THE NATURE OF ENZYMES AND SITE OF ENZYME ACTION 69 



species, at least, can form fat from carbohydrate (Stephenson and Whetham 

 1922, 1923). 



The Nature of Enzymes and Site of Enzyme Action. 



The elucidation and the nature of bacterial enzymes, their mode of action, 

 and even the identity of certain enzymes, which in our present state of know- 

 ledge are often little more than names attached to something catalysing a recog- 

 nizable reaction, must await the isolation of enzymes in a pure state. For the 

 most part enzymes appear to be proteins or closely associated with proteins. They 

 differ in their susceptibility to changes of hydrogen-ion concentration, salt concen- 

 tration, temperature, and to exposure to various chemicals. For example, Quastel 

 and Wooldridge (1927a, h) found that in general the dehydrogenases of Bad. coli 

 behaved in the same way when the bacterial suspensions were subjected to increasing 

 temperature, pH, exposure to nitrite, benzene, toluene, phenol, ether, chloroform 

 and propyl alcohol. The first affected were those acting on alanine, glycerol, 

 glycol, the sugars and glutamic acid ; next, those acting on lactic, succinic and 

 fumaric acid ; and finally the formic and acetic dehydrogenases. With strong 

 solutions of KCN and to a certain extent with HgOj, the picture was reversed, 

 the formic and acetic dehydrogenating systems being the least resistant. Extend- 

 ing these studies to non-toxic inhibitors, the authors (1928) found that the lactic 

 dehydrogenase of Bacf. coli was specifically inhibited by compounds having in 

 common the groups ■ — CO-COOH or ■ — CHOH-COOH, and the succinic dehydro- 

 genase by compounds with the groups =C-CHOH-COOH or =C-CH2C00H in 

 common. On these groups presumably depends the specific adsorption of the 

 compounds at that part of the enzymic surface responsible for the activation of the 

 lactic or the succinic acid, with a consequent inhibition of dehydrogenase 

 activity. 



The multiplicity of enzymic activities exhibited by a simple species — Bad. coli, 

 for instance, is capable of activating over fifty types of dehydrogenations, and 

 both micrococci and streptococci exhibit a wide variety of dehydrogenase activity 

 ■ — inspired the doubt as to whether each enzyme would prove to be a separate 

 chemical entity, on the grounds that the single small bacterial cells would not 

 be large enough to contain them (Grey 1924). The problem of accommodating a 

 large variety of enzymes in a bacterium is not, however, as difiicult as first appears. 

 To some extent the production of an enzyme in quantity is conditioned by the 

 presence of a substrate in the bacterial environment (see Chapter 9) ; so that 

 in the absence of the substrate it is necessary to postulate the presence only of a 

 few molecules of a given enzyme in order that the cell may exhibit the enzyme 

 activity when the substrate is added. The volume of an enzyme molecule, if 

 we take pepsin, with a molecular weight of about 37,000, as a model, will be of 

 the order of 20 to 30 mju^. The volume of an average cell of Bact. coli is about 

 700 X 10^ mju^. Even if we assume a major part of this volume is occupied by 

 non-enzymic material, there is ample room for several thousand enzyme molecules. 



In some cases, however, the multiplicity of enzymes may be more apparent 

 than real. Bernheim, Bernheim and Webster (1935) observed that suspensions 

 of Proteus vulgaris were able to oxidize practically all the known "natural" 

 (Z-form) amino-acids. Stumpf and Green (1944) found that this activity was 

 due to a number of enzymes, all but one of which were relatively unstable, and 

 disappeared with ageing of the suspension. The remaining stable enzyme, which 

 they also found in Bad. aerogenes and Ps. pyocyanea, had nevertheless a wide 



