Page:Astounding Science Fiction v54n06 (1955-02).djvu/132

hooked together in a chain to form a protein molecule is also known. Finally, in the case of many proteins, including hemoglobin, we know exactly how many of each amino acid the molecule contains. Most of the problem seems to be licked. The only thing left is to figure out the exact order in which the different amino acids occur along the protein chain.

To show what we mean, let's suppose we have a very small protein molecule made up of four different amino acids: a, b, c, and d. These four amino acids can be arranged in twenty-four different ways, as shown in Figure 5. Each arrangement represents a molecule with distinct properties of its own. The situation is then similar to that in the case of heme. Each of the twenty-four possible molecules can be synthesized and its properties compared with the natural product. One of the twenty-four must be the right one.

 

a-b-c-d- a-b-d-c- a-c-b-d- a-c-d-b- a-d-b-c- a-d-c-b-	b-a-c-d- b-a-d-c- b-c-a-d- b-c-d-a- b-d-a-c- b-d-c-a-	c-b-a-d- c-b-d-a- c-a-b-d- c-a-d-b- c-d-b-a- c-d-a-b-	d-b-c-a- d-b-a-c- d-c-b-a- d-c-a-b- d-a-b-c- d-a-c-b-
Figure 5. The different arrangements of four amino acids in a protein chain.

 

To be sure, hemoglobin has somewhat more than four amino acids in its molecule so the number of possible arrangements is to be expected to be somewhat more than twenty-four. Still, proteins are so important that biochemists would be willing to go to an unusual amount of effort to solve the problem of their structure and the mere presence of additional arrangements might not discourage them. Trial and error might be a little more tedious than in the case of heme, but, given time enough, it ought to be as sure as death and taxes.

Or should it?

To begin with, hemoglobin is a protein of only average size. Its molecule is made up of five hundred thirty-nine amino acids of exactly twenty different varieties and the number of each amino acid present is known. There is no need to name each amino acid. We can accomplish all that is necessary for our purposes by lettering them from a to t inclusive. There are seventy-five amino acids of type a present in the molecule, fifty-four of type b, fifty of type c and so on. One possible arrangement of the five hundred thirty-nine amino acids is shown in Figure 6.

Obviously the letters in Figure 6 can be written down in quite a few different arrangements and the reader may well shiver a bit at the thought of trying to write down all possible combinations and then counting them. Fortu-