Glycolysis other pathways

Glycolysis Energy production from glucose, small metabolites for other pathways ATP... 

Glycolysis also provides intermediates for other pathways. In the Iher, ycolysis is part of the process by which excess glucose is converted to fatty acids for storage. Glycol)rais is shown in Figure 1-12-2. Important enzymes in glycolysis include ... 

One role of glycolysis is to provide intermediates which are substrates for other pathways (Figure 6.2). Roles in... 

This compartmentation of processes in the liver cells occurs with other pathways glycolysis occurs mainly in the perivenous cells whereas gluconeogenesis occurs mainly in the periportal cells (Chapter 6). 

PM A). Neutrophils stimulated in this way produced a 40-minute heat burst of 23.5 nW per cell, approximately fourfold more than resting cells over a similar period. The rate of glycolysis, as measured by lactate evolution, was not altered substantially but the flux through the pentose phosphate pathway was increased by nearly 500% to give the necessary NADPH. All of the heat dissipated was accounted for by measurements of oxygen consumption, lactate production and C02 evolution in the glucose carbon 1/carbon 6 ratio. Enthalpy recovery was, therefore, 1.0 which indicated that no other pathway was induced by neutrophil activation—an enhanced flux in the pentose phosphate pathway to produce NADPH was responsible solely for the observed metabolic burst. 

Glycolysis presumably evolved as a process for ATP generation after carbohydrates such as glucose were being produced in significant quantities by other pathways. Glycolysis will be discussed in detail in Chapter 16. 

In the non-oxidative portion of the Pentose Phosphate Pathway a series of sugar interconversions takes the RU-5-P to intermediates of other pathways Ribose-5-P for nucleotide biosynthesis, and F-6-P and Ga-3-P for glycolysis/ gluconeogenesis. All of these reactions are near equilibrium, with fluxes driven by supply and use of the three intermediates listed above. 

As was the case with glycolysis and gluconeogenesis, it would be futile for the cell to carry out glycogen synthesis and degradation simultaneously. The results achieved by the action of one pathway would be imdone by the other. This problem is avoided by a series of hormonal controls that activate the enzymes of one pathway while inactivating the enzymes of the other pathway. 

Biosynthetic intermediates from glycolysis (Figure 13.10) Regulatory relationships with other pathways (Figure 13.11)... 

Note that several of the allosteric regulators are products of other metabolic pathways or are made in other metabolic pathways. These include acetyl-CoA, AMP, F2,6BP, and GIP, (readily converted into G6P). By having regulation dependent on other pathways, glycolysis is coordinately controlled with these pathways as well. 

Acetyl CoA serves as a common point of convergence for the major pathways of fuel oxidation. It is generated directly from the (3-oxidation of fatty acids and degradation of the ketone bodies (3-hydroxybutyrate and acetoacetate (Fig. 20.14). It is also formed from acetate, which can arise from the diet or from ethanol oxidation. Glucose and other carbohydrates enter glycolysis, a pathway common to all cells, and are oxidized to pyruvate. The amino acids alanine and serine are also converted to pyruvate. Pyruvate is oxidized to acetyl CoA by the pyruvate dehydrogenase complex. A number of amino acids, such as leucine and isoleucine are also oxidized to acetyl CoA. Thus, the final oxidation of acetyl CoA to CO2 in the TCA cycle is the last step in all the major pathways of fuel oxidation. 

The fate of pyruvate depends on the route used for NADH oxidation. If NADH is reoxidized in a shuttle system, pyruvate can be used for other pathways, one of which is oxidation to acetyl-CoA and entry into the TCA cycle for complete oxidation. Alternatively, in anaerobic glycolysis, pyruvate is reduced to lactate and diverted away from other potential pathways. Thus, the use of the shuttle systems allows for more ATP to be generated than by anaerobic glycolysis by both oxidizing the cytoplasmically derived NADH in the electron transport chain and by allowing pyruvate to be oxidized completely to CO2. 

The function of the citrate-anion allosteric site is to integrate glycolysis with other pathways. For example, the inhibition of PFK-1 by citrate may play a role in decreasing glycolytic flux in the heart dnring the oxidation of fatty acids. 

Lactic acidosis can also result from inhibition of lactate utilization in gluconeoge-nesis (e.g., hereditary fructose intolerance, which is due to a defective aldolase gene). If other pathways that use glucose-6-P are blocked, glucose-6-P can be shunted into glycolysis and lactate production (e.g., glucose 6-phosphatase deficiency). 

Entry of 2-carbon units is carried out by pyruvate dehydrogenase and citrate synthase in the first phase of the TCA cycle. Pyruvate from glycolysis or other pathways enters the TCA cycle through the action... 

It has been suggested that the key elanent in the development of diabetic retinopathy is metabolism of glucose by other pathways than the normal route through glycolysis. This might include an involvement of aldose reductase with a consequent accumulation of sorbitol to be deposited in the intercellular matrix of the tissues. The significance of this pathway is supported by the fact that activation of aldose reductase in experimental animals fed on a diet... 

---