Triose phosphate compounds

These catalysts facilitate the interconversion of isomeric compounds and include racemases, optimerases, cis-trans isomerases, intramolecular oxidoreductases and intramolecular transferases. Scheme 10.14 shows the conversion of an aldehyde to a ketone by triose phosphate isomerase. 

FIGURE 20-10 Third stage of C02 assimilation. This schematic diagram shows the interconversions of triose phosphates and pentose phosphates. Black dots represent the number of carbons in each compound. The starting materials are glyceraldehyde 3-phosphate and dihydroxyacetone phosphate. Reactions catalyzed by transaldolase ( and ) and transketolase ((3) and ) produce pentose phosphates that are converted to ribulose 1,5-bisphosphate—ribose... 

FIGURE 20-37 Pools of pentose phosphates, triose phosphates, and hexose phosphates. The compounds in each pool are readily interconvertible by reactions that have small standard free-energy changes. 

The metabolic pool that consists of fructose-1,6-bisphosphate and the two triose phosphates—glyceralde-hyde-3-phosphate and dihydroxyacetone phosphate (DHAP)—is somewhat different from the other two pools of intermediates in glycolysis because of the nature of the chemical relationships between these compounds. In the other pools the relative concentrations of the component compounds at equilibrium are independent of the absolute concentrations. Because of the cleavage of one substrate into two products, the relative concentrations of fructose-1,6-bisphosphate and the triose phosphates are functions of the actual concentrations. For such reactions, the relative concentrations of the split products must increase with dilution. (For the reaction A v B + C, the equilibrium constant is equal to [B][C]/[A], If the concentration of A decreases, for example, by a factor of 4, equilibrium is... 

Affinity labels are molecules that are structurally similar to the substrate for the enzyme that covalently modify active site residues. They are thus more specific for the enzyme active site than are group-specific reagents. Tosyl-l-phenylalanine chloromethyl ketone (TPCK) is a substrate analog for chymotrypsin (Figure 8.21). TPCK binds at the active site and then reacts irreversibly with a histidine residue at that site, inhibiting the enzyme. The compound 3-bromoacetol is an affinity label for the enzyme triose phosphate isomerase (TIM). It mimics the normal substrate, dihydroxyacetone phosphate, by binding at the active site then it covalently modifies the enzyme such that the enzyme is irreversibly inhibited (Figure 8.22). 

Another important hydrolytic enzyme of the gut is acid phosphatase Like enterokinasc, it is bound to the enierocyte facing the lumen and is present in the duodenum, jejunum, and ileum. Alkaline phosphatase, a zinc metalloenzyme, also occurs in the gut. Acid phosphatase and alkaline phosphatase catalyze the removal of phosphate groups from a wide variety of compounds in foods, for example, sugar phosphates, triose phosphates, nucleotides such as AMP, ADP, and ATP, pyrophosphate, and phosphorylaled amino adds, A number of sugar and triose phosphates are described in the section on glycolysis in Chapter 4,... 

Figure 4.24 shows the molecular structures of compounds in the glycolytic pathway, The same compounds occur in gluconeogenesis. Generally, G-6-F and pyruvate are considered to be the first and final compounds, respectively, of glycolysis. The triose phosphate intermediate, dihydroxyacetone phosphate, is of special interest to nutritional scientists, because both dietary fructose and glycerol enter the pathway at this point. 

Berry, R. S, Correlation of rates of intramolecular tunneling processes, with application to some group V compounds. J. Chem. Phys. 32, 933-938 (1960), Muirhead, H. Triose phosphate isomerase, pyruvate kinase and other ajfl-barrels. Trends Biochem. Sci. 8, 326-330 (1983). 

Fig.2.10 Phosphoglycerate utilization, (a) During the day. Photosynthesis in the chloroplast makes starch until there is no more room. The Calvin cycle continues to make triose phosphate, which exits the chloroplast in exchange for organic phosphate (Pi) entering the chloroplast and converting ADP to ATP. In the cytosol, the triose phosphate is mostly converted to sucrose but also to small amounts of other compounds such as amino acids for transport throughout the plant, (b) During the night. Phosphorylase is activated and it breaks up the starch to glucose 6-phosphate from which triose phosphate is made. The triose phosphate is exchanged for Pi. The Pi is a substrate for phosphorylase and keeps it active. Once in the cytosol, the triose phosphate is transferred mostly to mitochondria for respiration...

