microns. (A millimicron is one-billionth of a meter; a meter is forty inches.) The bulk of this molecule is "globin" which, by itself, is an unstable protein. It makes up ninetyseven per cent of the whole. Attached to the globin, and rendering the whole more stable, are four iron-bearing groups called "heme" (see Figtire 1).
Figure 1. Schematic representation of hemoglobin molecule.
Hemoglobin can be split apart into a heme fraction and a globin fraction without very much difficulty, and the two can be studied separately. Heme, being simpler in construction and quite stable in addition, was naturally the more intensively investigated of the two.
The heme molecule is flat and approximately circular in shape. In the very center of heme is an iron atom. Surrounding that iron atom are twenty carbon atoms and four nitrogen atoms—plus some hydrogens—arranged in four small rings that are themselves connected into one big ring. This wheels within wheels arrangement occurs in numerous compounds other than heme—notably in chlorophyll—and is called the "porphyrin ring." Establishing the structure of the porphyrin ring itself took some fancy footwork, but was a relatively straightforward matter.
Now, however, there enters an additional refinement. There are eight points in the porphyrin ring where groups of atoms called "side-chains" can be, and are, attached. In the heme molecule, the eight side-chains are of three variaties: four of one kind, two
Figure 2. Schematic representation of heme molecule.
(Note: The positions available for side-chain attachment are numbered 1 to 8. The small rings which are themselves combined to form the porphyrin ring are numbered I to IV. The symbol Fe stands for the iron atom.)
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Astounding Science-Fiction
