138 



PROTEINS 



of iron Fe+++ (the iron is ferrous Fe ++ in the original hemoglobin and 

 is oxidized to ferric Fe + + + in the process of isolating hemin) which is 

 joined to the four nitrogens of the pyrrole rings by partial valences and 

 to a chlorine atom to form the chloride salt. The four pyrrole groups, 

 without the side chains and iron, form a unit known as porphin, and 

 derivatives thereof are called porphyrins. Many porphyrins occur in 

 nature, e.g., coproporphyrin of feces, uroporphyrin of urine, and chloro- 

 phyll (p. 389) . Chlorophyll, the green pigment of plants, contains an ad- 

 ditional ring, differs from heme in its side chains, and has Mg at the center 

 instead of Fe. One of the propionic side chains in chlorophyll is linked 

 as an ester to the unsaturated alcohol phytol. That the most important 

 animal pigment and the most important plant pigment should have 

 related structures is something to be noted carefully. 



The outstanding chemical feature of heme (hemoglobin) is its ability 

 to combine with oxygen and thus serve as a transport agent in the blood. 

 Each heme combines with one molecule of oxygen. Since there are four 

 heme units in each molecule of hemoglobin, the reaction between hemo- 

 globin and oxygen may be represented thus: 



Hb-h402^Hb(02)4 

 Hemoglobin Oxyhemoglobin 



more conveniently, but less exactly, 



Hb + O2 ^ HbOa 



This reaction takes place in the lungs; in the tissues it is reversed. 



Hemoglobin also reacts with carbon monoxide to form a combination 

 that is several hundred times stronger than that with oxygen. It there- 

 fore takes much oxygen to displace the carbon monoxide from the hemo- 

 globin complex and makes breathing of carbon monoxide a very dan- 

 gerous matter. Hemoglobin also combines with carbon dioxide, and a 

 considerable part of the carbon dioxide contained in the blood is combined 

 with it. Hemoglobin is thus a carrier of gases both in going from the 

 lungs and in returning to them. 



If hemoglobin is exposed to mild oxidizing agents, for example, potassium 

 ferricyanide, the iron is converted from the ferrous state Fe ++ to the 

 ferric form Fe + + + . The resulting hemoglobin is called methemoglobin. 

 It can carry only one-half as much oxygen as hemoglobin and does not 

 readily release oxygen. 



The hemoglobins of different species differ in the globin part of the 

 molecule, but the heme part is the same in all. Note the difference in 

 crystalline structure of hemoglobins from different species (Figs. 5-1 to 

 5-4). 



A form of hemoglobin has recently been reported to occur in the 



