Single-displacement Mechanisms

Creatine Kinase Acts by a Random, Single-Displacement Mechanism... 

An example of a random, single-displacement mechanism is seen in the enzyme creatine kinase, a phosphoryl-transfer enzyme that uses ATP as a phosphoryl... 

NAD -Dependent Dehydrogenases Show Ordered Single-Displacement Mechanisms... 

The lack of glycosyl transfer reaction is the class of pectinolytic hydrolases is in agreement with the observed inversion of the anomeric configuration of the newly formed reducing ends of the products. All three polygalacturonases studied here utilize the single displacement mechanism of hydrolysis. 

In discussing possible mechanisms for the reactions catalyzed by E. coli glutaminase in Section I, it was concluded that either a two-step acylation-deacylation pathway or a one-step route, displacement by the ultimate nucleophile, could be accommodated by the results. It may be noted that any single displacement mechanism for a group transfer reaction requires that both incoming and outgoing substituent groups associate with the enzyme at the same time... 

Although each pair of reactions is chemically matched their reaction mechanisms are quite different. The first member of each pair, 21 and 23, follow sequential kinetic pathways involving ternary complexes and inversion of configuration [88,89]. They seem to proceed by single displacement mechanisms. Their chemically matched... 

The nature of the action of the mutarotase from P. notatum has been investigated extensively by Bentley and Bhate,140 and compared with the acid-, base-, and solvent-catalyzed reactions (see also, p. 31). Through use of lsO on C-l, it was shown that dehydrogenations do not occur on C-l. In addition, no dehydrogenation occurred at carbon-bound hydrogen atoms a single-displacement mechanism was thus eliminated. The enzyme probably transfers a proton, in a process similar to that usually involved in nonenzymically catalyzed muta-rotations. 

Two kinds of information about nucleotidyltransferases and phosphotransferases are obtained by use of substrates or substrate analogs with chiral P. The stereochemical course of phosphoryl transfer and nucleotidyl transfer gives important information about the reaction mechanism. If inversion of configuration at phosphorus is observed, it may be concluded that an uneven number of displacements at phosphorus occurs in the reaction mechanism. If retention of configuration at phosphorus is observed, it may be concluded that the mechanism entails an even number of displacements at phosphorus. Inversion corresponds to the single-displacement mechanism of Eq. (2), and retention indicates a mechanism such as that of Eq. (3) or Eqs. (4a) and (4b). 

The nucleotidyl transfer step is reaction (29a), which proceeds with inversion of configuration at phosphorus in all of the aminoacyl-tRNA synthetase reactions so far studied [for amino acids (aa) Phe, He, Tyr, and Met] (89-92). Stereochemical inversion shows that the nucleotidyl transfer mechanism involves an uneven number of substitutions on phosphorus. Since no other evidence of an adenylyl-enzyme can be found, aminoacyl activation most likely occurs by a single-displacement mechanism, with direct transfer of the AMP group from ATP to the carboxylate group of the amino acid within the enzyme-amino acid-ATP complex. 

Acetyl-CoA synthetase from mammalian tissues and yeast catalyzes the reaction of acetate with ATP and CoA to form acetyl-CoA by a chemical mechanism similar to that of the aminoacyl-tRNA synthetases. The catalytic pathway is similar to that of reactions (29a) and (29b), substituting acetate for the amino acid and CoA for tRNA (93). The activation of acetate via the intermediate acetyl adenylate also occurs with inversion of configuration at P of ATP (94). Thus, as for aminoacyl-tRNA synthetases, acetyl-CoA synthetase appears to catalyze the activation of acetate by a single-displacement mechanism. 

Much more research will be required to address the question of the true nature of the transition states for enzymic substitution at phosphorus. The catalytic reaction pathways are now well understood with respect to the question of double-displacement and single-displacement mechanisms. Information about the nature of the transition states will require much more information about the structures of active sites, as well as many more structure-function studies on the enzymes for comparison with mechanistic information about comparable nonenzymic reactions. 

Scheme 9 The single-displacement mechanism of inverting glycosyltransferases. R, nucleotide X, divalent metal ion or charged side chains.

There is a great deal of evidence in favor of Wilson s mechanism, which is also supported by 0 work 43). I and II fall into the classes of double and single displacement mechanisms, respectively, for which stereochemical and exchange criterions have been advanced by Koshland 43). 

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