Phosphorylation in glycolysis

In contrast to substrate level phosphorylation in glycolysis, mitochondrial oxidative phosphorylation is an efficient process in that it generates in excess of 30 ATP per mole of glucose. In essence, the movement of electrons along the respiratory chain or electron transport chain is coupled with phosphorylation of ADP. 

When grown under aerobic conditions, the yeast produces two ATP molecules from one molecule of glucose by substrate-level phosphorylation in glycolysis. The two molecules of pyruvate produced can then be completely oxidized to CO2, and each yields a further 15 molecules of ATP. This leads to a slow decrease in the concentration of glucose, a steady production of CO2, and relatively little change in the amount of ATP. Also, the two molecules of NADH can be reoxidized to NAD+ by the electron-transport system. (This produces yet more ATP, as discussed in Chap. 14.)... 

Which of the following could theoretically yield the maximum net number of molecules of ATP by substrate-level phosphorylation in glycolysis a molecule of sucrose, two molecules of glucose, or two molecules of fructose ... 

Cellular adenine nucleotides are compartmentalized by their very low diffusibility (due to their size and charge) with pools in the mitochondria, at the myofibrils, SR, and other sites of energy utilization. CK is located at those sites. Phosphocreatine is much smaller and less charged, and therefore much more mobile in cells than ATP. ATP produced by substrate-level phosphorylation in glycolysis may be used to rephosphorylate creatine in the sarcoplasm ... 

The enzyme pyruvate kinase (PK), one of the sites of ADP phosphorylation in glycolysis, provides a good example of adaptation via alteration of enzyme properties. This is because it is modified by the binding of allosteric effectors, it is phosphorylated in the liver and it binds regulatory proteins in the muscle. The reaction catalyzed by PK is as follows ... 

The hypothesis that a proton-motive force across the inner mitochondrial membrane is the immediate source of energy for ATP synthesis was proposed in 1961 by Peter Mitchell. Virtually all researchers working in oxidative phosphorylation and photosynthesis Initially opposed this chemlosmotic mechanism. They favored a mechanism similar to the well-elucidated substrate-level phosphorylation in glycolysis, in which oxidation of a substrate molecule is directly coupled to ATP synthesis. By analogy, electron transport through the... 

This reaction is one of two substrate level phosphorylations in glycolysis (the other is catalyzed by phosphoglycerate kinase). The enzyme is a key control point for glycolysis. 

If 1 molecule of glucose produces 2 molecules of ATP by substrate-level phosphorylation in glycolysis and the resulting 2 molecules of pyruvate can each yield 12.5 molecules of ATP when oxygen is available, how many glucose molecules will be necessary to produce 135 molecules of ATP by yeast grown under (a) aerobic and... 

Only about 5% of the ATP generated comes from substrate-level phosphorylation in glycolysis, and the remainder is obtained by the oxidative phosphorylation process. If the oxygen supply becomes temporarily inadequate, NADH is re-oxidised using pyruvic acid as the hydrogen acceptor. This enables glucose breakdown to work anaerobically. 

Figure 10.2 Substrate-level phosphorylation in glycolysis produces ATP.

Dihydroxyacetone phosphate is of course an intermediate in glycolysis. D-Gly-ceraldehyde can be phosphorylated by triose kinase in the presence of ATP to form D-glyceraldehyde-3-phosphate, another glycolytic intermediate. 

FIGURE 20.1 Pyruvate produced hi glycolysis is oxidized in the tricarboxylic acid (TCA) cycle. Electrons liberated in this oxidation flow through the electron transport chain and drive the synthesis of ATP in oxidative phosphorylation. In eukaryotic cells, this overall process occurs in mitochondria. 

Whereas ATP made in glycolysis and the TCA cycle is the result of substrate-level phosphorylation, NADH-dependent ATP synthesis is the result of oxidative phosphorylation. Electrons stored in the form of the reduced coenzymes, NADH or [FADHa], are passed through an elaborate and highly orga-... 

Because the 2 NADH formed in glycolysis are transported by the glycerol phosphate shuttle in this case, they each yield only 1.5 ATP, as already described. On the other hand, if these 2 NADH take part in the malate-aspartate shuttle, each yields 2.5 ATP, giving a total (in this case) of 32 ATP formed per glucose oxidized. Most of the ATP—26 out of 30 or 28 out of 32—is produced by oxidative phosphorylation only 4 ATP molecules result from direct synthesis during glycolysis and the TCA cycle. 

Nevertheless, using GC-based technologies, the quantification of several important intermediates of central metabolism, especially phosphorylated intermediates, is not very reliable, presumably because these compounds and their derivatives are not thermostable. For an analysis of these groups of metabolites, an LC-MS (liquid chromatography or HPLC coupled to MS) is more suitable, because it eliminates the need for volatility and thermostability and thereby eliminates the need for derivatization. Using a triple quadrupole MS, most of the intermediates in glycolysis, in the pentose phosphate pathway, and in the tricarboxylic acid cycle were measured in E. coli [214]. 

Start with the easy ones Glucose [compound (iv) ] should be familiar to you and it is one of only two substrates in glycolysis which is not phosphorylated the other one being pyruvate [compound (i)]. 

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