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Essays in Biochemistry- 



ground state) . 9 This absorption seems to be proportional to the num- 

 ber of peptide bonds per mole indicating little interaction between 

 them. The great energy of excitation makes it unlikely that these 

 states are used in the interaction of the substrate and the enzyme. 



In analogy with the crystalline solids, the electrons of the protein 

 molecule could move in a periodic field arising from the regular linear 



t 



Energy 



Fig. 1. Hypothetical energy levels of an enzyme. 



arrangement of the peptide bonds of the polypeptide chain. The 

 electrons in this excited state would be distributed in a large number 

 of discrete levels which taken together form an energy band (see 

 Fig. 1). The lower edge of this band could approach the ground state 

 rather closely. The width of this band and therefore of the minimum 

 excitation energy of its lower edge would depend on the number of 

 peptide bonds which cooperate, on the size of the protein. This band 

 is similar to that postulated to explain electrical conduction in semi- 

 conductors. In these materials, conduction is possible because of the 

 existence of a vacant electron band just above a filled band. It is 

 suggestive in this regard that electrical conduction in semiconductors 



