Transcript Bioinorganic Chemistry

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.
K Mb 
[Mb(O 2 )]
[Mb][O 2 ]
K Hb 
[Hb (O 2 ) 4 ]
[Hb ][O 2 ]2.8
• The Bohr effect.
– Increased acidity favors the
release of O2 from Hb(O2)4