Which of the following is NOT correct about a competitive inhibitor of an enzyme?

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Question: Which of the following is NOT correct about a competitive inhibitor of an enzyme?

Option 1: 'Its binding to an enzyme is minimised in the presence of large excess of substrate.' This statement is largely accurate for competitive inhibition: when substrate concentration is very high, the substrate outcompetes the inhibitor for the active site, reducing inhibitor binding effectively. So this option describes a correct property of competitive inhibitors.

Option 2: 'It bears a close structural resemblance to the substrate.' This is a classic feature of many competitive inhibitors: they mimic the substrate's geometry to fit into the active site, blocking substrate access. This option is correct.

Option 3: 'It forms an enzyme-inhibitor complex that does not lead to catalysis.' Indeed, the enzyme-inhibitor complex formed at the active site blocks catalysis by preventing substrate turnover. This option correctly describes the mechanism.

Option 4: 'It binds at a site on the enzyme only when the substrate has been bound.' This statement is misleading. Competitive inhibitors typically bind to the active site itself, competing with substrate, and do not require substrate binding to occur; they can bind in the absence of substrate and prevent substrate binding. Hence, this claim misstates the binding behavior and is not correct.

Option 5: 'It binds only at the active site.' This aligns with the classic view of competitive inhibitors: occupancy of the active site by inhibitor prevents substrate binding, and binding occurs at the active site itself. This option is correct.

Summary: Options 1, 2, 3, and 5 describe correct aspects of competitive inhibitors, while option 4 presents an inaccurate statement about the binding requirement. The incorrect statement is option 4. The explanation above walks through each choice, clarifying why the others align with competitive inhibition and why the fourth option does not.

PHR1122 S2 2025 PA D5.2 CYL - Enzyme inhibitors

Which of the following is NOT correct about a competitive inhibitor of an enzyme?

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