ENZYME REACTIONS AT SURFACKS 33 



enzyme, in and about the substrate structure. Very little seems to be known about 

 these phenomena. The action of chymotrypsin on substrate adsorbed on kaolinite 

 was shown to involve the preliminary formation of a reactive enzyme-substrate- 

 kaolinite complex, and proteolysis of the adsorbed complex occurs at a rate com- 

 parable to the action of chymotrypsin on the same substrate in solution. It would 

 seem that the enzyme is able to move about on the face of the adsorbant at a rate 

 comparable to that in solution. Addition of substrate to chymotrypsin previously 

 adsorbed on kaolinite resulted in a slower reaction rate, however, showing that 

 structural details are important. Trurnit(46) studied the proteolytic activity of 

 chymotrypsin on adsorbed serum albumin at a solid-liquid interface and found 

 that the reaction velocity increased with increasing substrate thickness. He also 

 concluded that the various theories developed for the kinetics of enzyme reactions 

 in solution could not be applied to a system where one of two reaction partners is 

 in a solid phase. By introducing diffusion as the rate-limiting factor it was possi- 

 ble to derive equations which described the initial phase of adsorption and reac- 

 tion. Experitnentally, enzyme reactions at surfaces or in gels may not go to com- 

 pletion for structural reasons; these observations have so far been given ad hoc 

 explanations (31, 36). 



Mazia and Hayashi (35, 36) found albumin fibers were hydrolyzed by added 

 pepsin in solution faster than albumin in solution, and that 20:1 albumin-pepsin 

 libers were hydrolyzed extremely rapidly. Since it does not seem likely that a 

 pepsin molecule can be in contact with 20 equal-in-size substrate molecules in a 

 fiber, this rapid rate is not easily explained within the framework of contemporary 

 conceptual schemes. It has also been established that the enzyme activity of a 

 trypsin ergosterol complex is higher than that of pure trypsin (39), where a 

 catalase-cellulose derivative is less active than the soluble enzyme (38). Taken all 

 in all, we may infer that the amount of an enzyme in a structured system does 

 not alone dictate the reaction kinetics, since the rate will depend on the structural 

 restrictions and on the relative amounts of enzyme and substrate (36, 46). 



Finally, unlike adsorption of a globular protein at a solid-liquid interface (28), 

 adsorption at an oil-water interface involves a drastic reorientation of polypeptide 

 chains, with the breaking of many intramolecular bonds (15). Presumably a rigid 

 structure as one moiety of an interface prevents tangential motion of the interface 

 as adsorption takes place. The zeta potential of an enzyme at an oil-water inter- 

 face may be 2-3 times greater than in solution (11) and, through an influence on 

 interfacial pH, can contribute to apparent differences on activities of enzymes in 

 solution and at these interfaces (15). The situation is enormously complicated 

 when more than one interfacial layer of adsorbed enzyme is present. 



SUMMARY 



Many of the enzyme reactions in nature take place on surfaces or in ordered 

 structures. At interfaces the 'concentrations" of reactants generally differ from 



