Sedoheptulose 1,7-bisphosphate

T) Ribulose 1,5-bisphosphate (2 Carbon dioxide (3) 3-Phosphoglycerate (4 1,3-Bisphosphoglycerate (5 Clyceraldehyde 3-phosphate Dihydroxyacetone phosphate (7) Fructose 1,6-bisphosphate Fructose 6-phosphate (9) Erythrose 4-phosphate Sedoheptulose 1,7-bisphosphate ( Sedoheptulose 7-phosphate Xylulose 5-phosphate Ribose 5-phosphate Ribulose 5-phosphate ( ) Glucose 6-phosphate... 

Beyond the classical glycolytic enzyme (See Aldolase), numerous enzymes catalyze aldol and aldol-like con-densation/cleavage reactions. Among them are the following (1) Sedoheptulose-l,7-bisphosphate aldolase, which converts sedoheptulose 1,7-bisphosphate to ery-throse 4-phosphate and dihydroxyacetone phosphate. 

Some of the many differences include the involvement of erythrose-4-phosphate and dihydroxyacetone phosphate in the production of sedoheptulose-1,7-bisphosphate, phosphorylation of ribulose-5-phos-phate to ribulose-l,5-bisphosphate, the addition of C02 to ribulose-l,5-bisphosphate, and the production of two glyceraldehyde-3-phosphates. 

Fig. 1. The reductive pentose phosphate cycle (RPP). The solid lines indicate reactions of the RPP cycle. The number of lines per arrow indicates the number of times each reaction occurs for one complete turn of the cycle in which three molecules of COj are converted to one molecule of G3P. Each reaction of the cycle occurs at least once. The double dashed lines indicate the principal reactions removing intermediate compounds of the cycle for biosynthesis. Abbreviations RuBP, ribulose 1,5-bis-phosphate PGA, 3-phosphoglycerate DPGA, 1,3-diphosphoglycerate, FBP, fructose 1,6-bisphos-phate F6P, fructose 6-phosphate SBP, sedoheptulose 1,7-bisphosphate S7P, sedoheptulose 7-phosphate Xu5P, xylulose 5-phosphate R5P, ribose 5-phosphate Ru5P, ribulose 5-phosphate TPP, thiamine pyrophosphate. From Ref. 1.

Variation on a theme. Sedoheptulose 1,7-bisphosphate is an intermediate in the Calvin cycle but not in the pentose phosphate pathway. What is the enzymatic basis of this difference ... 

Despite the fact that many heptoses are by far less prominent in Nature than hexoses these monosaccharides are found both as metabolic intermediates, and as structural carbohydrates of bacterial cell walls.D-Sedoheptulose 7-phosphate is an important intermediate of the pentose cycle, and D-sedoheptulose 1,7-bisphosphate is present in plants as an intermediate of the dark phase of photosynthetic reactions. L-Glycero-D-manno-heptose was isolated from the oligosaccharides obtained by partial acid hydrolysis of the lipopolysaccharide from Escherichia coli K-12 strain W3100 [153] and Haemophilus influenzae [154]. Both L-glycero-D-wtanno-heptose and D-glycero-D-ma o-heptose were isolated from the lipopolysaccharide of Vibrio parahaemolyticus [155]. 

D-Fructose 1,6-diphosphatase utilizes both D-fructose 1,6-bisphos-phate and sedoheptulose 1,7-bisphosphate as substrate, but the affinity for D-fructose 1,6-bisphosphate is the greater. However, D-fructose 1,6-diphosphatase isolated from Candida utilis380 or spinach chloroplast379 is specific for D-fructose 1,6-bisphosphate, and does not attack sedoheptulose 1,7-bisphosphate.379 380 The enzyme from C. utilis has an alkaline pH optimum (9.0-9.5), and a requirement for Mg2+ or Mn2+. At pH 7.5-8.0, the enzyme is inactive, unless EDTA is added.380... 

It is also an intermediate in the Calvin cycle, resulting from the action of sedoheptulose-1,7-bisphosphatase on sedoheptulose-1,7-bisphosphate. 

Erythrose-4-phosphate + Dihydroxyacetone phosphate <=> Sedoheptulose-1,7-Bisphosphate (catalyzed by aldolase)... 

In the event, as there is now one more carbon in erythrose 4-phosphate than in glyceraldehyde 3-phosphate, a seven-carbon sugar, sedoheptulose 1,7-bisphosphate,... 

Briefly, two of the five glyceraldehyde 3-phosphates are isomerized to glycerone phosphate, one of which reacts with a third glyceraldehyde 3-phosphate under the influence of fructose-bisphosphate aldolase (Section 11.2) to yield fructose 1,6-bis-phosphate which is dephosphorylated.to fructose 6-phosphate (Section 11.7). Transketolase catalyses a two-carbon unit transfer between fructose 6-phosphate and a fourth glyceraldehyde 3-phosphate to yield erythrose 4-phosphate and xylulose 5-phosphate. An aldol condensation of erythrose 4-phosphate with the second glycerone phosphate, catalyst by fructose-bisphosphate aldolase, produces sedoheptulose 1,7-bisphosphate which on dephosphorylation yields sedoheptulose 7-phos-phate. A second transketolase reaction utilizes sedoheptulose 7-phosphate and a flfth glyceraldehyde 3-phosphate to produce xylulose 5-phosphate and ribose 5-phosphate. The epimerization of both xylulose 5-phosphates and the isomerization of ribose 5-phosphate (Section 11.9) produces ribulose 5-phosphates which are phosphorylated to regenerate three ribulose 1,5-bisphosphate molecules. 

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