Overall, the hydrogen stored in NADPH is used to reduce C02 to carbohydrate units (0H2O). This is not a direct reaction because the C02 is first combined with a C5 compound, ribulose diphosphate (RDP), which then spontaneously splits into two identical C3 molecules, phosphoglyceric acid (PGA). Most of the PGA is used to synthesize further RDP but some is reduced by NADPH, using energy supplied by the ATP/ADP system, to give triose phosphate, which in turn is converted into the glucose phosphate from which various carbohydrates are synthesized. This assimilatory path is known as the Calvin cycle and is involved in all autotrophic carbon fixation, whether photosynthetic or chemosynthetic. 

We have already considered the photosynthetic formation of carbohydrates (Box 1.10) and have seen that the basic building blocks are C3 compounds — triose phosphates—D-glyceraldehyde-3-phosphate... 

Green plants contain in their chloroplasts unique enzymatic machinery that catalyzes the conversion of CO2 to simple (reduced) organic compounds, a process called CO2 assimilation. This process has also been called CO2 fixation or carbon fixation, but we reserve these terms for the specific reaction in which CO2 is incorporated (fixed) into a three-carbon organic compound, the triose phosphate 3- " H phosphoglycerate. This simple... 

Glyceraldehyde 3-phosphate is on the direct pathway of glycolysis, whereas dihydroxyacetone phosphate is not. Unless a means exists to convert dihydroxyacetone phosphate into glyceraldehyde 3-phosphate, a three-carbon fragment useful for generating ATP will be lost. These compounds are isomers that can be readily interconverted dihydroxyacetone phosphate is a ketose, whereas glyceraldehyde 3-phosphate is an aldose. The isomerization of these three-carbon phosphory-lated sugars is catalyzed by triose phosphate isomerase (TIM ... 

Fructose-1,6-bisP is cleaved into two phosphorylated 3-carbon compounds (triose phosphates) by aldolase (see Fig. 22.5). Dihydroxyacetone phosphate (DHAP) is isomerized to glyceraldehyde 3-phosphate (glyceraldehyde-3-P), which is a triose phosphate. Thus, for every mole of glucose entering glycolysis, 2 moles of glyceraldehyde-3-P continue through the pathway. 

Why are these isotopes important in biochemistry and medicine The isotopes we have mentioned occur at very low natural abundance , e.g. in the world around us only about 1 carbon atom in 10 (a million million) is C. However, with the advent of nuclear physics and specifically the Manhattan Project, the atomic bomb project in World War 11, radioactive isotopes started to be produced artificially, and this meant that chemical compounds could be radioactively labelled , either uniformly (e.g. in every carbon position) or selectively (i.e. with radioactive enrichment in particular positions). In the case of carbohydrate metabolism, it was possible to study the relative importance of glycolysis and PPP by comparing the release of radioactivity from glucose, specifically labelled either in carbon 1 or in carbon 6. If you look at Topic 28, you will see that in the initial reactions of the PPP the CO2 that is produced comes entirely from the Cl position. Over time, as the later molecular rearrangements come into play, C6 atoms could also eventually be released but not initially. On the other hand, if you revisit Topics 13 and 14, you will see that, because the sugar phosphate is split down the middle into two triose phosphate halves that are then handled identically, CO2 released in the oxidation of pyruvate to acetyl CoA will be derived equally from Cl and C6. This allows biochemists to assess the relative activities of PPP and glycolysis in different tissues or in the same tissue over time. This is how it was possible to estimate (Topic 28) that 30% of glucose breakdown in liver is via PPP. 

The equilibrium constant of the aldolase reaction depends greatly on temperature. At low temperatures the condensation is more favored, whereas the amount of triose at equilibrium increases with rising temperatures. The equilibrium constant is evaluated as (dihydroxyacetone phosphate) (phosphoglyceraldehyde)/(HDP) = IT, = 6 X 10 at 28 C. At first glance it appears that this implies that very little triose exists at equilibrium at this temperature. This is an example of reactions in which one compound is converted to two, and closer examination shows that the percentage conversion in such cases is a function of the absolute concentration. Thus, with 1 M HDP, only a fraction of 1 per cent is split at equilibrium, whereas at 10 M, approximately half is converted to triose phosphates. The equilibrium constant is markedly affected by temperature. Lower temperatures favor the condensation to HDP, while higher temperatures cause the reaction to shift toward increased formation of triose phosphates. 

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. 

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