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 does NOT describe the iron-porphyrin complex formed between Fe²⁺ and the four nitrogen atoms in Figure 1?

Question

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 does NOT describe the iron-porphyrin complex formed between Fe²⁺ and the four nitrogen atoms in Figure 1?

Quizplus · Accepted Answer

11ecba2d_11ab_a946_a3bf_63ab949a57b5_MD0008_00 Coordinate covalent bonds are a special type of bond between a central atom, such as a metal, and a ligand. Both electrons in the bond come from the ligand. Although classified as covalent bonds, coordination bonds do share some of the characteristics of ionic bonds. For example, the metal ion maintains its oxidation state, as in the case of an ionic bond, but the ligands are not charged and are usually electronegative, as in covalent bonds. The metal and its ligands together form a complex, and the number of coordinate bonds to the metal is known as the coordination number. The metal ion Fe²⁺ is positively charged whereas two of the donor atoms (nitrogen) have a negative formal charge (due to the loss of an H atom), resulting in a net charge of 0 for the iron-porphyrin complex. Unlike ionic bonds, the donor atoms do not give electrons to the positively charged metal. Instead counterions often surround the complex in solution to balance the metal's positive charge. (Choice A) The nitrogen atoms in the porphyrin ligand each have a lone pair of electrons that provide both bonding electrons in each coordinate covalent bond. (Choice C) The lone pairs of nitrogen's electrons interact with iron's d orbitals to form the coordinate bond. In a coordination bond, the d orbitals are no longer degenerate and have distinct shapes, some of which point directly at the negatively charged ligands. (Choice D) The strength of the coordination bond does depend on which d orbitals have electrons in them. Educational objective: Coordinate covalent bonds are a special bond between a central atom and its ligands. The number of coordinate bonds indicates the coordination number and the ligands provide bonding electrons, which interact with the metal's d orbitals to form the coordinate covalent bond.

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