THE CATALYST ENTELECHY IN DIFFERENTIATION 199 



Classes (a) and (b) have similar iron-containing prosthetic groups, 

 and can take up one molecule of oxygen for each iron atom. In class 

 (c) the prosthetic group contains copper, and these proteins can take 

 up one molecule of oxygen to two atoms of copper. In class (d) the 

 prosthetic group also contains iron, but the oxygen-binding capacity is 

 one oxygen molecule to three atoms of iron. The cellular respiratory 

 proteins cytochrome-C and myoglobin both have iron in their pros- 

 thetic group and have about the same molecular weight (17,000) one- 

 fourth that of hemoglobin (68,000) which has four iron atoms per 

 molecule, and is found in the blood of all vertebrates except the lowest 

 class, the Cyclostomata. Mammalian hemoglobin dissociates reversibly 

 upon addition of certain amino compounds, e.g., urea, acetamide, 

 formamide. 



"The hemocyanins form an interesting class with a number of inner 

 connections. The molecular weights of the hemocyanin molecules 

 found in the blood of certain species are always simple multiples of 

 the lowest well defined component. Thus, for the Malacostraca the 

 relationship is 1:2 and for the Gastropoda 2:8:16:24. Moreover, the 

 weights of all the well-defined hemocyanin molecules seem to be simple 

 multiples of the lowest among them. In most cases the hemocyanin 

 components of certain species are interconnected by reversible pH- 

 influenced dissociation-aggregation reactions. At certain pH values 

 a profound change in the number and percentage of the components 

 takes place. The shift in pH necessary to bring about reaction is not 

 more than a few tenths of a unit. Consequently, the forces holding 

 dissociable parts of the molecule together must be very feeble. 



"Not only the molecular weights of the hemocyanins but also the 

 mass of most protein molecules — even those belonging to chemically 

 different substances — show a similar relationship. This remarkable 

 regularity points to a common plan for building up protein molecules. 

 Certain amino acids may be exchanged for others, and this may cause 

 slight deviations from the rule of simple multiples; but on the whole, 

 only a very limited number of masses seems to be possible. Probably 

 the protein molecule is built up by successive aggregations of definite 

 units, but only a few aggregates are stable. The higher the molecular 

 weight the fewer are the possibilities of stable aggregation. The steps 

 between the existing molecules, therefore, become larger and larger 

 as the weight increases." 



All this indicates how delicate is the physico-chemical mech- 

 anism controlling the particle size of proteins, which may affect 

 catalyst formation, and thereby determine the pH and the forma- 

 tion or deflocculative liberation of specific substances that may 

 exert profound effects on protein structure and function, e.g., as 



