Glycolysis metabolic pools

Fructose-1,6-bisphosphate and the Two Triose Phosphates Constitute the Second Metabolic Pool in Glycolysis... 

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... 

Relationships in glycolysis and gluconeogenesis. Points at which ATP is produced or consumed are indicated. Compounds in the same metabolic pools are indicated by purple boxes. Three small pseudocycles (la, II, III) in the paired sequences occur between glycogen and pyruvate, or between glycogen and glucose (lb, II, III). Only enzymes that are unique to either glycolysis or gluconeogenesis are indicated (screened in blue). 

The organization of glycolysis and gluconeogenesis as a series of connected metabolic pools makes it possible for most of the same enzymes to function in both directions. Only the reactions connecting the metabolic pools require different enzymes and a coupling to the ATP-ADP system to make them thermodynamically feasible in the direction of gluconeogenesis. 

The tightly regulated pathway specifying aromatic amino acid biosynthesis within the plastid compartment implies maintenance of an amino acid pool to mediate regulation. Thus, we have concluded that loss to the cytoplasm of aromatic amino acids synthesized in the chloroplast compartment is unlikely (13). Yet a source of aromatic amino acids is needed in the cytosol to support protein synthesis. Furthermore, since the enzyme systems of the general phenylpropanoid pathway and its specialized branches of secondary metabolism are located in the cytosol (17), aromatic amino acids (especially L-phenylalanine) are also required in the cytosol as initial substrates for secondary metabolism. The simplest possibility would be that a second, complete pathway of aromatic amino acid biosynthesis exists in the cytosol. Ample precedent has been established for duplicate, major biochemical pathways (glycolysis and oxidative pentose phosphate cycle) of higher plants that are separated from one another in the plastid and cytosolic compartments (18). Evidence to support the hypothesis for a cytosolic pathway (1,13) and the various approaches underway to prove or disprove the dual-pathway hypothesis are summarized in this paper. 

Fig. 2 The red blood cell has played a special role in the development of mathematical models of metabolism given its relative simplicity and the detailed knowledge about its molecular components. The model comprises 44 enzymatic reactions and membrane transport systems and 34 metabolites and ions. The model includes glycolysis, the Rapaport-Leubering shunt, the pentose phosphate pathway, nucleotide metabolism reactions, the sodium/potassium pump, and other membrane transport processes. Analysis of the dynamic model using phase planes, temporal decomposition, and statistical analysis shows that hRBC metabolism is characterized by the formation of pseudoequilibrium concentration states pools or aggregates of concentration variables. (From Ref...

A large intracellular pool of a) ions (caused by a negative effect of severe limitation of b) on protein turnover) and an increased respiratory activity, which in part is not coupled to c) synthesis, stimulates metabolic flux throu glycolysis without significant metabolic control. This, togettier with d) pyruvate carboxylase and the peculiarities in the operation of the TCA-cyde, results in elevat cellular concentrations of e>. This in turn enhances dtric add accumulation by inhibiting i) dehydrogenase. 

At very low values of EC, when AMP is elevated it is deaminated via AMP deaminase to inosine monophosphate (IMP). This further displaces the adenylate kinase reaction in the direction of ATP synthesis. The IMP is dephosphorylated by nucleotide phosphatase, and the inosine is phosphorylyzed via purine nucleotide phosphorylase, releasing hypoxanthine and ribose 1-phosphate. The latter is metabolized via the pentose phosphate pathway, and most of the carbon atoms enter glycolysis. Because this course of events depletes the overall adenine nucleotide pool, and hence the scope for ATP production in the longer term, it represents a metabolic last ditch stand by the cell to extract energy even from the energy currency itself ... 

Even though AMP-induced activation of glycolysis and glycogenolysis is rapid, cellular reserves of ATP would only allow vigorous contraction of skeletal muscle for 1 s. An instantly available store of high-energy phosphate is provided by creatine phosphate. Creatine kinase is exceptionally active and maintains the reaction between creatine and ATP, and creatine phosphate and ADP, in rapid dynamic equilibriiun. This pool of creatine phosphate serves as a buffer for ATP that occurs without any activation of metabolic pathways. 

Metabolic networks can be analysed using 2D [ C, H] COSY (NMR) measurements of C-labeled metabolites. A framework is presented whereby the steady state reaction rates are deduced from conventional isotopomer balances. This model is reduced by removing redundant nodes and lumping equilibrium pools. Conversion of the balances to the recently introduced bondomer notation further reduces the complexity. When the reduction approaches are applied to the glycolysis and pentose phosphate pathway, the number of equations is reduced by a factor of three without loss of information. 

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