Hemoglobin M (HbM) is a rare hemoglobin variant in which certain histidine residues are substituted by tyrosine, stabilizing heme iron in the ferric (Fe3+) state. A newborn with HbM presents with cyanosis unresponsive to oxygen therapy. Methemoglobin levels are 40%. Why does 2,3-BPG fail to shift the oxygen dissociation curve rightward in this patient?

• A 2,3-BPG cannot bind to fetal hemoglobin gamma chains
• B The ferric (met) heme subunits cannot bind 2,3-BPG in the central cavity
• C Met-Hb lacks the allosteric communication between alpha and beta subunits for 2,3-BPG binding
• D Tyrosine-substituted subunits are permanently locked in the R (oxy) conformation regardless of 2,3-BPG ✓

Explanation

In HbM, the tyrosine substitution forms a stable iron-tyrosinate complex that fixes the affected subunit(s) in the ferric (met) state. Met-heme cannot bind oxygen. The unaffected subunits remain functional but the tertiary/quaternary conformational changes driven by 2,3-BPG require cooperative communication with all four subunits. The locked subunits disrupt the T (deoxy) to R (oxy) transition and the allosteric response to 2,3-BPG. Additionally, met-heme has a higher oxygen affinity for the remaining subunits, shifting the curve leftward — methylene blue (which reduces methemoglobin via NADPH) is ineffective in HbM because the iron is prevented from reduction by the tyrosinate bond.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.