Triose phosphates, acylation

TRIACYLGLYCEROLS PHOSPHOGLYCEROLS ARE FORMED BY ACYLATION OF TRIOSE PHOSPHATES... 

However, (26) should bind strongly to class II aldolases on account of its chelating ability and this has been found to be the case with rabbit muscle fructose diphosphate aldolase where (26) functions as a competitive inhibitor. Triose phosphate isomerase is also strongly inhibited by (26) this may be due to its similarity to the cw-eriediolate (28), which is an intermediate in the reaction pathway of this enzyme. Acyldihydroxyacetone phosphates are important intermediates in the biosynthesis of glycerolipids and the acyl... 

The glycolytic pathway includes three such reactions glucose 6-phosphate isomer-ase (1,2-proton transfer), triose phosphate isomerase (1,2-proton transfer), and eno-lase (yS-elimination/dehydration). The tricarboxylic acid cycle includes four citrate synthase (Claisen condensation), aconitase (j5-elimination/dehydration followed by yS-addition/hydration), succinate dehydrogenase (hydride transfer initiated by a-proton abstraction), and fumarase (j5-elimination/dehydration). Many more reactions are found in diverse catabolic and anabolic pathways. Some enzyme-catalyzed proton abstraction reactions are facilitated by organic cofactors, e.g., pyridoxal phosphate-dependent enzymes such as amino acid racemases and transaminases and flavin cofactor-dependent enzymes such as acyl-C-A dehydrogenases others. 

Reactions of Triose Phosphate Dehydrogenase. This enzyme is not specific in its reaction with phosphoglyceraldehyde. The nonphospho-rylated compound is oxidized also, but only at 0.1 per cent of the rate found with the natural substrate. Acetaldehyde, propionaldehyde and butyraldehyde are also oxidized, but at still slower rates. The corresponding acyl phosphates are formed when these substrates are oxidized in the presence of inorganic phosphate. If arsenate is substituted for phosphate, the reactions proceed not to equilibrium, but to completion, with the formation of free acids. The formation of free acids is presumed to be the result of rapid spontaneous hydrolysis of unstable acyl arsenates, as proposed for other arsenolysis reactions. 

Triose Phosphate Dehydrogenases in Plants. Triose phosphate dehydrogenase, as described earlier, exists in many organisms. In green plants two additional enzymes have been reported. One is a very similar enzyme that has TPN in place of DPN. The other also uses TPN, but has no requirement for inorganic phosphate or any other acyl acceptor. This enzyme apparently catalyzes an irreversible formation of phospho-glyceric acid. 

---