On the Bigness of Enzymes 



237 



is enhanced by addition of impurities (prosthetic groups) which supply 

 unfilled electron levels used to facilitate electron flow. Similar phe- 

 nomena exist in the photoconduction of MoS 2 ; a compound known to 

 have catalytic activity. 



This excited state must be one which cannot be observed by absorp- 

 tion spectroscopy, since proteins do not absorb light above 1850 A. 

 except for that absorption due to the R groups. If the excited state 



h :o : 



H 



(a) 



(b) 



Fig. 2. Interaction of substrate with enzyme. 



were a triplet one, it should not give rise to an absorption band, since 

 transitions from the singlet ground state to the excited triplet by light 

 absorption would be forbidden by the usual transition rules. 



Electrons in this excited state would not be associated with any 

 particular peptide bond, since they would be distributed over all the 

 peptide bonds. Their average contribution to the electron density of 

 any peptide bond would be very low. On this hypothesis, as a suitable 

 substrate containing a peptide bond approaches a proteolytic enzyme 

 the Coulombic and van der Waals forces would bring the substrate 

 up in a suitable position relative to the enzyme. In this position, as 

 the carbonyl group of the peptide bond approaches the enzyme with 

 the C=0 bond pointing toward the enzyme (see Fig. 2), two of the 

 7T electrons of the carbonyl group are transferred to the low-lying 



