Heme Binding in Hemoglobin Steps:

• Hemoglobin consists of four subunits, each with a heme group containing Fe²⁺ iron at its center.
• The iron in heme reversibly binds molecular oxygen (O₂) for transport.
• Carbon monoxide (CO) also binds to the heme iron, but with higher affinity, causing toxicity.
• Carbon dioxide (CO₂) does not bind heme; it reacts with amino groups on the globin chain to form carbaminohemoglobin.

Why B is correct:

• Option B (1 and 3) identifies O₂ and CO as the gases that directly coordinate with the heme iron, per hemoglobin's molecular structure.

Why the others are wrong:

• A includes 2 (CO₂), which binds the protein, not heme.
• C omits 3 (CO), ignoring its known binding to heme iron.
• D includes 2 (CO₂) and excludes 1 (O₂), missing the primary transport function.

Final answer: B