MACROMOLECULES 123 



isolated from yeast and other types of cell. The "old yellow enzyme" 

 plays a catalytic role in the oxidation of glucose-6-phosphate. Another 

 macromolecule of the same type is the "new yellow enzyme" in which the 

 flavin is flavin-adenine dinucleotide (FAD), and which plays a similar 

 catalytic role to the "old yellow enz}'me", the two being present in the 

 same cells. Corresponding to the flavoproteins there is a whole series of 

 metalloflavoproteins resulting from the chelation of copper, iron or 

 molybdenum. 



B. Metalloproteins in which the Metal is Bound Directly to 



THE Protein 



In proteins, the carboxyl and amino groups are, for the most part, 

 combined in peptide linkages, so that it is chiefly the polar side chains 

 which form complexes with metals. If we refer back to Fig. 13, we have a 

 list of side chains of this type. They are those whose pK is below 10. 

 Those groups whose pK is above 10 are such strong bases that they cannot 

 form bonds with metals. 



Most protein molecules, then, have many points where complexes may 

 be formed with metals. Among these metals it is worthwhile to distinguish 

 those which appear to be coordinated strongly and by many diflferent polar 

 side-chains : such are mercury, silver, copper and zinc. The alkaline earth 

 metals, like calcium, seem to be bound primarily by free carboxyl groups, 

 or, in the phosphoproteins such as casein, by phosphate groups. 



The most abundant metals in the biosphere (Na, K, Ca, Mg), which are 

 bound strongly to proteins, are those whose internal electronic levels are 

 full whilst the external ones are not. They have the very stable electronic 

 structure found in the rare gases. They therefore bind to functional groups 

 by electrostatic attraction whilst the transition elements tend to use their 

 incompletely filled inner orbits to form covalent bonds. 



(a) Proteins Binding Copper 



As in the case of the association of a haem and a protein, the nature of 

 which association controls the diflFerent properties and functions of the 

 macromolecule, macromolecules of proteins and copper diff^er from each 

 other, both in their properties and in their functions. Examples of this 

 type of association are the phenolases and the haemocyanins. 



1. Phenolase {phenol oxidase) 



This molecule of protein and copper has a double function, as a catalyst 

 in the o-hydroxylation of phenols and in the dehydrogenation of 

 o-diphenols. 



