Carbohydrates in Aromatic Compound Biosynthesis 267 



The high conversion of SDP to DHS cannot be due to direct cycliza- 

 tion of the seven-carbon chain of SDP. Carbon atoms 7, 1, and 2 of 

 SA were shown to arise from G- (3, 4) - (2, 5) - (1, 6) , respectively. Such 

 a sequence of glucose carbon atoms in carbons 1, 2, and 3 of heptose 

 has never been observed. The reverse order, i.e., G- (1, 6) - (2, 5) - (3, 4) , 

 would be expected in carbon atoms 1, 2, and 3 of heptose. 12 ' 20 



Since S-2, 1, 7 are derived from a three-carbon product of glycolysis, 

 but not pyruvate, it was surmised that carbons 1, 2, and 3 of SDP 

 are converted to phosphoenolpyruvate and then condensed with tetrose 

 phosphate to a seven-carbon precursor of DHS. In support of this 

 scheme, it was found (Table 1) that the conversion of SDP to DHS 

 was completely inhibited by fluoride and iodoacetate but that it was 

 completely restored by phosphoenolpyruvate. 22 Moreover, synthetic 

 D-erythrose-4-phosphate 23 and phosphoenolpyruvate were almost quan- 

 titatively converted to DHS (Table 1). The efficient conversion of 

 SDP to DHS may therefore be explained on the basis of the following 

 reactions: 



SDP ^ D-Erythrose-4-phosphate + Dihydroxyacetone phosphate 20 



Dihydroxyacetone phosphate ^ Phosphoenolpyruvate 



D-Erythrose-4-phosphate + Phosphoenolpyruvate — » — > DHS 



Although the details of the reactions are still unknown, it may be 

 postulated at present that the pathway from glucose to DHS is in part 

 as shown in the diagram below. 



ch 2 op 



I 



c=o 



HOCH 



Glycolysis 



CHOH 



I 

 CHOH 



I 

 CH2OP 



CH2OH 



I 



c=o 



HOCH 



CHOH 

 I 



CHOH 

 I 

 CH2OP 



Reactions 

 1 to 5 



Figs. 3 

 and 4 



COOH 

 I 

 C— O— P 



II 



CH 2 



CHO 



CHOH 

 I 



CHOH 

 I 

 CH 2 OP 



D-Erythrose- 

 4-phosphate 



OH 



It is of interest that several classes of alicyclic compounds, e.g., the 

 carotenoids and steroids, in the biosynthesis of which an isoprene unit 

 appears to be involved, are derived from acetate. On the other hand, 



