On the Bigness of Enzymes 23") 



some substitute such as phosphoric acid) to a molecule or those involv- 

 ing an oxidoreduction. 



All the enzymes involved in the latter group of reactions seem to 

 be composed of a metal-containing prosthetic group attached to a 

 protein. Many samples are known composed of an iron porphyrin 

 attached to a protein. Four of the six bonds of the octahedral con- 

 figuration of the iron atom are firmly bound to the porphyrin; the fifth 

 bond is used to bind the iron porphyrin to the specific protein; the 

 sixth one can be loosely bonded to a water molecule or to the substrate. 

 The role of iron and of the proteins in such enzymes, although unknown 

 in detail, is obviously related to the transfer of electrons from the 

 substrate to the enzyme; the role of the porphyrin is not so obvious. 

 It is possible that the geometry of the iron-porphyrin complex is impor- 

 tant chemically. When the iron atom complexes with the planar tetra- 

 valent porphyrin molecule, of necessity it leaves two of the six valences 

 free. Since the valences of the iron atom are directed to the six vertices 

 of an octahedron the two free valences are on the opposite sides of the 

 plane containing the iron atom and the porphyrin. It is well known 

 that bifunctional molecules complex more strongly with iron than 

 monofunctional molecules. Were there two adjacent free spaces in the 

 octahedral structure of the iron atom, it would rapidly be filled by the 

 bifunctional amino acids always present in the cells. Because of this 

 particular structure it is not possible for the hematoporphyrin enzymes 

 to be inhibited by an amino acid. For the same reason it is likely 

 that the copper enzymes are so constructed that there is not more than 

 one space in the valence shell of copper for combination with its 

 substrate. 



An analogy between metal catalysts of the transition groups with 

 enzymes is not possible, since enzymes have neither vacant d orbitals 

 nor do they show the phenomena resulting from such partially filled 

 shells. Nevertheless it seems unlikely that there is a complete di- 

 chotomy in the kind of mechanisms that are employed for catalysis 

 in the living and the non-living world. 



All enzymes are characterized by their largeness and their protein 

 nature. Despite a thorough search no example of an enzyme has been 

 found which is not a protein or a large molecule. The backbone of 

 the proteins is a series of repeating peptide bonds separated from each 

 other by the CHR groups. These groups destroy the possibility of 

 conjugation of the double bonds of the peptides. There is evidence 

 that the electrons of the peptide bond can be raised to an excited 

 state by absorption of light of 1850 A. (154,000 cal./mole above the 



