Hexose isomerase

After fructose-1,6-bisphosphate is cleaved by aldolase to afford dihydroxyacetone phosphate (122) and glyceraldehyde-3-phosphate (124), the former is further converted to the latter by trlosephosphate isomerase (TIM). The structure of this enzyme has the prototypical TIM fold as an (a/ )s barrel (Figure 12), with the active site in one side of the barrel. This protein folding pattern is also found in the structures of hexose isomerases. 

The biochemistry of the lactic acid bacteria has received attention [4, 17-20]. Homofermentative strains such as the Pediococci use the glycolytic pathway for the dissimilation of carbohydrates, such as glucose, to yield pyruvic acid. Pyruvic acid acts as a hydrogen acceptor and is converted to lactic acid by means of an NADH-dependent lactic dehydrogenase. It is believed that the homofermentative strains use in addition the hexose monophosphate pathway and possibly a phosphoketolase pathway (Fig. 21.2) when pentoses are degraded. The heterofermentative strains on the other hand lack both aldolase and hexose isomerase, essential for the operation of the glycolytic pathway, while pyruvic acid will not readily function as a... 

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. 

The natural substrate for the dehydrogenase, glyceraldehyde-3-phosphate (G-3-P), had been synthesized earlier by Hermann Fischer, Emil Fischer s son, and Baer in 1932. In 1934 Meyerhof and Lohmann synthesized hexose diphosphate, establishing it to be fructose 1,6 bisphosphate (F-l, 6 bis P). With F-1,6 bisP as substrate and hydrazine to trap the aldehydic and ketonic products of the reaction, G-3-P was identified in the mixture of G-3-P and dihydroxyacetone phosphate which resulted. Triose phosphate isomerase was then isolated and the importance of phosphorylated 3C derivatives established. 

Fructose bisphosphate is cleaved by action of an aldolase (reaction 4) to give glyceraldehyde 3-phosphate and dihydroxyacetone phosphate. These two triose phosphates are then equilibrated by triose phosphate isomerase (reaction 5 see also Chapter 13). As a result, both halves of the hexose can be metabolized further via glyceraldehyde 3-P to pyruvate. The oxidation of glyceraldehyde 3-P to the corresponding carboxylic acid, 3-phosphoglyceric acid (Fig. 17-7, reactions 6 and 7), is coupled to synthesis of a molecule of ATP from ADP and P . This means that two molecules of ATP are formed per hexose cleaved, and that two molecules of NAD+ are converted to NADH in the process. 

The oxidative pentose phosphate cycle is often presented as a means for complete oxidation of hexoses to C02. For this to happen the C3 unit indicated as the product in Fig. 17-8A must be converted (through the action of aldolase, a phosphatase, and hexose phosphate isomerase) back to one-half of a molecule of glucose-6-P which can enter the cycle at the beginning. On the other hand, alternative ways of degrading the C3 product glyceraldehyde-P are available. For example, using glycolytic enzymes, it can be oxidized to pyruvate and to C02 via the citric acid cycle. 

Subsequent Reactions Catalyzing 6-Deoxyhexose Formation 3,5-Epimerase and Reductase. After the formation of the 4-keto intermediate, a step common to all deoxyhexose formations, at least two and possibly three additional enzymes are necessary for the biosynthesis of the end product. This sequence of transformations is illustrated in Figure 3. The first step is the conversion of the 4-keto-6-deoxy-hexose intermediate described above. The intermediates in brackets are postulated and are assumed to be bound to the enzyme. An enzyme (or enzymes), referred to below as 3,5-isomerase, catalyzes epimerizations at carbons 3 and 5, probably via the enediol form. The epimerizations are followed by a... 

Transketolases are characterized by their ability to transfer a two-carbon unit from a ketose to an aldehyde. The C3 and C7 sugar-phosphates can subsequently be converted to a C4 and a Csugar-phosphate, erythrose 4-phosphate (3.17) and fructose 6-phosphate (3.2), respectively. This reaction is catalyzed by a transaldolase, which transfers a three-carbon glyceraldehyde unit from an aldose to a ketose. Erythrose-4-phosphate (3.17) can be used in the shikimate pathway (see Section 6). A second transketolase reaction can generate a second fructose-6-phosphate (3.2) and glyceraldehyde-3-phosphate (3.4) residue from erythrose-4-phosphate (3.17) and xylulose-5-phosphate (3.15). Hexose-phosphate isomerase converts the... 

