is true in the sense that GTP binding does induce a conformational change, but it does not uniquely capture the essential description of ATCase’s organization and regulation. Option 2 contains a misconception about the T state’s substrate affinity, and Option 4 reverses the regulatory effect of CTP. The most structurally defining true statement among the choices is that ATCase contains both catalytic and regulatory subunits.

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Aspartate transcarbamoylase (ATCase) is an example of an allosteric enzyme. Which of the following is TRUE of ATCase?

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