Phosphate transfers, bond cleavage

There are also examples in which phosphate esters of saturated alcohols are reductively deoxygenated.229 Mechanistic studies of the cleavage of aryl dialkyl phosphates have indicated that the crucial C-O bond cleavage occurs after transfer of two electrons.230... 

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. 

Both catalyze chain cleavage and transfer reactions (Eqs. 17-14 and 17-15) that involve the same group of substrates. These enzymes use the two basic types of C-C bond cleavage, adjacent to a carbonyl group (a) and one carbon removed from a carbonyl group ((3). Both types are needed in the pentose phosphate pathways just as they are in the citric acid cycle. The enzymes of the pentose phosphate pathway are found in the cytoplasm of both animal and plant cells.n7c Mammalian cells appear to have an additional set that is active in the endoplasmic reticulum and plants have another set in the chloroplasts.117c... 

AH fungal RNases (T, T , Ni, Ui, and U2) treated in this section catalyze the reaction shown in Fig. 1. The first step (phosphate transfer) is the cleavage of the phosphodiester bond between the 3 and 5 positions of the ribose moities in the RNA chain with the formation of nucleoside 2, 3 -cyclic phosphates and oligonucleotides with 2, 3 -cyclic phosphate at 3 terminal. The nature of the phosphodiester bonds to be cleaved depends on the base specificity of the enzyme. This phosphoryl transfer step is reversible. In the second step (hydrolysis), these terminal cyclic phosphate groups are hydrolyzed with the formation of corresponding 3 -phosphates. Because the first-step is usually faster than the second step, more or less accumulation of the cyclic phosphate may be observed. 

The results of that experiment allow one to synthesize a-D-ribose-l-[l80U]-phosphate which can be employed to determine the position of bond cleavage by other enzymes whose role is transfer of phosphate (Pj) to water or to another acceptor. We report results on a. the position of bond cleavage in R-l-P by PNP from human erythrocytes and E. coli as well as by alkaline phosphatase, acid phosphatase, formic acid andb. the position of bond making in ribose-5-phosphate by phosphoglucomutase. The earlier experiment from this laboratory employed the equilibration ... 

Position of Bond Cleavage or Formation in Some Other Phosphate Transfers... 

Mechanistically, it was first proposed that the enzyme—metal—substrate (EMS) complex involved first-sphere coordination of substrate to enzyme-bound metal ion. For transition metal ions, 3d orbitals were suggested to facilitate catalysis by promoting delocalization of electrons in the substrate, thereby aiding bond lengthening and breakage, especially for proteases catalyzing peptide bond cleavage . For phosphate transfer enzymes, a transition between rr-levels in the donor-acceptor (substrate-metal) was proposed as the mechanistic role for metal in catalysis . 

Thiamine pyrophosphate (TPP) is the coenzyme required by enzymes that catalyze the transfer of a two-carbon fragment. Biotin is the coenzyme required by enzymes that catalyze carboxylation of a carbon adjacent to a carbonyl group. Pyridoxal phosphate (PLP) is the coenzyme required by enzymes that catalyze certain transformations of amino acids decarboxylation, transamination, racemiza-tion, C —Cp bond cleavage, and a,j8-elimination. In a transimination reaction, one imine is converted into another imine in a transamination reaction, the amino group is removed from a substrate and transferred to another molecule. 

Uncatalyzed Reactions of Phosphate Monoesters. Phosphate monoesters in physiologically relevant pH ranges exist as either the monoanion or dianion. A large body of evidence indicates that uncatalyzed phosphoryl transfer reactions of both ionic forms takes place by loose transition states that are characterized by extensive bond cleavage to the leaving group and a small degree of bond formation to the nucleophile (for reviews of the evidence, see References (3,4)). Despite loose transition states, these reactions are concerted, and free metaphosphate is not formed in protic solvents (5). 

Fig. 3. Two mechanistic possibilities for phosphoryl transfer of the monoanionic species of a phosphate monoester (a) stepwise transfer of the proton to the bridge oxygen atom in a preequilibrium step (b) proton transfer concerted with P-0 bond cleavage.

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