Glucose breakdown

In contrast to experiments which attempted to identify phosphorylated intermediates on the look and see what is there basis, others were designed to investigate possible pathways of glucose breakdown suggested from its known chemical reactivity. Treating glucose in vitro... 

Kiewer (20) indicated that for the vinifera grapes studied, glucose breakdown apparently occurred when grapes were in an overripe condition which resulted in a sharp decrease in the glucose/fructose ratio. Normal ratio during the ripe period was considered to be ca. 1.00 0.10. This was in accord with the observation with Concord (25) that sucrose was synthesized in the leaves and translocated to other vine parts where hydrolysis resulted in 1 1 ratio of glucose to fructose. 

Most of these varieties conform to this observation. The somewhat lower ratio for hot-press Concord must could be attributed to an increased irate of glucose breakdown, as reported for vinifera fruit since some overripe fruit may be processed near the end of the harvest. However, the Ives appears to be an exception because of its consistently low ratio. The vineyard characteristics of this variety are such that it is normally harvested at the early-ripe to ripe stage. Thus the low glucose concentration found in the must may be the result of factors other than the glucose breakdown observed in overripe fruit. The low glucose content in the juice may be related to the extremely high anthocyanidin digluco-side content of the berry. 

The free energy change for glucose breakdown alone is ... 

Glucose breakdown and tryptophan biosynthesis illustrate pathways consisting of a linear series of reactions. In chapter 12 we will see that the glycolytic pathway also contains branches so that common intermediates can proceed along diverging routes after the branchpoint. We will also see that the glycolytic pathway is organized so that some of the reactions within the pathway can go in either the forward or the reverse direction. 

The cycling of lactate and glucose between muscle and liver in the Cori cycle. Under conditions of intense activity the muscle operates anaerobically so that the end product of glucose breakdown is lactate. This product can pass through the bloodstream to the liver and be converted to glucose, which can be returned to the muscle. 

The acetyl CoA that gets on the ferris wheel can be continually replenished through glucose breakdown, or, mainly, through fatty acid degradation (oxidation), or by transformation of certain amino acids. What, however, produces the seats of the ferris wheel, or replenishes them when necessary The seats cannot be replaced by acetyl CoA, which is merely a passenger. The chemicals of the ferris wheel can be restored in part by certain amino acids that can convert to Krebs cycle intermediates. There also is an important side step in which pyruvate can be directly convert to oxaloacetate (D-8). 

The pathway of glucose breakdown is indicated in Figure 4.25 by heavy dots. Steps that are Inactivated by a rise in the glucagon/Insulin ratio are indicated by boldface Xs. As explained, PEPCK (more accurately, cytosolic PEPCK) activity is controlled by cellular levels of cAMP in the following manner. Cyclic AMP combines with a special protein (CREBP) to form a complex that binds to DN A. Where does the cAMP/CREBP complex bind The complex binds to a short stretch of... 

Controlling the wx of substrates is another means of regulating metabolism. Glucose breakdown can take place in many cells only if insulin is present to promote the entry of glucose into the cell. The transfer of substrates from one compartment of a cell to another (e.g., from the cytoplasm to mitochondria) can serve as a control point. 

Color determination of the reaction core from the humidity series show an increase in color formation with increasing humidity, however, to a reduced extent compared to 1-amino propan-2-one production (Figure 7). As might be expected the formation of the transamination product of 2-oxopropanal originating from glucose breakdown and die recombination with the amino acid only partially represents those potentially similar chemical reactions which are responsible for color formation. 

It is hardly surprising then that for many years now the study of the behaviour of such slices has continued to provide a goldmine of useful information. The detailed mechanisms of glucose breakdown, amino acid interconversions, and fatty acid synthesis have been resolved in slice preparations. And, more and more, it is becoming possible to use them to tackle much more general problems of the regulation and control of the whole pattern of cell... 

Glucose polymerizes to form glycogen when the organism has no immediate need for the enei derived from glucose breakdown. 

These electrons are subsequently transferred to cytochrome h, and then to the terminal electron acceptor, Fe oxide in contact with the cell membrane. This reaction is mediated by the enzyme Fe reductase. The net amount of energy released during glucose oxidation using Mn(IV) and Fe(III) is approximately 67 and 15%, respectively, of that released from glucose breakdown stored in the reduced end products such as Mn(II) and Fe(II). Reduction of Mn oxides occurs in wetland soils with Eh range of +200 to +300 mV, as compared to -100 to +100 mV for Fe oxide reduction. 

The glycerol from fat breakdown feeds into the glucose breakdown/synthesis pathways at the level of glyceraldehyde 3-phosphate. It is important, however, to note that the fatty acids themselves cannot yield glucose. They form acetyl CoA, but in humans there is no route back from acetyl CoA to pyruvate. The conversion of pyruvate into acetyl CoA is effectively a one-way valve. Thus, it is easy to convert excess carbohydrate into fat but not the other way round. 

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. 

---