Bioinorganic Chemistry Chapter 16

Bioinorganic Chemistry Only a few representative examples will be presented for these bioinorganic compounds and their actions. Table 16-1 (mammalian biochemistry) Examine the importance of iron. A relatively new area of study for bioinorganic molecules is the incorporation of MO calculations.

Porphyrins and Related Complexes in Bioinorganic Molecules A porphine ring has a square planar geometry with a “pocket” in the center. Examine Fig. 16-1. A metalloporphyrin complex can result by incorporating a metal atom into the pocket (look at heme from Rasmol). Axial sites are available for other ligands. Structure, specificity, and reactivity are changed by differing the side chains, metal ions, and surrounding species.

Hemoglobin and Myoglobin Oxygen transfer and storage agents in the blood and muscle tissue. Hemoglobin transports oxygen (O2) from the lungs/gills to tissues and muscles. Myoglobin stores oxygen (O2) in the muscles and tissues. Oxygen commonly transfers from the hemoglobin to the myoglobin for later use.

Hemoglobin Made up of four globin protein subunits ( and ). Each protein partially encloses a heme group. Each heme group is in a porphyrin pocket. One axial position of the iron is bound to an imidazole nitrogen from the protein. One axial position is available/vacant or has H2O bound to it. Dissolved O2 can bind reversibly to this axial position. http://www.umass.edu/microbio/chime/hemoglob/

Hemoglobin In hemoglobin, the Fe(II) does not become oxidized to Fe(III) or Fe(IV). Occurs readily in simpler systems (see Figure on the next page). There needs to be reversible binding of the O2 without oxidation. A free heme also oxidizes in an aqueous environment. Why doesn’t oxidation occur in hemoglobin by O2 or H2O?

Hemoglobin (Figures)

Hemoglobin In nonoxygenated hemoglobin, the Fe(II) is about 70 pm out of the plane of the porphyrin nitrogens (show with Chime). Bonding O2 or CO in the sixth position causes the iron to be come planar. Fe(II) becomes diamagnetic Oxygen bonds at an angle of ~130 degrees (show with Chime). Explain these structural changes upon bonding.

Hemoglobin There is a considerable amount of  backbonding from the metal to the O2. Can be described as Fe(III)-O2- Why is the O2 bent? The energy changes very little with this angle. suggestions

Hemoglobin Cooperativity The function of hemoglobin is to bind O2 at high oxygen pressure and carry it through the blood to needed areas (and myoglobin for storage). Hb + 4O2  Hb(O2)4 Hb(O2)4 + 4Mb  4Mb(O2) + Hb As one iron binds an oxygen molecule in Hb, the molecular shape changes to make binding of additional oxygen molecules easier. In a similar fashion, initial removal of oxygen triggers the release of the remaining oxygens.

Hemoglobin At low partial pressures of O2, Mb has a much greater affinity for O2. The Bohr effect. Increased acidity favors the release of O2 from Hb(O2)4