POLYPEPTIDES AND PROTEINS 23I 



compounds, as polymers of amino acids having extremely 

 specific structures. As well as this we can to a certain, though 

 admittedly very limited, extent relate this structure to 

 enzymic and other biologically important properties of pro- 

 teins. This will enable us to understand their extremely great 

 significance in the metabolic process of life. Many organic 

 substances of different kinds entering into the composition of 

 living protoplasm can only readily take part in its metabolism 

 after they have interacted with the proteins of the proto- 

 plasm to form extremely active complexes (enzyme-substrate 

 complexes). In the absence of such interaction the chemical 

 reactions of which these substances are capable take place 

 too slowly at ordinary temperatures for them to have any 

 significance in the rapidly moving process of life. Hence the 

 metabolic course followed by any organic compound will 

 depend not only on the peculiarities of its molecular struc- 

 ture, its chemical potentialities, but also on the specific 

 enzymic activity of those proteins of the protoplasm with 

 which the compound is involved in the general metabolism. 



Thus, in proteins (enzymes) living material has both 

 powerful catalysts to accelerate chemical processes and an 

 internal chemical apparatus whereby these processes are 

 directed along completely determinate paths co-ordinated 

 with each other in a definite sequence and forming the 

 orderly arrangement of processes characteristic of metabol- 

 ism. On the basis of this organisation there also takes place, 

 in particular, the constant regeneration of proteins, their 

 self-reproduction, by virtue of which, to use Engels' words, 

 the protein body " while being the result of ordinary chemi- 

 cal processes, is distinguished from all others by being a self- 

 acting, permanent chemical process ".^ 



This presentation is, of course, radically different in prin- 

 ciple from those hypotheses formulated at the end of the 

 nineteenth century which identified protoplasm with pro- 

 teins and referred to the so-called ' living protein molecule '. 

 In these hypotheses, which were discussed more fully in 

 Chapter III of this book, some workers attempted to treat 

 protoplasm as a whole, as a single chemical substance, as a 

 gigantic protein molecule endowed with life (E. Pfliiger, 

 1875^; F. Bottazzi, 1911'; N. N. Iwanoff, 1925'°; H. G. 



