[CITATION][C] The attractions of proteins for small molecules and ions

G Scatchard - Annals of the New York Academy of Sciences, 1949 - Wiley Online Library
G Scatchard
Annals of the New York Academy of Sciences, 1949Wiley Online Library
The number and variety of known compounrjs between proteins and small molecules are
increasing rapidly and make a fascinating story. For instance, there are the compounds of
iron, which is carried in our blood plasma by a globulin, two atoms of iron to each molecule
of globulin held in a rather tight salt-lie binding? which is stored as ferric hydroxide by ferritin
much as water is held by a sponge? and which functions in hemoglobin, four iron atoms in
tight porphyrin complexes in each protein molecule. Or, there are many compounds of …
The number and variety of known compounrjs between proteins and small molecules are increasing rapidly and make a fascinating story. For instance, there are the compounds of iron, which is carried in our blood plasma by a globulin, two atoms of iron to each molecule of globulin held in a rather tight salt-lie binding? which is stored as ferric hydroxide by ferritin much as water is held by a sponge? and which functions in hemoglobin, four iron atoms in tight porphyrin complexes in each protein molecule. Or, there are many compounds of serum albumin, which was used during the war by many chemists, most of whom found at least one 6ew compound. This molecule, which has about a hundred carboxyl radicals, each of which can take on a proton, and about the same number of ammonium radicals, each of which can dissociate a proton, has one single radical which combines with mercuric ion so firmly that two albumin molecules will share one mercury atom if there are not enough to go ar~ und.~ At the present stage of rapid growth of known compounds, it seems more profitable for me to make no attempt to catalogue the various classes of compounds, but to discuss the general principles involved, in the hope that this will make more useful the information which is accumulating so rapidy from so many laboratories.
We want to know of each molecule or ion whicb can combine with a protein molecule,/‘How many? How tightly? Where? Why?” The answer to the first two questions, and sometimes to the third, can be furnished by the physical chemist, but he will often need to team with an organic chemist to determine the effect of altering specified groups to find if they are reactive. The determination of function iç a complicated problem which may be the business of the physiologist or physiological chemist. But the answers to both of the more complicated problems will depend on the answers to the simpler questions,“HOW many?” and “How tightly bound?” If the various groups on a protein molecule act independently, we can apply the law of mass action as though each group were on a separate molecule, 4 and the strength of binding can be expressed as the constant for each group. Often, a single constant will express the behavior of severa1 groups. If the constants are widely spread, as those for the reaction of hydrogen ion with carboxylate ions, with imidazoles and with amines, the interpretation is simple. If the separation is less, it is very difficult to distinguish the case of different intrinsic affinities from the case of interaction among the groups. We know that such interaction occurs in simple moleculeç in which a reac-
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