Kinases transfer

Kinases are the class of enzymes which transfer the terminal phosphate group from ATP to a substrate. In particular, tyrosine kinases transfer the phosphate to a tyrosine residue... 

In the last step, pyruvate kinase transfers this residue to ADP. The remaining enol pyruvate is immediately rearranged into pyruvate, which is much more stable. Along with step [7] and the thiokinase reaction in the tricarboxylic acid cycle (see p. 136), the pyruvate kinase reaction is one of the three reactions in animal metabolism that are able to produce ATP independently of the respiratory chain. 

The serine/threonine protein kinases transfer a phosphate from ATP to a serine or threonine residue in the target protein. This class of enzymes are generally associated with cytoplasmic signaling events. 

The tyrosine protein kinases transfer phosphate from ATP to tyrosine residues in the target protein and are generally associated with receptors that become activated after binding a growth factor or other ligand. 

FIGURE 15-14 Protein phosphorylation and dephosphorylation. Protein kinases transfer a phosphoryl group from ATP to a Ser, Thr, or Tyr residue in a protein. Protein phosphatases remove the phosphoryl group as P . 

Although inversion was not observed with the E. colt alkaline phosphatase, it has been observed for ribonucleases and many other hydrolytic enzymes and for most kinases transferring phospho groups from ATP. The difference lies in the existence of a phospho-enzyme intermediate in the action of alkaline phosphatase (see Eq. 12-38). Each of the two phosphotransferase steps in the phosphatase action apparently occurs with inversion. The simplest interpretation of all the experimental results is that phosphotransferases usually act by in-line -like mechanisms which may involve metaphosphate-ion-like transition states that are constrained to react with an incoming nucleophile to give inversion. An adjacent attack with pseudorotation would probably retain the original configuration and is therefore excluded. 

Kinases transfer phospho groups from polyphosphates such as ATP to oxygen, nitrogen, or sulfur... 

Most kinases transfer chiral phospho groups with inversion and fail to catalyze partial exchange reactions that would indicate phosphoenzyme intermediates. However, nucleoside diphosphate kinase contains an active site histidine which is phosphorylated to form a phosphoenzyme.869 The enzyme catalyzes phosphorylation of nucleoside diphosphates other than ADP by a nucleotide triphosphate, usually ATP. 

Available evidence (14,15) favors the pathway for pyruvate kinase by way of phosphorylation of pyruvate enol. Furthermore, J. Knowles and his coworkers (16,17), using chiral thiophosphates and chiral (160,170,180) phosphate have shown that pyruvate kinase transfers phosphate from phosphoenolpyruvate to ADP with stereochemical inversion at phosphorus. Since monomeric metaphosphate is presumably planar, a chemical reaction by way of that ion should proceed with racemization. In the active site of an enzyme, however, all components might be held so rigidly that racemization need not occur. Furthermore, no information is yet available on the detailed mechanism of reactions catalyzed by cytidine synthetase our own experiments, designed to distinguish among the mechanisms here discussed, are as yet incomplete. 

An alternative to the two-stage end-labeling protocol involves a phosphate exchange reaction catalyzed by polynucleotide kinase. In the presence of excess amounts of ADP, polynucleotide kinase transfers a 5 -phosphoryl residue from DNA or RNA to ADP in a reversal of the phosphorylation reaction (5,12). At pH... 

A kinase transfers a phosphoryl group from one molecule to another... 

Other examples of crossover activity are known. ASA has been shown to possess cyclic phosphodiesterase activity. Adenylate kinase transfers the 7-phosphoryl group of ATP, but can also accept as a substrate the sulfuryl analogue 7-sulfiiryl-ADP. Sulfoenolpyruvate, the sulfate analogue of phosphoenolpyruvate, is a substrate for pyruvate kinase, producing pyruvate and adenosine-5 -sulfatopyrophosphate. Many other examples probably remain undiscovered. 

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