A microbial glucose isomerase (EC 5.3.1.5) that catalyzes the conversion of D-glucose into D-fructose was able to convert 5-deoxy-D-xy/o-hexose (40) into the 2-hexulopyranose 51.104... 

M. H. Fechter and A. E. Stfitz, Synthetic application of glucose isomerase Isomerization of C-5-modified (2R,3R,4R)-configured hexoses into the corresponding 2-ketoses, Carbohydr. Res., 319 (1999) 55-62. 

Answer Problem 1 outlines the steps in glycolysis involving fructose 1,6-bisphosphate, glyceraldehyde 3-phosphate, and dihydroxyacetone phosphate. Keep in mind that the aldolase reaction is readily reversible and the triose phosphate isomerase reaction catalyzes extremely rapid interconversion of its substrates. Thus, the label at C-l of glyceraldehyde 3-phosphate would equilibrate with C-l of dihydroxyacetone phosphate (AG ° = 7.5 kJ/mol). Because the aldolase reaction has AG ° = -23.8 kJ/mol in the direction of hexose formation, fructose 1,6-bisphosphate would be readily formed, and labeled in C-3 and C-4 (see Fig. 14-6). 

Two metabolic patterns are discernible from the results. Carbon atoms 2, 1, and 7 of shikimate (VI) are derived almost equally from G-1,6, G-2,5, and G-3,4, respectively. In the Embden-Meyerhof pathway of hexose metabolism (see Fig. 2), D-fructose 1,6-diphosphate is cleaved to 1,3-dihydroxy-2-propanone phosphate (G-1,2,3) and D-glycerose 3-phosphate (G-4,5,6), and the two trioses are interconverted by triose phosphate isomerase. The observed randomization of label between Cl and C6, C2 and C5, and C3 and C4 of hexose therefore implies that C2, Cl, and C7 of shikimate are derived from a 3-carbon intermediate of glycolysis. The small but significant preponderance of G-6 over G-1, of G-5 over G-2, and, presumably, of G-4 over G-3, can be explained by recent observations that, in the aldolase cleavage of D-fructose 1,6-diphosphate, the 1,3-dihy-... 

The glucose analog, 3-deoxy-3-C-methylene-D-nfco-hexose (102) is a substrate of a D-xylose isomerase. The efficient synthesis from 3-deoxy-3-C-methylene-glucofuranose (101 prepared by the classical method according to [92]) was described recently (O Scheme 44) [93]. 

The pentose phosphate pathway also catalyzes the interconversion of three-, four-, five-, six-, and seven-carbon sugars in a series of non-oxidative reactions. All these reactions occur in the cytosol, and in plants part of the pentose phosphate pathway also participates in the formation of hexoses from CO2 in photosynthesis. Thus, D-ribulose 5-phosphate can be directly converted into D-ribose 5-phosphate by phosphopentose isomerase, or to D-xylulose 5-phosphate by phosphopentose epimerase. D-Xylulose 5-phosphate can then be combined with D-ribose 5-phosphate to give rise to sedoheptulose 7-phosphate and glyceraldehyde-3-phosphate. This reaction is a transfer of a two-carbon unit catalyzed by transketolase. Both products of this reaction can be further converted into erythrose 4-phosphate and fructose 6-phosphate. The four-carbon sugar phosphate erythrose 4-phosphate can then enter into another transketolase-catalyzed reaction with the D-xylulose 5-phosphate to form glyceraldehyde 3-phosphate and fructose 6-phosphate, both of which can finally enter glycolysis. 

G6P isomerase (GPI EC 5.3.1.9) (also known as phospho-giucose isomerase [PGI]), catalyzes the interconversion of G6P and fructose 6-phosphate (F6P), the second step of the EMP. As a result of this reversible reaction, products of the hexose monophosphate pathway can be recycled to G6P. Besides being a housekeeping enzyme of glycolysis, GPI exerts outside the cell cytokine properties and is involved in several extracellular processes. In addition, autoantibodies against GPI seem to be involved in rheumatoid arthritis. GPI is a crucial enzyme, since GPI knockout mice die in the embryological state. ... 

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