Metalloflavoproteins: In at least one known iron-containing flavoprotein, the iron is present in which form?

Introduction / Context:Many electron-transfer proteins combine flavin cofactors (FMN/FAD) with iron centers. The nature of the iron (heme vs nonheme) influences redox properties and spectroscopic signatures.

Given Data / Assumptions:

• Flavoproteins contain flavin prosthetic groups.
• Some also contain iron as iron–sulfur clusters (nonheme iron).
• Heme iron is coordinated within porphyrin rings; nonheme iron is not.

Concept / Approach:Examples like NADH dehydrogenase (Complex I) and other dehydrogenases are flavoproteins that also house multiple iron–sulfur centers. These iron centers are nonheme; they participate in stepwise single-electron transfers complementary to flavin chemistry.

Step-by-Step Solution:Identify iron type in representative flavoproteins: Fe–S clusters (e.g., [2Fe–2S], [4Fe–4S]).Classify as nonheme iron since no porphyrin is present.Conclude that the iron is present as a nonheme type in such metalloflavoproteins.

Verification / Alternative check:EPR/Mössbauer and UV–Vis spectra distinguish Fe–S cluster signals from heme-type absorbance bands, confirming nonheme identity.

Why Other Options Are Wrong:

• Heme-protein: not universal for flavoproteins; the cited example specifically features nonheme iron.
• Both: not applicable to the specific case asked.
• Flavin moiety: the iron is not part of the flavin ring.

Common Pitfalls:Assuming all iron in electron transport is heme-based; many key carriers are nonheme Fe–S proteins.

Final Answer:nonheme type