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.
-The figure below shows Zn²⁺ in place of Fe²⁺ to form ZPP, which cannot bind to His F8 or O₂. Based on the structure of ZPP, which of the following is the coordination number for Zn²⁺?

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.
-The figure below shows Zn²⁺ in place of Fe²⁺ to form ZPP, which cannot bind to His F8 or O₂. Based on the structure of ZPP, which of the following is the coordination number for Zn²⁺?

Quizplus · Accepted Answer

11ecba2d_11ac_e1cd_a3bf_b311f1910340_MD0008_00 A complex ion is formed when a central atom, usually a metal ion, is bonded to ligands through a coordinate covalent bond. The number of coordinate bonds to the central atom is known as the coordination number. The four nitrogen atoms within the cavity of the PPIX ring form coordinate covalent bonds with the central atom. In ZPP, the central atom is Zn²⁺ and it forms four coordinate covalent bonds, making its coordination number 4. (Choice A) Even though Zn²⁺ has a 2+ oxidation state, it is coordinated to four nitrogen atoms via the lone pairs of nitrogen's electrons. Coordinate covalent bonds do not necessarily balance the charge on the central atoms. This characteristic is different from an ionic bond, such as ZnCl₂, in which Zn²⁺ bonds to the number of Cl⁻ ions required to balance its charge. (Choice C) Although Fe²⁺ has a coordination number of 6, Zn²⁺ has a coordination number of 4. A metal's preferred coordination number has to do with several factors, including the size of the metal ion, the number of electrons in the metal's d orbitals, and the size of the ligands. In heme, the central atom is Fe²⁺, and it forms not only four coordinate covalent bonds with the nitrogen atoms but also coordinate covalent bonds with His F8 and O₂. (Choice D) The image of ZPP and the information in the passage provide enough information to know how many coordinate bonds are formed with Zn²⁺. The central atom in ZPP, Zn²⁺, prefers a coordination number of 4 so it does not bind to His F8 or to O₂, but it does form coordinate covalent bonds with the four nitrogen atoms in PPIX. Educational objective: A complex ion consists of a central atom, typically a metal ion, and its surrounding ligands, which form coordinate covalent bonds to the metal. The number of coordinate bonds to the central atom is the coordination number.

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