Triose phosphate dehydrogenase

Glyceraldehyde-3-phosphate dehydrogenase (NADP+) (nonphosphorylating) [EC 1.2.1.9], also referred to as triose-phosphate dehydrogenase, catalyzes the reaction of D-glyceraldehyde 3-phosphate with NADP+ and water to produce 3-phospho-D-glycerate and NADPH. 

Another enzyme of the carbon reduction cycle activates PGA, converting it to phosphoryl-3-phosphoglyceric acid (IV). This acid anhydride can then be reduced in a subsequent enzymatic step mediated by triose phosphate dehydrogenase. For its reducing agent, this enzyme uses nicotinamide adenine dinucleotide phosphate (NADPH) and thereby converts the carboxylic acid to... 

Reactions (9), (10), and (13)-(15) bring about the conversion of PGA to fructose-6-phosphate (VI). The corresponding glycolytic enzymes would be (9) phosphoglyceryl kinase (10) triose phosphate dehydrogenase (13) triose phosphate isomerase (14) aldolase. Equation (15) would require a phosphatase. 

Role of NAD as a coenzyme or cosubstrate in dehydrogenation and hydrogenation. Enz. I triose-phosphate dehydrogenase, Enz. II = alcohol dehydrogenase S and S-H2 = oxidized and reduced substrate, respectively P and P-H2 = oxidized and reduced products. 

Triose phosphate dehydrogenase has been crystallized from rabbit muscle, and even after repeated crystallization, it contains two moles of bound NAD per mole of protein. The nucleotide can be removed by passing the enzyme through charcoal, but this treatment renders the enzyme preparation unstable. Both the stability and the activity of the enzyme can be restored by adding NAD to the medium. The presence of NAD in the enzyme molecule can readily be demonstrated by ultraviolet spectrophotometry. The exact mode of attachment of NAD to the enzyme molecule is not known, but it has been established that the enzyme contains SH groups belonging to cysteine residues—cysteine probably is part of the tripeptide gluta-... 

The fate of dihydroxyacetone phosphate is also varied—it may be transformed to a-glycerophosphate, or through the reaction catalyzed by the triose isomerase, it may yield D-glyceraldehyde-3-phosphate, which in the presence of triose phosphate dehydrogenase and phosphoglycerate kinase yields 3-phosphoglycerate. This degradation pathway is complete only in liver, and it is not known to what extent it operates in muscle. 

Triose phosphate isomerase Triose phosphate dehydrogenase... 

Triose phosphate dehydrogenase has been studied both as a chemical molecule and as a catalyst. These studies were made possible by the availability of gram quantities of crystalline enzyme from rabbit muscle, by the procedure of Cori, Slein, and Cori, and from yeast, by the procedure of Warburg and Christian. Krebs has isolated four fractions from yeast with equivalent specific activity, one of which is the enzyme crystallized by Warburg and Christian. The four components make up about 5 per cent of the total extractable protein of the yeast. 

Composition of Triose Phosphate Dehydrogenase. The animal and yeast enz3qnes are similar but not identical in certain respects. The amino acid compositions are similar, and both have N-terminal valine residues. The two are distinct immunologically. The most striking difference is the imusual binding of DPN in the rabbit enzyme. After repeated recrystallizations, each mole of this protdn contains two moles of firmly bound DPN. This DPN can be removed by treatment of the enzyme with charcoal. The charcoal-treated enzyme is less stable and more soluble than the original enzyme. 

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. 

DPNH-X. DPNH is rapidly destroyed by acid at room temperature. A similar destruction of DPNH was found to be catalyzed by triose phosphate dehydrogenase specifically, and by no other dehydrogenases tested. The pH optimum for the destructive reaction is near 5, in contrast to the optimum for the oxidative reaction at 8.5. The reaction product is identified by an absorption peak at 290 mjti and by an altered optical rotation. This product is called DPNH-X, and is distinguished from the acid degradation product by its rotation and by its enzymatic activity. A kinase has been found that catalyzes the reaction ... 

Aspartyl phosphate is reduced to aspartic-/3-semialdehyde (ASA) by aspartic semialdehyde dehydrogenase. The reaction resembles that catalyzed by triose phosphate dehydrogenase, but specifically requires TPN (III). The equilibrium constant is about 3 X 10 , so that at neutral... 

Effect of lodoaeetate on Fermentation, Triose Phosphate Dehydrogenase, and Small Molecular Mercaptans. 174... 

Glutathione, a Prosthetic Group of Triose Phosphate Dehydrogenase. 177... 

As most of you know, when mammalian triose phosphate dehydrogenase (TPD) is crystallized, DPN is found to be firmly attached to the protein. The enzyme can be crystallized in either an active (SH) or inactive (S—S) form, both having the same low dissociation constant for the bound DPN. In addition, the active enz3rme may be obtained with bound DPNH. 

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