Succinyl coenzyme phosphorylation

A High Phosphoryl-Transfer Potential Compound Is Generated from Succinyl Coenzyme A... 

The breakdown of succinyl coenzyme A may also be linked to the phosphorylation of GDP and IDP to yield GTP and ITP in the presence of inorganic phosphate. The enzyme catalyzing that reaction has been named the phosphorylating enzyme, and has been prepared in a crude form from heart muscle. This preparation also contains another enzyme, nucleoside diphosphate kinase, which catalyzes the transfer of phosphorus from GTP or ITP to ADP to yield ATP. The phosphorylation of ADP coupled to the oxidation of a-ketoglutarate is the only substrate level phosphorylation in the Krebs cycle, and, as can be expected, it is not inhibited by dinitrophenol. When O-labeled phosphate is used in the reaction, the label appears... 

The subsequent cleavage of the thio-ester succinylCoA into succinate and coenzyme A by succinic acid-CoA ligase (succinyl CoA synthetase, succinic thiokinase) is strongly exergonic and is used to synthesize a phosphoric acid anhydride bond ( substrate level phosphorylation , see p. 124). However, it is not ATP that is produced here as is otherwise usually the case, but instead guanosine triphosphate (CTP). However, GTP can be converted into ATP by a nucleoside diphosphate kinase (not shown). 

Figure 17,13 Reaction mechanism of succinyl CoA synthetase. The reaction proceeds through a phosphorylated enzyme intermediate. (1) Orthophosphate displaces coenzyme A, which generates another energy-rich compound, succinyl phosphate. (2) A histidine residue removes the phosphoryl group with the concomitant generation of succinate and phosphohistidine. (3) The phosphohistidine residue then swings over to a bound nucleoside diphosphate, and (4) the phosphoryl group is transferred to form the nucleoside triphosphate.

Eugene Kennedy and Albert Lehninger showed in 1948 that, in eukaryotes, the entire set of reactions of the citric acid cycle takes place in mitochondria. Isolated mitochondria were found to contain not only all the enzymes and coenzymes required for the citric acid cycle, but also all the enzymes and proteins necessary for the last stage of respiration—electron transfer and ATP synthesis by oxidative phosphorylation. As we shall see in later chapters, mitochondria also contain the enzymes for the oxidation of fatty acids and some amino acids to acetyl-CoA, and the oxidative degradation of other amino acids to a-ketoglutarate, succinyl-CoA, or oxaloacetate. Thus, in nonphotosynthetic eukaryotes, the mitochondrion is the site of most energy-yielding... 

The products suggest activation as a phosphoryl rather than as a nucleotidyl derivative. Both succinyl phosphate and thiophosphoryl coenzyme A have been suggested as intermediates. However, neither is included, at least as a freely dissociable intermediate, in current formulations of this reaction. An enzyme-bound phosphoryl histidine intermediate is thought to be involved, as is some sort of activated enzyme-CoA complex. Many aspects of the enzyme mechanism are still in doubt, but the sequence below is consistent with most available data. 

Succinyl CoA may yield succinate by at least three different pathways (see Fig. 1-15). It may be hydrolyzed by the action of succinyl CoA deacylase the cleavage of the coenzyme may be linked to the phosphorylation of IDP or GDP or the coenzyme may be transferred from succinate to a keto acid. 

So, the biosynthesis of methionine (Met, M), the first of the essential amino adds to be considered (Scheme 12.13), begins by the conversion of aspartate (Asp, D) to aspartate semialdehyde in the same way glutamate (Glu, E) was converted to glutamate semialdehyde (vide supra. Scheme 12.6). Phosphorylation on the terminal carboxylate of aspartate (Asp, D) by ATP in the presence of aspartate kinase (EC 2.7.2.4) and subsequent reduction of the aspart-4 yl phosphate by NADPH in the presence of aspartate semialdehyde dehydrogenase (EC 1.2.1.11) yields the aspartate semialdehyde. The aspartate semialdehyde is further reduced to homoserine (homoserine oxoreductase, EC 1.1.1.3) and the latter is succinylated by succinyl-CoA with the liberation of coenzyme A (CoA-SH) in the presence of homoserine O-succinyl-transferase (EC 2.3.1.46). Then, reaction with cysteine (Cys, C) in the presence of cystathionine y-synthase (EC 2.5.1.48) produces cystathionine and succinate. In the presence of the pyridoxal phosphate protein cystathionine P-lyase (EC 4.4.1.8), both ammonia and pyruvate are lost from cystathionine and homocysteine is produced. Finally, methylation on sulfur to generate methionine (Met, M) occurs by the donation of the methyl from 5-methyltetrahydrofolate in the presence of methonine synthase (EC 2.1.1.13). 

Energy of oxidation is conserved as it were in the form of a thioester bond between the succinyl group and CoA. Sanadi et oZ. have shown that the interaction of succinyl CoA with inorganic phosphate involves the following steps where CoP represents the phosphorylating coenzyme ... 

Succinyl CoA—ADP phosphorylation system—the nature of the reaction involving the newly discovered coenzyme (CoP) is not clear yet. 

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