ENERGY TRANSFERS AND BIOLOGICAL OXIDATIONS 163 



2CH3CO— S— CoA -> CH:,COCHoCO— S— CoA + CoA— SH 



acetoacetyl coenzyme A 



where the energy ot one acetyl CoA has been used to make the new 

 carbon-carbon bond in the acetoacetyl coenzyme A. This substance is 

 a possible intermecHate in the synthesis of fats by many cells. A more 

 recent theory of fatty acid biosynthesfs postulates a reduction of one 

 acetyl coenzyme A molecule, then condensation of this product with 

 a reactant from the other acetyl coenzyme A (see page 458) to yield 

 /?-hydroxybutyryl coenzyme A plus coenzyme A. This mechanism 

 provides for reduction before condensation instead of the other way 

 around, as above when acetoacetyl coenzyme A is formed. Acetyl 

 coenzyme A is formed by the reversal of reactions like the above in 

 the metabolism of fats and by a reaction of pyruvate mentioned 

 earlier. 



CH3COCOO- + HS— CoA + DPN+ -^ 



CH3CO— S— CoA + CO2 + DPNH 



If not converted to fatty acids, the acetyl CoA may be converted to 

 citrate as shown on pages 171 and 450, and from citrate the energy 

 is recovered by later reactions. 



Note that DPN+ is reduced in the above reaction of pyruvate. 

 This reduction utilizes part of the energy available when pyruvate 

 is decarboxylated. Thereafter the DPNH formed (see page 140 for 

 the structures) is reoxidized and the energy transferred to other 

 substances. One of the most important steps is thought to be 



DPNH + HOPOa^ + ADP + H3O+ + FAD -^ 



DPN+ + ATP + FADH2 + H2O 



where this obviously very complex process provides another source 

 of ATP. Still other subsequent steps provide additional ATP from 

 ADP and HOPOy=, although little is known about these steps. The 

 structure of FAD (flaAine adenine dinucleotide) appears on page 172. 

 The foregoing illustrates how chemical energy may be transferred 

 and utilized for particular purposes. To some extent energy is stored 

 in these special substances, ATP, CH3CO — S — CoA, and DPNH. 

 However, only limited quantities of these compounds accumidate in 

 normal cells. Rather the energy is stored in other reserve forms like 

 fats, starch, or glycogen. In addition, some species store energy in 

 special forms. The vertebrates employ this reaction (shown on the 

 following page): 



