Acetyl phosphate cleavage

As for the acetyl phosphate monoanion, a metaphosphate mechanism has also been proposed 78) for the carbamoyl phosphate monoanion 119. Once again, an intramolecular proton transfer to the carbonyl group is feasible. The dianion likewise decomposes in a unimolecular reaction but not with spontaneous formation of POf as does the acetyl phosphate dianion, but to HPOj and cyanic acid. Support for this mechanism comes from isotopic labeling proof of C—O bond cleavage and from the formation of carbamoyl azide in the presence of azide ions. 

A reaction that is related to that of transketolase but is likely to function via acetyl-TDP is phosphoketolase, whose action is required in the energy metabolism of some bacteria (Eq. 14-23). A product of phosphoketolase is acetyl phosphate, whose cleavage can be coupled to synthesis of ATP. Phosphoketolase presumably catalyzes an a cleavage to the thiamin-containing enamine shown in Fig. 14-3. A possible mechanism of formation of acetyl phosphate is elimination of HzO from this enamine, tautomerization to 2-acetylthiamin, and reaction of the latter with inorganic phosphate. 

Glycine reductase is a complex enzyme530 564 566 that catalyzes the reductive cleavage of glycine to acetyl phosphate and ammonia (Eq. 15-61) with the... 

Some lactic acid bacteria of the genus Lactobacillus, as well as Leuconostoc mesenteroides and Zymomonas mobilis, carry out the heterolactic fermentation (Eq. 17-33) which is based on the reactions of the pentose phosphate pathway. These organisms lack aldolase, the key enzyme necessary for cleavage of fructose 1,6-bisphosphate to the triose phosphates. Glucose is converted to ribulose 5-P using the oxidative reactions of the pentose phosphate pathway. The ribulose-phosphate is cleaved by phosphoketolase (Eq. 14-23) to acetyl-phosphate and glyceraldehyde 3-phosphate, which are converted to ethanol and lactate, respectively. The overall yield is only one ATP per glucose fermented. 

Most of the compounds under discussion are quite labile in acid, but with two important exceptions they are resistant to mild alkaline hydrolysis (0.1-1 N base, 25°C). One exception is the mixed anhydrides of phosphate and carboxylic acids, such as acetyl phosphate. They are very susceptible to alkaline cleavage or cleavage by nucleophiles such as hydroxylamine. The other relatively alkali-labile phosphate compounds are phosphodiesters, in which the phosphate is attached to one of two vicinal hydroxyl groups (as in RNA or derivatives of phosphatidic acid). In these instances, the neigh-... 

Thiosugars can be rapidly synthesized when sulfur-substituted aldehydes are condensed with DHAP under the influence of FDP aldolase.42 The synthesis of these heterocycles is completed by phosphate cleavage, acetylation and reduction of the resultant ketone (Scheme 5.20). Cyclitols, another interesting class of biologically active compounds, have been prepared through the reaction of phosphonate- and nitro-substituted aldehydes with DHAP under the influence of FDP aldolase (Scheme 5.21) 43,436... 

For example, a number of studies have been made on the metal ion-catalyzed hydrolyses of acetyl phosphate and acetyl phenyl phosphate, " but the role of the metal ion in these processes remains uncertain. Investigations of catalysis by exchange labile metal ions e.g. Ca" and Mg") have yielded conflicting results and both the nature and distribution of the kinetically significant species, as well as the positions of bond cleavage, have yet to be determined unequivocally. Chelation, charge neutralization and attack by metal-bound hydroxide have variously been proposed as important factors in acyl phosphate hydrolysis. 

Pyruvate formate-lyase (EC 2.3.1.54 formate acetyltransferase PEL) catalyzes the key reaction in anaerobic glucose metabolism in bacteria, the coenzyme A-dependent dismutation of pyruvate into acetyl-CoA and formate. The reaction, first reported by Werkman and co-workers in the early 1940s (169, 170), was described as the phosphoroclastic cleavage of pyruvate because acetyl phosphate was detected as a product. Following the discovery of CoA and the elucidation of its role in acetyl transfer reactions (77/, 772), the intermediacy of acetyl-CoA in pyruvate dismutation was realized the overall reaction catalyzed by PFL is generally described by two half-reactions (Scheme 37). 

