238 Essays in Biochemistry 



triplet state of the enzyme. The energy required for the promotion 

 of the two electrons is in part supplied by the bond which can now 

 form between the electron-deficient oxygen atom and a free electron 

 pair of a nitrogen atom of a peptide bond of the enzyme. This require- 

 ment for a lone pair of electrons of a trivalent nitrogen atom may 

 explain why proteins exist in nature rather than the analogous com- 

 pounds formed from lactic acid derivatives. It would be difficult to 

 form an analogous bond between two oxygen atoms. 



The carbon atom of the peptide bond of the substrate has by this 

 transfer of electrons become positively charged (electron deficient) . 

 The positively charged carbon atom then attracts by Coulombic forces 

 either a hydroxyl ion or a water molecule (see Fig. 2a). As the 

 hydroxy! (or water molecule) approaches, the C-N bond weakens. 

 Depending on the particular conditions the approaching group may 

 either be reflected, in which case nothing has happened, or the C-N 

 bond may progressively weaken as the OH~ group approaches the 

 positively charged carbon atom until the new OH - group has com- 

 pletely replaced the original amino group. It will be noted that this 

 mechanism postulates that the essential enzymatic step is the polariza- 

 tion of the carbonyl group. Depolarization and evaporation of the 

 substrate from the enzyme surface restores the system to its original 

 catalytic condition. Such a mechanism explains hydrolysis of peptide 

 bonds, transpeptidation, or O 18 exchanges in amino acids, 3 and a similar 

 mechanism can obviously explain other hydrolytic reactions. 



Some members of this class of enzymes require inorganic ions for 

 their action. In many cases these ions (Ca ++ , Mg++, etc.) are not 

 very good complexing agents. They may function by supplying a set 

 of energy levels which facilitate transfer of electrons from the substrate 

 to the triplet state of the active enzyme (see Fig. 1). Specificity 

 toward the metal required will be great since the energy levels of the 

 excited protein and the same excited levels of the hydrated metal ion 

 should be approximately equal. An analogous explanation could be 

 offered to explain the concentration of potassium within the living cell. 

 Some enzymes involved in the maintenance of the energy flow in the 

 living cell may have an excited state approximately the same distance 

 above the ground state of the protein as the 4p state of potassium is 

 above the filled 3p shell. Sodium will not substitute for potassium 

 since the energy difference between the corresponding states of sodium 

 (3p and 2p) is much greater than between those of potassium (4p 

 and 3p). 



