Molecidar Structure in Protoplasm 



67 



limit residue specificity; for example, if a hydroxyl group is needed 

 adjacent to an arginine residue ending, serine may be too short and 

 only a tyrosine residue would suffice to furnish the hydroxyl. 



Again, in some instances, a large specific area consisting of as 

 many as four to six specific residues properly placed may be required 

 on the surface in order to provide for not only the active prosthetic 

 group but also for the substrate molecule with which it reacts; for 



Fig. 14. Photographs of models showing dimensional relationship between: 

 (A) Ascorbic acid and adjacent amino acid residues; (B) Hexose diphosphate 

 and adjacent residues. 



example, in the case of the pyridine nucleotides (110) the protein 

 determines the kind of substrate molecule with which it may react. 

 A further instance, one in which the conception of a prosthetic 

 group attached to the face of a particle gives way to a more complex 

 structure, is seen in the enzyme, cytochrome. It has been said (111) 

 "the heme residue is linked in two different ways to the protein, viz., 

 by stable main valency bonds between the protein and one or two 

 vinyl side chains of the porphyrin, as well as by the usual coordinate 

 bonds between the central iron atom and some hemochromogen- 

 forming groupings in the protein component." To accommodate this 

 complex attachment the flat prosthetic group, shown previously in 

 Figure 11, would have to be partially enclosed by the protein chains 

 (129) . From such examples as these, where experimental evidence 



