BIOCHEMICAL REACTIONS AND THEIR CATALYSTS 121 



alteration in the appearance of the gel. The associated complex of enzymes 

 is believed to be localized in certain morphological structures occurring 

 in cytoplasm, the mitochondria. 26, 27 



Most of the individual reactions utilized in the synthesis of the units 

 for cell structure and in the production of energy are only component 

 parts of some process involving a large number of reactions. Usually the 

 intermediate compounds formed by the reactions are utilized immediately. 

 In many instances the concentration of these intermediates under normal 

 conditions is so low that it is difficult to demonstrate their existence ; also 

 some of the processes in which such intermediates occur are known to 

 proceed very rapidly. The simplest explanation to account for the rapidity 

 of a long process, when the concentration of the reactants for a number 

 of the individual reactions is so low, is that the enzymes are in some way 

 associated together in the cell. 



There probably are also special means by which labile intermediates 

 and by-products arising in one part of the cell can be transported to 

 another. If enzyme systems containing cofactors that are easily dissoci- 

 able are used, it is then possible that a reaction can be carried on in one 

 part of the cell to furnish chemical intermediates or energy for utilization 

 elsewhere within the cell. For example, adenosine diphosphate, the co- 

 enzyme for most reactions in which a high-energy phosphate bond is 

 generated, is converted in these reactions to adenosine triphosphate. This 

 form of the coenzyme can dissociate and is free to transport its energy 

 "unit" to the other parts of the cell where it can be utilized. Hydrogen 

 atoms arising from the numerous dehydrogenations of cellular reactions 

 can be transported to other parts of the cell by dissociable coenzymes. 

 It may be found that several of the highly reactive labile substances like 

 the activated acetate derivative and the single carbon units are trans- 

 ported from one part of the cell to another by the coenzymes of panto- 

 thenic acid and p-aminobenzoic acid. Thus the complete series of enzymes 

 needed for a particular cell function need not all be located exactly at the 

 site of the function. 



One of the present goals of enzyme chemists is the establishment of 

 the exact location of the enzyme systems within cells. Some information 

 has already been obtained by carrying out vitamin and enzyme analyses 

 on the fractions obtained by separating cells into their gross components. 

 Any procedure, however, that disrupts cell organization even slightly will 

 not give all the information needed to explain what actually takes place 

 within normal cells. 



Hence, now that the relationship between most of the vitamins and 

 their chemical functions has been established, many investigators are 

 seriously engaged in an equally challenging study — that of associating 



