ENZYME REACTIONS AT SURFACES 3 1 



In this connection it is ot interest to note that the formation of new cell wall 

 material must be confined to the plasma surface, and hence enzyme systeins in- 

 volved in carbohydrate metabolism are present in the surface layer. 



SOME REMARKS ON INTRACELLULAR ENZYMES 



Enzymes in cells are probably associated for the most part with specialized 

 structures, the inner cell membranes, reticular networks (giving rise to microsomes 

 by rupture), mitochondria, etc. (21). If this were not the case, it is difficult to see 

 how reaction sequences could be controlled (43). The state of the enzyme in or 

 on such a particle may depend on the available hydrions near by and on the 

 orientation of the enzyme. The orientation with respect to adjacent protein 

 molecules inay determine the amount of intermolecular protein bonding which 

 exists and hence the H^ + enzyme equilibria. Huennekens has compared the pH 

 optimum of coniugated (pH 7-8) and dissociated (pH 9.5) malic oxidase still 

 attached to particles of the cyclophorase system (23), and found a difference 

 which may depend in part on this factor (see also Dickman and Speyer(io)). 

 Also, the degree of swelling of mitochondria influences cytochrome activity (43). 



In very small intact cells, such as those of bacteria, the concept of pH is without 

 useful meaning (14). Thimann cites the interesting example that a 0.5 /x diameter 

 microbial cell with a continuum of pH throughout the cell and its surroundings 

 of pH = 7 would only have room for 3.6 hydrions. What aliout the chances for 

 such a free ion in a similar cell at pH 8! Under such circumstances, with the ac- 

 tivity of an enzyme depending on the dissociation of say i H"^ per molecule, the 

 observed dissociation may actually involve a switch of i H"^ from one protein 

 molecule to another, or displacement by another cation. For example, in the 

 utilization of potassium by B. lactis aerogenes, potassium appears to play the part 

 of an enzyme-activator. Eddy and Hinshelwood based a quantitative treatment of 

 the competition between K^ and H^ for an array of negative sites on an enzyme 

 surface, on the assumption that a certain critical area of K^-activated sites is 

 necessary for growth to continue (12). 



In spite of the fact that the idea of a pH inside a bacterial cell is nebulous, one 

 must ultiinately explain differences among bacteria. For example. Gale (17) has 

 suggested that the difference in pH^ optima for amino acid decarboxylases acting 

 in bulk and in intact washed cells arises because the value of pHb for cells is 

 necessary in order to have an intracellular pH equal to the true pH of optimum 

 activity. By contrast. Few ct al. (13) believe that there is a constancy of pH at 

 the site of the intracellular catalase in M. lysoddkj.icus despite changes in pH of 

 the external medium. 



For cells the size of yeast, intracellular pH (pHj) begins to take on some mean- 

 ing. The pHi of ( resting) S. ccrcvisiac is 5.8 as a whole and the buffering power 

 of the cell is considerable. Thus the enzyme system fermenting glucose is nearly 



