Quite often the synthesis of an enzyme at the beginning of a biochemical pathway is

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Question restatement: 'Quite often the synthesis of an enzyme at the beginning of a biochemical pathway is'

Option 1: 'enhanced by the end product of the pathway.' While feedback mechanisms can regulate pathways, the common regulatory theme for the initial enzyme in many pathways is not enhancement by the final product. Instead, end products often restrain early steps to prevent overproduction, so this option describes the opposite of the usual control, making it unlikely.

Option 2: 'inhibited by the end product of the pathway.' This aligns with classic feedback inhibition: when the end product accumulates, it inhibits the first enzyme in the pathway to decrease flux and conserve resources. This is a well-established regulatory strategy in biochemistry, reflecting a negative feedback loop that maintains metabolic balance.

Option 3: 'inhibited by the substrate of the pathway.' In some rare cases substrate inhibition can occur, but it is not the common regulatory theme for initiating enzyme synthesis in most biochemical pathways. The typical broad mechanism is product-based feedback rather than substrate-based inhibition, so this option is not generally correct.

Option 4: 'induced by the constitutive gene of the pathway.' Constitutive genes are expressed at a constant level, but induction refers to regulatory control that boosts expression in response to signals. Since the question specifies synthesis at the beginning of a pathway being regulated by end-product feedback, relying on constitutive expression contradicts the regulated feedback concept and is thus inaccurate in this context.

Further reflection: The central idea across options 1, 3, and 4 conflicts with the standard principle of end-product (product-level) feedback inhibition controlling the first committed step in a pathway. The most consistent and widely observed regulatory motif is that the end product inhibits the initial enzyme to prevent unnecessary accumulation. This negative feedback ensures metabolic efficiency and prevents futile cycles.

In summary, among the choices, option describing inhibition by the end product captures the canonical regulatory mechanism that governs the initiation of enzyme synthesis in many biochemical pathways, whereas the other options describe scenarios that are either contrary to common regulatory patterns or describe less typical or irrelevant controls.

BIOL 1009 (001-006/050-055) Midterm Exam III

Quite often the synthesis of an enzyme at the beginning of a biochemical pathway is

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