Phosphoryl group transfer kinases

Phosphoryl group transfer reactions either add or remove phosphoryl groups to or from cellular metabolites and macromolecules, and play a major role in biochemistry. Phosphoryl transfer is the most common enzymatic function coded by the yeast genome, and in addition to its importance in intermediary metabohsm (Chapter 5) the reaction is catalysed by a large number of central regulatory enzymes which often are part of signalhng cascades, such as protein kinases, protein phosphatases, ATPases, and GTPases. 

FIGURE 10,2 (a) yeast hexokinase (b) in its complex with glucose. (From Voet Voet, 2004 pp. 1591. Copyright 2004 with permission 

FIGURE 10.3 In the phosphoryl transfer reaction catalysed by hexokinase, the y-phosphoryl group of ATP inversion of configuration. (Adapted from Voet Voet, 2004 pp. 1591.) 

FIGURE 10.5 Stereoview of the reaction running through a bipyramidal pentavalent phosphorus atom. The y-phosphoryl group before and after the transfer is in a transparent mode. A putative Mg was placed at the expected position between Asp 10 and the p and y-phosphoryl groups. (From Grueninger Schultz, 2006. Copyright 2006 with permission from Elsevier.) 

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Phosphoryl group transfers from ATP result in an accumulation of ADP for example, when muscle is contracting vigorously, ADP accumulates and interferes with ATP-dependent contraction. During periods of intense demand for ATP, the cell lowers the ADP concentration, and at the same time acquires ATP, by the action of adenylate kinase ... 

Figure 9.45. Phosphoryl Group Transfer by Nucleoside Monophosphate Kinases. These enzymes catalyze the interconversion of a nucleoside triphosphate (here, ATP) and a nucleoside monophosphate (NMP) into two nucleoside diphosphates by the transfer of a phosphoryl group (shown in red).

Nucleotide Monophosphate Kinases Catalyze Phosphoryl-Group Transfer Without Promoting Hydrolysis... 

Phosphoryl group transfer. In this reaction ATP is synthesized as phos-phoglycerate kinase catalyzes the transfer of the high-energy phosphoryl group of glycerate-l,3-bisphosphate to ADP ... 

Mechanistically related enzymes that are ubiquitous in their requirement for a metal ion are those that catalyze phosphoryl group transfer reactions. These enzymes include the (phospho)kinases and phosphatases, which catalyze the transfer of a phosphoryl group (PO3) from the y-position of a nucleotide to an acceptor molecule (the phosphatases are a special case wherein the acceptor is the solvent water) nucleotidyltransferases and nucleases, which catalyze the transfer of a nucleoside phosphodiester to an acceptor molecule (the nucleases and phosphodiesterases are special cases wherein the acceptor is the solvent water) and phosphomutases. In all of these cases, the mechanism of phosphoryl group transfer can occur through one of several postulated pathways. The possible mechanistic extremes are those described as dissociative, analogous to an Sn 1 reaction. 

The catalytic pathway is best described as a random binding kinetic mechanism involving the formation of the ternary complex E-acetyl-P-ADP, with direct phosphoryl group transfer between enzyme-bound substrates to form the product ternary complex E-acetate-ATP. The formation and decomposition of these ternary complexes involve only noncovalent binding interactions of the enzyme with the substrates and products. The stereochemistry is inconsistent with a mechanism in which the phosphoryl group is transferred to an enzymic nucleophile as a step in the interconversion of the ternary complexes. The case of acetate kinase is one in which the stereochemical course of phosphoryl group transfer essentially discredited a double-displacement mechanism that had been reasonably well supported by other evidence. 

Phosphoryl group transfer from creatine phosphate to ADP is catalyzed by the enzyme creatine kinase. 

FIGURE 9.45 Phosphoryl group transfer by nucleoside monophosphate kinases. 

One of the best studied enzymes involving phosphoryl group transfer is pyruvate kinase, which catalyzes the transfer of a phosphoryl group from phosphoenolpyruvate to ADP. Pyruvate kinase requires a mono- and a di-valent cation for activation, and will be discussed in more detail in Section 62.1.3. It is well known that is the most effective monovalent activator. Figure 7 shows how the activity of the enzyme varies with the ionic radius of the monovalent cation. It is clear that NH4 (1.43 A), Rb (1.47 A) and Tl (1.47 A) are able to replace (1.33 A), while (0.68 A), Na" (0.97 A) and Cs" (1.67 A) are ineffective. Studies in which has been replaced by Tl have been most informative. Tl has an affinity some 50 times greater than for the potassium site, and so its binding could be detected more readily by equilibrium dialysis techniques. This showed that four TT cations were bound per molecule of enzyme, which has four subunits. 

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