Succinyl phosphate, hydrolysis

In both cases, the mixed anhydride is used to synthesize ATP from ADP. Hydrolysis of the anhydride liberates more energy than the hydrolysis of ATP to ADP and, therefore, can be linked to the enzymic synthesis of ATP from ADP. This may be shown mechanistically as a hydroxyl group on ADP acting as nucleophile towards the mixed anhydride, and in each case a new phosphoric anhydride is formed. In the case of succinyl phosphate, it turns out that GDP rather than ADP attacks the acyl phosphate, and ATP production is a later step (see Section 15.3). These are enzymic reactions therefore, the reaction and the nature of the product are closely controlled. We need not concern ourselves why attack should be on the P=0 rather than on the C=0. 

When we investigate this substrate-level phosphorylation reaction in detail, we find it also involves a molecule of phosphate. Phosphate reacts initially with succinyl-CoA, converting the thioester into an acyl phosphate, which is, of course, a mixed anhydride (see Box 7.27). It is actually hydrolysis... 

The phosphorylation of GDP in this reaction is an example of substrate-level phosphorylation, and this is the only reaction in the citric acid cycle to produce a high-energy phosphate bond directly. The energy for this phosphorylation is derived from the hydrolysis of the thioester bond of succinyl-CoA. Subsequently, GTP phosphorylates ADP, catalyzed by nucleoside diphosphokinase, but this reaction... 

ALA synthase is a pyridoxal phosphate-dependent enzyme and promotes Schiff-base formation between its coenzyme and glycine (67 in Fig. 37). Nucleophilicity at C-2 of the glycine could be generated either by decarboxylation or by abstraction of a proton. In the first case 5-aminolaevulinic acid would retain both methylene protons of glycine, in the second, one of the protons would be lost to the medium (Fig. 37). Acylation of the pyridoxal-bound intermediate (68 or 69) by succinyl-CoA would constitute the next step and this could be followed either by direct hydrolysis of the Schiff-base or by decarboxylation with subsequent hydrolysis depending on which course was chosen in the first stage of the reaction. 

STEPS s-6 Hydrolysis and dehydrogenation of succinyl CoA. Succinyl CoA is hydrolyzed to succinate in step 5. The reaction is catalyzed by succinyl CoA synthetase and is coupled with phosphorylation of guanosine diphosphate (GDP) to give guanosine triphosphate (GTP). The overall transformation is similar to that of step 8 in glycolysis (Figure 29.4), in which a thiol ester is converted into an acyl phosphate and a phosphate group is then transferred to ADP. 

The reaction mechanism consists of formation of a Schiff base by pyridoxal phosphate with a reactive amino group of the enzyme entry of glycine and formation of an enzyme-pyridoxal phosphate-glycine-Schiff base complex loss of a proton from the a carbon of glycine with the generation of a carbanion condensation of the carbanion with succinyl-CoA to yield an enzyme-bound intermediate (a-amino-yS-ketoadipic acid) decarboxylation of this intermediate to ALA and liberation of the bound ALA by hydrolysis. ALA synthesis does not occur in mature erythrocytes. 

As usual, standard syntheses of nucleotides are not included in this survey. Using adenosine 5 -[(i )- 0, 0, 0]phosphate, it has been shown that the conversion of adenosine 5 -phosphate to cyclic AMP and the enzymatic hydrolysis of the latter to the former both occur with retention of configuration at phosphorus. Cyclic AMP can be used to prepare 2 -0-substituted adenosine derivatives 2 -0-succinyl adenosine was synthesized by acylation followed by enzymatic dephosphorylation on equilibration the 3 -0-succinyl isomer was slightly favoured (54%). ... 

In reaction 5, succinyl-CoA undergoes hydrolysis to succinate and CoA. The energy released is used to add a phosphate group (Pi) to GDP (guanosine diphosphate), which yields GTP (guanosine triphosphate), a high-energy compound similar to ATP. 

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