every other arrangement. Only one of the fifteen is the arrangement found in heme.
Which one?
A German chemist called Fisher was faced with that problem and he solved it in the most straightforward possible manner. He wrote down the fifteen possible arrangements on pieces of paper, numbering them arbitrarily from one to fifteen. He then, in effect, called out his sixty graduate students, marshaled them into platoons of four apiece, and gave each platoon one of the arrangements. Instructions were for each to synthesize the protoporphyrin with the particular arrangement pictured.
Figure 4. Side-chain arrangement in protoporphyrin IX.
The students got to work. As each protoporphyrin was formed, its properties were compared with those of the natural protoporphyrin obtained from hemoglobin. It turned out that only one of the synthetic protoporphyrins matched the natural product. It was the one that Fisher had happened to assign the number 9, and it has the side-chain arrangement shown in Figure 4. Since then, generations of medical students and biochemists have memorized the formula of the natural product and learned to call it "Protoporphyrin IX." (It is my personal experience that few students show any curiosity at all as to why the IX.)
Score a tremendous victory for pure trial and error!
Now let's tackle the globin portion of the hemoglobin molecule. Globin is, as has been said, protein in nature, and proteins are by far the most important chemicals in living tissue. There is no question but that most or all of the secrets of life lie hidden in the details of protein structure. A biochemist who could learn the exact structure of some protein would be an awfully happy biochemist. So let's get some notion as to what it would take to achieve that desirable end.
All protein molecules are made up of relatively small compounds called "amino acids," which are strung together in the molecule like beads on a string. There are about twenty different amino acids occurring in proteins and the structure of each one of them is exactly known. Furthermore, the
exact manner in which amino acids are
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Astounding Science-Fiction
