Click on thumbnail to see full-size!	The Porphyrin Ring chelates an Iron atom.  Porphyrins are widely distributed in nature. They occur in hemoglobin and myoglobin. However they are also found in chlorophylls (the green pigments in plant which trap light energy), peroxidases,  Vitamin B12, and the cytochromes of the electron transport chain. The essential characteristic of the porphyrin ring is the ability to chelate a number of metals such as iron (Fe), copper (Cu),  zinc (Zn), nickel (Ni), or magnesium (Mg) in the case of chlorophyll. In myoglobin and hemoglobin, there are 4 heterocyclic nitrogens which bind the iron atom. Another important characteristic is that - because of its aromaticity - the porphyrin is very hydrophobic!
Click on thumbnail to see full-size!	The Iron atom binds molecular oxygen (O2).  Iron and Copper are readily oxidized to iron oxides which have a red color. This is what make a-globin, b-globin and myoglobin good oxygen carriers. The thumbnail to the left shows how molecular oxygen is bound is bound to one side of the Fe2+ atom. Unfortunately, carbon monoxide binds to the iron even more strongly than oxygen. This is why carbon monoxide is so toxic - it removes oxygen from the blood stream and asphyxiates the individual.   In order to view the stereo image in 3-D, look at it from a distance which is so close that you can't focus. At this point, you should see 3 images. Then slowly move out, trying to focus on the third image in the center. When this is brought into focus, it will appear 3-dimensional.
Click on thumbnail to see full-size!	The tertiary structure of myoglobin forms a binding pocket for the Porphyrin Ring.  Myoglobin is composed of 8 alpha-helices, which fold on each other to form a hydrophilic exterior (shown in green) and a hydrophobic interior (yellow). A cleft in the protein gives access to this hydrophobic interior. Recall that the porphyrin ring is very hydrophobic. It can "escape" from the aqueous environment of the cells and tissues by binding into the cleft of myoglobin!
Click on thumbnail to see full-size!	A stereo space-filling view of the Myoglobin binding pocket. The thumbnail to the left shows a space-filling stereo view of myoglobin. This shows the real shape of the binding pocket.
Click on thumbnail to see full-size!	A Porphyrin Ring in the Myoglobin binding pocket. The thumbnail to the left shows the polarity of the alpha helices, with the porphyrin ring inserted into the binding pocket.
Click on thumbnail to see full-size!	A stereo space-filling view of the Porphyrin Ring in the Myoglobin binding pocket. The thumbnail to the left shows a space-filling model, with the porphyrin ring inserted into the binding pocket.