本题目来源于试卷: Interaction & Interdependence C.1 Enzymes & Metabolism，类别为 IB生物学
[单选题]
A metabolic pathway that wj(tg:5sg zr1zh r /o+ conve :94v;ril 3s( d bsk vtixg/+h g0w-ncirts threonine to the end-product isoleucine consists of five reactions. Threonine is converted to various intermediate products sequentially by different enzymes xhc vv;(gn:sig0i-r49l3 ikwbs/dt+ before it is converted to isoleucine by the last enzyme in the pathway.
In the following diagram, the enzymes have been labelled as E1, E2, E3, E4 and E5.
Which of the following shows how this pathway is regulated?
A. Isoleucine inhibits E1, the enzyme that catalyses the first step of the pathway.
B. Isoleucine inhibits both E1 and E2, the enzymes that catalyse the first two steps.
C. Isoleucine inhibits E5, the enzyme that catalyses the last step.
D. Isoleucine inhibits all the enzymes in the pathway.
参考答案: A
本题详细解析:
Isoleucine, the end product, inhibits E1 (threonine dehg2 c *yw h,a dscd*y./yp22pmza69zumydratase) by binding to the allosteric site of the enzyme. E1 catalyses the first reaction of the pathway. When there is an excess of the end-product isoleucine, it inhibits the first enzyme and prevents the buildup of intermediate products.
Isoleucine inhibits E1 but not E2, so Choice B is incorrect.
Isoleucine does not inhibit E5, so Choice C is incorrect.
Isoleucine does not inhibit2w2 p2 m*,huy6mp*.gzcdyz9csa d ay/ all the enzymes in the pathway, only E1. Choice D is therefore not correct.
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