ENZYMK REACTIONS AT SURI-ACliS 



27 



Table i. Valuf.s of A pH = pHi, — pHs iok somi colloidal and uiological surfaces in 



AQUEOUS SVSTIMS AT UOOM 1 IM I> IK AIL R1-; 

 SURFACE 



L>sozyme 



Kaolinite 



Lysozyme on kaolinite 



Pseiidomonas iicnigiuosa 



Yeast 



Erythrocytes 



Mitochondria (kidney) 



Roots 



* The ionic strength i is of fundamcnta 

 They are lower the higher the ionic strength. 



The value ApH has been found to be virtually identical for ovalbumin between 

 pH 3 and 6 by either method of calculation. This is remarkable when one re- 

 members that the first approach requires an assumption about 8 ( based on Debye- 

 Hiickel theory) and the results of the second approach give the surface pH essen- 

 tially at the plane of shear, and this plane inay be several Angstroms away from 

 the surface ionogenic groups (9). In solutions between o.i n and o.ooi n, 8 varies 

 from about 10 A to 100 A, so the attack on a substrate molecule by an enzyme of 

 molecular weight of about 25,000 most certainly involves an environment char- 

 acterized more by pHs than by pHj, in such solutions.^ 



100 



>- 



80 



Fig. 2. Effect of pH on invertase activity of 



yeast cells and isolated enzyme [1,2] and on 



chymotryptic activity in solution or adsorbed on 

 kaolinite [3,4]. (Refs. 29, 48.) 



< 



> 



I- 

 < 



LJ 40 



20. 



1,3= in vitro 

 2 = in VIVO 

 4 = on Koolinite 



2 4 6 8 10 



pH of SOLUTION or SUSPENSION 



A dramatic example of the influence of pH^ on the hydrolysis of lysozyme 

 (denatured) is shown in figure 2. A comparison of the action of chymotrypsin on 

 lysozyme in solution and on the surface of kaolinite particles (about i /x in size) 

 in suspension reveals that the pHb of half maximum activity for the suspension is 

 shifted about two units toward higher pH, showing that chymotrypsin behaves in 



3 The protein error of indicators has been semiquantitati\cly handled trom this point of 

 view (8). 



