100 



I. M. KLOTZ 



was shown before with zinc (Fig. 5), and you will notice that even at pH 5 

 the extent of binding is decreasing. If we take a metal such as cadmium, which 

 is known to have a lesser afhnity for hydroxyl than either zinc or copper, the 

 photometric titration curve continues to go up until pH 9 and then it drops 

 precipitously. So specific effects result when these metals are present. The zinc 

 would act very differently in an experiment, however, with a zinc enzyme at 

 pH 7. The zinc would have unusual properties, compared to the other two 

 metals which I have chosen as examples, which are really due to its hydrolytic 

 equilibria or to its interaction with the solvent. In other words, we have spe- 

 cific effects here which are essentially due to the environment rather than to the 

 protein itself. 



There is something else in this curve (Fig. 9), though, which also shows a 

 certain specificity which some of you will surely notice. In the zinc curve (Fig. 

 5) after the rise, once the drop came in, there was a continuous drop. In contrast 

 in the case of copper we have here (Fig. 9) a plateau near pH 9 and then a sub- 

 sequent second drop which is somewhat puzzling until you examine the struc- 

 ture of this complex schematically and in more detail. Let us turn to the next 

 illustration (Fig. 10). 



I have assumed, because my reference point is so strongly colored toward 

 the dye, that the OH could go in only at the Me — D bond (top. Fig. 10) 

 but, of course, if you look at the full structure you really have to ask the ques- 

 tion: Where will the OH go in? It can combine with the metal either by dis- 

 placing the metal-protein bond or by displacing the metal-dye bond. I think 

 it is pretty clear that which place it will go depends upon which bond is the 

 weaker. It will go to the weaker one. 



If we look at some of the data which we have for zinc and copper, evidently 



P— Me — D 



T ?t 



0H~ OH" 



pULJOlCu-^^^^D 



OH- 



P + Cu-D 

 I 

 OH 



0H~ 

 Cu(0H)2 +D 



p4ii03_ 2^02,0010 



P-Zn 

 OH 



OH- 

 + D 



Fig. 10 



