472 DIGESTION 



days. For this reason it is more difficult to demonstrate the presence of 

 proteinases in plant tissues than other enzymes and more difficult to study 

 their action even after suitable extracts have been prepared. 



Many plant enzymes are classified as proteinases which more properly 

 come under the heading of peptidases (Table 43). Following an older 

 terminology' such enzymes are often called erepsins, and have been found to be 

 of wide distribution in plants. 



Synthetic Action of Enzymes. — In the foregoing discussion our atten- 

 tion has been directed to the hydrolytic action of enzymes. Many and per- 

 haps all hydrolytic enzymes also catalyze the corresponding condensation re- 

 action. The role of enzymes in the synthetic reactions of plants should 

 be stressed, although it has been found much more difficult to study this 

 phase of their activity experimentally than their hydrolytic effects. Prob- 

 ably all of the condensation reactions involved in the synthesis of the many 

 types of carbohydrates, lipids, and proteins in plants proceed under the in- 

 fluence of enzymes. It is doubtful, however, if there is actually a specific 

 enzyme for every reaction which occurs in a living organism, as is sometimes 

 stated. However, that the key reactions occurring in any organism — those 

 which determine the metabolic pattern of that organism — proceed under the 

 controlling influence of enzyme catalysis seems beyond doubt. 



Factors Affecting Enzymatic Reactions. — The influence of various fac- 

 tors upon the reactions of the enzymes of the higher plants has been studied 

 almost entirely in vitro. The usual procedure has been to prepare a more 

 or less purified extract of the enzyme to be studied, then to bring this ex- 

 tract into contact with the substrate under controlled conditions, and finally 

 to determine the rate of the ensuing reaction. Enzymatic reactions as they 

 occur in living cells are influenced by conditions within the protoplasmic 

 matrix, and in this respect differ very greatly from such reactions occurring 

 in vitro. Since the conditions which influence the action of an enzyme in an 

 artificial medium are usually very different from those obtaining in a living 

 cell, it is certain that the effects of various factors on extracted enzymes 

 are quantitatively unlike those which they would have on that same enzyme 

 in vivo, although qualitatively the effects are undoubtedly very similar. 



I. Temperature. — The rate at which an enzymatic reaction will proceed 

 is influenced not only by the temperature, but also by the length of time 

 which the reaction mixture has already been at that temperature. In other 

 words, as in the process of photosynthesis, a "time factor" must be recognized 

 in considering the effects of temperature upon the rate of an enzymatic reac- 

 tion. In general the initial velocity of enzymatic reactions is accelerated 

 with increase in temperature until a certain "optimum" is attained. The 



