Phosphorylation substrate level

Substrate level phosphorylation refers to those reactions associated with the generation of energy by the transfer of phosphate groups in metabolism, and is exemplified by fermentative metabolism where it is the sole source of energy (e.g. yeast and bacteria growing in anaerobic conditions). 

The sequence of biochemical steps by which glucose is degraded to pyruvic acid is called glycolysis or the Embden-Meyerhof-Parnas (EMP) Pathway, and is shown 

The Embden-Meyerhof-Parnas pathway. The six carbon substrate, (glucose) yields two three carbon intermediates to produce two moles of pyruvate 

Apart from the energy and reducing power derived from these reactions, the pathway also provides carbon skeletons for the synthesis of cellular structures. The energy provided by this pathway is used to drive the synthetic processes of the cell, and the EMP pathway is the major route for glucose metabolism. However, there are specialised alternative routes by which glucose may be metabolised and the reader is referred to specialised biochemical texts for an account of these(l7,28). 

The overall glycolytic sequence from glucose to lactic acid can be written as follows QH,A + 2Pi + 2ADP - 2C3H603 + 2 ATP + 2H20 (AG = - 135.7 MJ/kmol) 

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All six carbons of glucose are liberated as CO2, and a total of four molecules of ATP are formed thus far in substrate-level phosphorylations. The 12 reduced coenzymes produced up to this point can eventually produce a maximum of 34 molecules of ATP in the electron transport and oxidative phosphorylation pathways. A stoichiometric relationship for these subsequent processes is 1... 

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-... 

Where es = number of substrate-level phosphorylations per mole of carbon utilised. For oxidative phosphorylation we can write ... 

You will note that the equation used to determine P/O does not take into account ATP synthesis via substrate level phosphorylation, which is a limitation of the P/O estimation. 

Succinyl-CoA is converted to succinate by the enzyme succinate thiokinase (succinyl-CoA synthetase). This is the only example in the citric acid cycle of substrate-level phosphorylation. Tissues in which glu-coneogenesis occurs (the hver and kidney) contain two isoenzymes of succinate thiokinase, one specific for GDP and the other for ADP. The GTP formed is used for the decarboxylation of oxaloacetate to phos-phoenolpymvate in gluconeogenesis and provides a regulatory hnk between citric acid cycle activity and the withdrawal of oxaloacetate for gluconeogenesis. Nongluconeogenic tissues have only the isoenzyme that uses ADP. 

As a result of oxidations catalyzed by the dehydrogenases of the citric acid cycle, three molecules of NADH and one of FADHj are produced for each molecule of acetyl-CoA catabohzed in one mrn of the cycle. These reducing equivalents are transferred to the respiratory chain (Figure 16-2), where reoxidation of each NADH results in formation of 3 ATP and reoxidation of FADHj in formation of 2 ATP. In addition, 1 ATP (or GTP) is formed by substrate-level phosphorylation catalyzed by succinate thiokinase. 

ATP synthase reaction has been calculated as approximately 51.6 kJ. It follows that the total energy captured in ATP per mole of glucose oxidized is 1961 kJ, or approximately 68% of the energy of combustion. Most of the ATP is formed by oxidative phosphorylation resulting from the reoxidation of reduced coenzymes by the respiratory chain. The remainder is formed by substrate-level phosphorylation (Table 17—1). 

Respiratory chain oxidation of 2 NADH Phosphorylation at substrate level Phosphorylation at substrate level... 

In environments lacking a suitable external electron acceptor - such as dioxygen, sulfate, or ferric iron - respiration is not possible. Here, many organic compounds may be metabolized by fermenting microorganisms. Microbes of this class may create ATP by a direct coupling mechanism, using a process known as substrate level phosphorylation, SLP with an ion translocation mechanism like that employed by respirers, as already described or by a combination of SLP and ion translocation.1... 

By the mid-1950s, therefore, it had become clear that oxidation in the tricarboxylic acid cycle yielded ATP. The steps had also been identified in the electron transport chain where this apparently took place. Most biochemists expected oxidative phosphorylation would occur analogously to substrate level phosphorylation, a view that was tenaciously and acrimoniously defended. Most hypotheses entailed the formation of some high-energy intermediate X Y which, in the presence of ADP and P( would release X and Y and yield ATP. A formulation of the chemical coupling hypothesis was introduced by Slater in 1953,... 

Substrate level phosphorylation involves the transfer of phosphate directly from a high energy compound (shown below as X-P) to ADP. This illustrates the concept of reaction coupling (Section 2.2.5) thus ... 

Examples of substrate level phosphorylation are to be found in glycolysis. Phos-phoglycerate kinase (PGK) and pyruvate kinase (PK) catalyse the following reactions ... 

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. 

The CNS is not the only vulnerable tissue as red cells also rely upon a constant supply of glucose to maintain structure and function. Because they lack mitochondria, and therefore the mechanism to produce ATP via oxidative phosphorylation, RBCs are entirely dependent upon anaerobic glucose metabolism to synthesize ATP through substrate level phosphorylation. 

The loss of AMP especially in active muscles is partly ameliorated by recycling of IMP via adenylosuccinate. Furthermore, because AMP is an important allosteric activator of PFK, regeneration of AMP ensures that glycolysis is fully active and able to provide pyruvate for the TCA cycle and some ATP via substrate level phosphorylation. 

The scheme (Fig. 15.1) thus explained the production of both sulfate and sulfur in equimolar amounts from thiosulfate oxidation. In showing adenylylsulfate as an intermediate, it also provided a feasible route for the conservation of energy from sulfite oxidation by a substrate-level phosphorylation mechanism, in which ADP sulfurylase and adenylate kinase give rise to ATP ... 

Energy Conservation by Substrate-Level Phosphorylation and Its Coupling to Carbon Dioxide Fixation... 

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