Biological oxidants — Which of the following is NOT considered a significant biological oxidizing agent in standard metabolism?

Introduction / Context:
Redox cofactors drive metabolic pathways by accepting or donating electrons. Identifying physiologically important oxidizing agents clarifies electron flow in catabolism and respiration.

Given Data / Assumptions:

• NAD+ and FAD are classic oxidized cofactors that accept electrons.
• O2 is the terminal electron acceptor in aerobic respiration.
• Fe3+ and Fe2+ interconvert in proteins, but free ferric ion is not a common soluble oxidant in cytosolic biochemistry.

Concept / Approach:
In cells, NAD+ and FAD accept electrons to become NADH and FADH2, respectively. Molecular oxygen accepts electrons at Complex IV to form water. While iron cycles in heme and iron–sulfur centers, “free” Fe3+ is not broadly deployed as a primary soluble oxidant in standard metabolic reactions due to tight sequestration by proteins (to prevent radical chemistry) and limited availability. Therefore, compared with NAD+, FAD, and O2, Fe3+ is not regarded as a significant general-purpose biological oxidizing agent.

Step-by-Step Solution:

Verification / Alternative check:
Standard biochemistry texts list NAD+/FAD and O2 as principal oxidants in cytosolic and mitochondrial metabolism; iron participates as cofactors rather than free oxidizing equivalents.

Why Other Options Are Wrong:

• NAD+ is a primary oxidant accepting hydride ions in numerous pathways.
• FAD serves similarly in certain dehydrogenases.
• O2 is the terminal oxidant in aerobic organisms.

Common Pitfalls:
Confusing protein-bound electron carriers (e.g., cytochromes containing Fe) with freely diffusing oxidants like NAD+ or O2.

Final Answer:
Fe3+