188 THE BIOCHEMISTRY OF B VITAMINS 



bonds are established. The condensations carried out by biological sys- 

 tems resemble the reactions which are commonly employed by organic 

 chemists, and many of the limitations concerning the types of reactive 

 compounds and the manner in which they unite are common both to 

 laboratory syntheses and to those occurring intracellularly. 



The earliest enzymatic condensation to be recognized was the one in 

 which hexoses are formed from trioses by a typical aldol condensation: 



HHH OH OH HHHOH 



HC— C — C + HC — C— CH =f=^ HC— C — C — C— C— CH 



Hh b H i> Hh Ah A A i 



HO— P— OH HO— P— OH HO— P— OH HO— P— OH 



O OOO 



phosphoaldo- phosphoketo- diphosphoketohexose 



triose triose 



In this reaction, the condensation takes place between a molecule con- 

 taining a carbonyl group and one containing an active hydrogen atom, 

 i.e., one bonded to a carbon atom alpha to a carbonyl group. This type of 

 reaction (aldol condensation) occurs readily in vitro with alkali as a 

 catalyst; the enzyme (aldolase) which mediates the reaction pictured 

 above has never been resolved into dissociable components, and is believed 

 to contain no recognized vitamin. The reaction is one of the essential 

 steps in the general process by which most organisms metabolize hexoses 

 or synthesize them from metabolic products. The isomerization of the 

 phosphate ester of the aldotriose to produce the corresponding ketose is 

 an essential reaction that must precede the condensation depicted, for the 

 condensation of two aldotrioses would give a branched chain hexose. Like- 

 wise, the isomerization of the glucose diphosphate to the corresponding 

 fructose ester must occur prior to the cleavage, since an aldose cleavage 

 of glucose could only occur between the a- and /?-carbon atoms, producing 

 a biose and tetrose. 



A second type of condensation often used by organic chemists is the 

 so-called Claisen type, a condensation which involves (1) the carbonyl 

 group of an acid anhydride or ester and (2) a carbon and a hydrogen 

 atom alpha to a carbonyl group. The enzymatic condensations by which 

 fatty acids, sterols, amino acid precursors, and probably several other 

 important products are elaborated are of this type. 



In Claisen type condensations in biological systems one of the reacting 

 molecules is usually (if not always) an acyl phosphate, the mixed acid 

 anhydride of phosphoric acid and an organic acid. In most reactions, it is 



