3] CONFIGURATION OF GLOBULAR PROTEINS 55 



side (above pH 10) is sufficiently steep to suggest that a similar change in 

 configuration occurs there. 



A detailed study of the kinetics of the acid reaction, in both directions, 

 has been made by Steinhardt and Zaiser.^"'^^^-^^^'^^'^ Combination of the 

 data establishes the pH-dependence of the equilibrium between the compact 

 and expanded form. This information is more complete than similar informa- 

 tion for any other protein, although it is unfortunate, in view of the probable 

 importance of electrostatic forces as a driving force of the reaction, that 

 all experimental studies have been performed at a single ionic strength. Both 

 the equilibrium and the rate constant for formation of the expanded con- 

 figuration depend strongly on pH, the former being roughly proportional 

 to (H+)^, the latter to (H"'")^"^. This marked dependence on pH has been 

 used as basis for the hypothesis that a number of critical hydrogen bonds 

 must be ruptured to initiate the reaction. This hypothesis, however, is entirely 

 speculative. As Kauzmann^^ has shown, kinetic behavior similar to that 

 observed for hemoglobin will result if the reaction depends solely on an 

 increase in electrosiaiic interaction energy consequent upon a decrease in 

 pH. Kauzmanns argument applies with equal force to the equilibrium of 

 the react on. 



Conalbumin. Conalbumin undergoes a major change in configuration below 

 pH 4,^°^ which results in loss of solubility under conditions where the original 

 form of the molecule is soluble. That the product of the reaction is of the 

 simple polyelectrolyte type is shown by the viscosity and sedimentation data 

 of Phelps and Cann.^^ Whereas the compact form has an intrinsic viscosity 

 of 3-8 cc./gram., the intrinsic viscosity, after the change in configuration, 

 is much larger and strongly dependent on ionic strength. At pH 3-0, [17] = 

 16-0, 11-0 and 8-4 cc./gram, respectively, at ionic strengths 0-02, 0-07 and 

 0-10. The sedimentation coefficient decreases with increasing charge, as is 

 shown by Fig. 11, and the decrease is partly suppressed by ionic strength. 



On the alkaline side of the isoelectric point, near pH 10-5, there is a 

 break in the logarithmic plot of titration data for phenolic and amino 

 groups, ^*^^ which is much like the similar break shown for serum albumin 

 in Fig. 9. The net charge at this point is about the same as the net charge 

 at which the acid reaction begins. 



Wishnia^"^ has made a detailed study of the kinetics of the acid reaction, 

 both in the forward and reverse direction. An interesting feature is that 

 the reverse reaction shows a pH maximum. 



Pepsin. Pepsin undergoes irreversible loss of its enzymatic activity between 

 pH 6 and 7.'^^ Edelhoch^^ has studied the molecular change which accom- 

 panies this reaction. It appears that a small part of the molecule is split off. 

 The remaining portion clearly shows polyelectrolyte-like behavior. At pH 6, 

 the value of -qspjc for a dilute solution (i.e. essentially the intrinsic viscosity) 



