18:3/ Enzymes: Kinetics of Oxidations 345 



O 



// 



(1) CH 3 CH 2 OH + 2 ^CH 3 C + H 2 



\ 



OH 



(2) CH 3 CH 2 OH + O -> CH 3 CHO + H 2 



(2') CH 3 CH 2 OH + 2A -> CH 3 CHO + 2.4H 



As far as the alcohol molecule is concerned, it cannot distinguish reactions 

 (2) and (2'). Thus, any removal of atomic hydrogen is called a biological 

 oxidation. 



Biological oxidations occur within a watery suspending medium 

 having an appreciable content of free protons (H 4- ). These can attach 

 to the oxidized compound if it leads to a stable form. In contrast, if 

 one removes an electron from a biological compound, an H + may 

 dissociate, leaving the compound oxidized. Thus, removal of an 

 electron is completely equivalent to the removal of a hydrogen atom. 

 All biological oxidations may be considered to be the removal of electrons. 

 For example, reaction (1) may be rewritten 



CH 3 CH 2 OH + 2 -^CH 3 C:H 2 OH + O" + O 



CH 3 C % H 2 OH -» CH 3 C— + 2H + 2H + + O - " ->- H 2 



\ 

 OH 



O 



// 

 CH 3 C— + O -> CH 3 C 



\ \ 



OH OH 



In a long chain of successive oxidations, one may consider each oxidation 

 to be the removal of an electron. Thus, in the oxidation of glucose, 

 electrons are transported from the glucose molecule to the oxygen mole- 

 cules through a series of steps. This electron transport is usually implied 

 by the term "biological oxidation." 



Another type of division of biological oxidations is in terms of the 

 type of molecule oxidized. For example, there are glucose oxidation 

 pathways, carbohydrate oxidations, fatty acid oxidations, amino acid 

 oxidations, and purine and pyrimidine oxidations. Most of these 

 eventually convert the molecule being oxidized to C0 2 , H 2 0, and some 

 nitrogen compound. In humans, the nitrogen of amino acids is con- 

 verted to urea, whereas purine metabolism stops with the formation of 

 uric acid which is then eliminated. 



All of the oxidations within the living cell serve two purposes. The 

 first is to convert the chemical energy of the molecules being oxidized 

 into a form useful to drive intracellular syntheses, muscular contractions, 



