Passage Heme Is Found in Several Proteins, Including Hemoglobin in Blood

Passage
Heme is found in several proteins, including hemoglobin in blood and myoglobin in muscle.  It has a four-coordinate iron-bound porphyrin structure where ferrous iron (Fe²⁺) is bound to four nitrogen atoms through coordinate covalent bonds.  The most common heme found in the body is heme b in hemoglobin (Figure 1) .
Figure 1  Structure of heme b found in hemoglobinAlong with the four nitrogen ligands in the porphyrin structure, His F8 also binds to Fe²⁺ in the axial position, leaving the other axial position available to bind oxygen.  In this setting, iron's atomic d orbitals are no longer degenerate.  The energetic difference between the d orbitals depends on the ligands.In deoxygenated heme, iron sits out of the plane formed by the nitrogen ligands in the porphyrin because its d orbitals make it slightly too large to fit in the porphyrin ring.  However, when O₂ binds to heme, iron's electrons re-arrange themselves.  The d orbitals become smaller, and iron sits within the plane of the nitrogen atoms.When iron is scarce in the body, Zn²⁺ will coordinate to the ligands in the protoporphyrin structure through a nonenzymatic process, forming Zn-protoporphyrin IX (ZPP) .  ZPP and protoporphyrin IX (PPIX) have distinct emission spectra when excited in the blue range whereas heme does not.  Hematofluorometry can be used as a simple, inexpensive indicator for iron deficiency in whole blood by placing a blood sample in a fluorometer and measuring the amount of ZPP relative to the amount of heme.  Figure 2 illustrates the fluorescence emission spectra for ZPP and protoporphyrin IX measured from a blood sample when ZPP is excited at 425 nm and PPIX is excited at 407 nm.
Figure 2  ZPP and PPIX fluorescence emission spectra
Adapted from Hennig G, Gruber C, Vogeser M, et al. Dual-wavelength excitation for fluorescence-based quantification of zinc protoporphyrin IX and protoporphyrin IX in whole blood. J Biophotonics. 2014;7(7) :514-24.
-Which of the following statements best explains why hemoglobin is red when it binds to O₂?

A) O₂ interacts with iron's d orbitals.
B) O₂ is a nonpolar ligand.
C) O₂ has lone pairs of electrons.
D) O₂ changes the surrounding protein structure.

Correct Answer:

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