Antimycin A added to isolated mitochondria halts electron flow. Its precise site of action is:

• A Blocking electron transfer from iron-sulfur centers to ubiquinone at Complex I
• B Inhibiting the F0 proton channel of ATP synthase
• C Binding the binuclear copper center of Complex IV
• D Binding the Qi site of Complex III, preventing reoxidation of reduced cytochrome b ✓

Explanation

Antimycin A binds the quinone reduction (Qi) site of cytochrome bc1 (Complex III) on the matrix side, blocking transfer of electrons from heme bH to ubiquinone and stalling the Q cycle. Downstream carriers such as cytochrome c become fully oxidized while upstream b cytochromes become fully reduced. Stigmatellin and myxothiazol act on other sites of the same complex; option A describes piericidin-like action at Complex I.

Reference: Lehninger Principles of Biochemistry, 7th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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