Cleavage of RNA using nuclease P- yields a mixture of nucleoside monophosphates, contaminated with oligonucleotides and other materials. The AMP present in this mixture can be converted selectively to ATP by treatment with acetyl phosphate and a mixture of adenylate kinase and acetate kinase. The resulting mixture can be used directly, without purification to supply ATP for use in cofactor recycling. 

In addition to a multiple-turnover mechanism, inversion of stereochemistry may also result from the intermediacy of a phosphorylated enzyme carboxyl group (Scheme 3a).Breakdown via C—O bond cleavage results in overall stereochemical inversion. Hydrolysis of acetyl phosphate, catalysed by divalent metal ions, proceeds by predominant C—O cleavage (7.4 < pH < 8.2) (Klinman and Samuel, 1971). 

The mechanism of the clastic cleavages of pyruvate, producing acetyl phosphate and formate or CO2 and Ha, is still obscure. Biotin (Schuster and Lynen, 1960), folic acid (Delavier-Klutchko, 1959), and vitamin B12 derivatives (Rabinowitz, 1960) have recently been implicated in these reactions. It seems possible that these may also be examples of tightly coupled systems in which 2-acetylthiamine pyrophosphate is an intermediate. 

In Acetobacter xylimim, carbohydrate is degraded by a phosphohexoketolase pathway a specific phos-phohexoketolase catalyses the phc horolytic cleavage of fructose 6-phosphate to o-erythrose 4-phosphate and acetyl phosphate. 

Lactobacillus, Leuconostoc and Bifidobacterium employ the PK pathway as the central fermentative pathway. PK catalyses the cleavage of a ketose phosphate (donor substrate such as o-xylulose 5-phosphate), utilizing an inorganic phosphate (acceptor substrate) to produce an aldose phosphate released from the donor (first product such as glyceraldehyde 3-phosphate), water (second product) and acetyl phosphate (third product) (Figure 4.3). The first half of the reaction of PK is identical to that of TK. However, the subsequent reaction catalysed by PK is distinct from TK (Yevenes and Frey 2008). PK releases a water molecule (dehydration) from the a,p-dihydroxyethyl ThDP intermediate to form the acetyl ThDP intermediate, and then a nucleophilic attack of the acceptor substrate (phosphate) on the acetyl ThDP intermediate yields the third product (acetyl phosphate). The crystal structures of the intermediates before and after the dehydration step have been reported (Suzuki et al. 2010). 

Phosphoketolase catalyses cleavage of sugar phosphates utilizing an inorganic phosphate to produce water, acetyl phosphate, and a shortened sugar phosphate. 

Preparation of PhAcOZ amino acids proceeds from the chloroformate, and cleavage is accomplished enzymatically with penicillin G acylase (pH 7 phosphate buffer, 25°, NaHS03, 40-88% yield). In a related approach, the 4-ace-toxy derivative is used, but in this case deprotection is achieved using the lipase, acetyl esterase, from oranges (pH 7, NaCl buffer, 45°, 57-70% yield). 

Ketols can also be formed enzymatically by cleavage of an aldehyde (step a, Fig. 14-3) followed by condensation with a second aldehyde (step c, in reverse). An enzyme utilizing these steps is transketolase (Eq. 17-15),132b which is essential in the pentose phosphate pathways of metabolism and in photosynthesis. a-Diketones can be cleaved (step d) to a carboxylic acid plus active aldehyde, which can react either via a or c in reverse. These and other combinations of steps are often observed as side reactions of such enzymes as pyruvate decarboxylase. A related thiamin-dependent reaction is that of pyruvate and acetyl-CoA to give the a-diketone, diacetyl, CH3COCOCH3.133 The reaction can be viewed as a displacement of the CoA anion from acetyl-CoA by attack of thiamin-bound active acetaldehyde derived from pyruvate (reverse of step d, Fig. 14-3 with release of CoA). 

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