Time triose phosphate isomerase

The role of enediols in the conversion of L-glyceraldehyde 3-phosphate (26) into either dihydroxyacetone phosphate (27) or inorganic phosphate plus methylglyoxal (28) has been postulated for some time. Triose phosphate isomerase, which does not react with (26) directly, can convert it into dihydroxyacetone phosphate by... 

Triose phosphate isomerase (TPI) catalyzes the interconversion of glyceralde-hyde-3-phosphate and dihydoxyacetone phosphate and has an important role in glycolysis, gluconeogenesis, fatty acid synthesis, and the hexose monophosphate pathway. Red blood cell TPI activity measured in vitro is approximately 1000 times that of Hx, the least active glycolytic enzyme. TPI is a dimer of identical subunits, each of molecular weight 27,000, and does not utilize cofactors or metal ions. Posttranslational modification of one or both subunits may occur by deamidination, resulting in multiple forms of the enzymes and creating a complex multibanded pattern on electrophoresis. 

Many of the mechanistic aspects of glucose isomerase catalysed aldose-ketose interconversion have been under discussion for some time and are still not fully understood. By comparison with triose phosphate isomerase (TIM, EC 5.3.1.1) and glucose 6-phosphate isomerase (EC 5.3.1.9), the base-catalysed formation of an 1,2-enediol was invoked as the key step of the epimerisation based on the work of Rose and co-workers with tritium-labelled substrates [26]. An unexplained featme of the epimerisation process was that in contrast to isomerisations with triose phosphate isomerase no proton exchange with the medium could be observed with D-xylose isomerase, a fact that was attributed to the phosphate group of the former as a mediator for the exchange process [26]. Subsequently, additional important differences between triose phosphate isomerase and xylose isomerase were recognised. For example, D-xylose isomerase is appar-endy a very slow enzyme catalysing about five molecules per second per active site with an absolute requirement for divalent cations, while TIM does not need co-factors and operates at nearly 1000-fold the speed of D-xylose isomerase at... 

Nikoh, N., Iwabe, N., Kuma, K., Ohno, M., Sugiyama, T., Watanabe, Y., Yasui, K., Shi-cui, Z., Hori, K., Shimura, Y. and Miyata, T. (1997) An estimate of divergence time of Parazoa and Eumetazoa and that of Cephalochordata and Vertebrata by aldolase and triose phosphate isomerase clocks . Journal of Molecular Evolution, 45, 97-106. 

The study of these metabolic steps is quite active at the present time. As noted above, Dickens found that ribose-5-phosphate was fermented anaerobically to ethanol, a 2-carbon compound, inorganic phosphate, and CO2. Racker observed that extracts of E. colt converted ribose-5-phosphate to a triose phosphate, which could be analyzed in the presence of triose phosphate isomerase as dihydroxyacetone phosphate.Therefore, the products of the oxidative pathway eventually join the Embden-Meyerhof scheme at the triose phosphate stage, the major difference being the formation of 2 moles of triose phosphate in the latter pathway and only 1 mole via the phosphogluconate pathway. 

Phosphofructokinase, the enzyme that phosphory-lates fructose-1-phosphate to yield the diphosphate, the precursor of the triose phosphates, has a fate similar to that of hexokinase, except that its prenatal activity is only three times greater than that of adult liver, and the prenatal activity drops to adult values within 9 days after birth. Fructose-1,6-diphosphate, triose-P-isomerase, and glyceraldehyde phosphate dehydrogenase all have high fetal activities that slightly increase at the adult levels in the newborn. Thus, in the fetal liver the activity of these enzymes seems to favor the formation rather than the use of lactic acid. 

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