Nitrogenase, the Enzyme That Catalyzes Nitrogen Fixation, Is Inhibited Whenever

question 4

Multiple Choice

Nitrogenase, the enzyme that catalyzes nitrogen fixation, is inhibited whenever free O₂ reaches a critical concentration. Consequently, nitrogen fixation cannot occur in cells wherein photosynthesis produces free O₂. Consider the colonial aquatic cyanobacterium, Anabaena, whose heterocytes are described as having "…a thickened cell wall that restricts entry of O₂ produced by neighboring cells. Intracellular connections allow heterocysts to transport fixed nitrogen to neighboring cells in exchange for carbohydrates."
-Given that the enzymes that catalyze nitrogen fixation are inhibited by oxygen, what are two "strategies" that nitrogen-fixing prokaryotes might use to protect these enzymes from oxygen?
1) couple them with photosystem II (the photosystem that splits water molecules)
2) package them in membranes that are impermeable to all gases
3) be obligate anaerobes
4) be strict aerobes
5) package these enzymes in specialized cells or compartments that inhibit oxygen entry

• A) 1 and 4
• B) 2 and 4
• C) 2 and 5
• D) 3 and 4
• E) 3 and 5

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