Coordinated enzyme phosphorylation

The hammerhead ribozyme and leadzyme belong to the second class of ribozymes. The short extra sequences of the ribozymes form the so-called catalytic loop which acts as the enzyme. There are two likely functions for metal ions in the mechanism of action of hammerhead ribozymes formation of metal hydroxide groups or direct coordination to phosphoryl oxygens. 

A typical chemical system is the oxidative decarboxylation of malonic acid catalyzed by cerium ions and bromine, the so-called Zhabotinsky reaction this reaction in a given domain leads to the evolution of sustained oscillations and chemical waves. Furthermore, these states have been observed in a number of enzyme systems. The simplest case is the reaction catalyzed by the enzyme peroxidase. The reaction kinetics display either steady states, bistability, or oscillations. A more complex system is the ubiquitous process of glycolysis catalyzed by a sequence of coordinated enzyme reactions. In a given domain the process readily exhibits continuous oscillations of chemical concentrations and fluxes, which can be recorded by spectroscopic and electrometric techniques. The source of the periodicity is the enzyme phosphofructokinase, which catalyzes the phosphorylation of fructose-6-phosphate by ATP, resulting in the formation of fructose-1,6 biphosphate and ADP. The overall activity of the octameric enzyme is described by an allosteric model with fructose-6-phosphate, ATP, and AMP as controlling ligands. 

Simple additions of such labile aquo ions as Mg, Ca, Zn and Mn "1", which are of importance in enzymic phosphoryl transfer, have resulted in only very modest catalytic effects for reactions of phosphate species (3). The much more effective t Com unit possesses the special advantage that it remains intact for long periods, while trans/cis isomerization and substitution in the fifth and sixth coordination sites proceed at moderately rapid and generally convenient rates. These characteristics make it particularly suitable for use in model studies (4). 

Mildvan AS, Grisham CM (1974) The Role of Divalent Cations in the Mechanism of Enzyme Catalyzed Phosphoryl and Nucleotidyl. 20 1-21 Mingos DMP, Hawes JC (1985) Complementary Spherical Electron Density Model. 63 1-63 Mingos DMP, Johnston RL (1987) Theoretical Models of Cluster Bonding. 68 29-87 Mingos DMP, McGrady JE, Rohl AL (1992) Moments of Inertia in Cluster and Coordination Compounds. 79 1-54... 

The current state of Ser/Thr phosphorylation of a protein is determined by the relative activity of Ser/Thr-specific protein kinase and protein phosphatase. It is therefore imderstandable that the cell has had to develop special mechanisms to balance the two activities with one another, and, when needed, to allow kinase or phosphatase activity to dominate. One of the best investigated examples of coordinated activity of protein kinases and protein phosphatases is the regulation of glycogen metabolism in skeletal muscle. Glycogen metabolism is an example of how two different signals, namely a cAMP signal and a Ca signal meet in one metabolic pathway and control the activity of one and the same enzyme. 

The examples of phosphorylase kinase and protein phosphatase I illustrate some important principles of regulation of enzyme activity by phosphorylation and dephosphorylation events. They clearly indicate how different signal transduction paths can meet in key reactions of metabolism, how signals can be coordinated with one another and how common components of a regulation network can be activated by different signals. The following principles are highlighted ... 

Highly coordinated chemical reactions, carried out simultaneously in response to the continuously changing cellular environment, are mediated by several enzymeregulating mechanisms. Enzymes are regulated by covalent (e.g., phosphorylation) and proteolytic modifications, binding to stimulatory and inhibitory proteins, and... 

The development of anion coordination chemistry and anion receptor molecules has opened up the possibility to perform molecular catalysis on anionic substrates of chemical and biochemical interest, such as adenosine triphosphate. The catalysis of phosphoryl transfer is of particular interest, namely in view of the crucial role of such processes in biology and of the numerous enzymes that catalyse them. 

There are two main differences between the binary and ternary complexes. In the binary complex, all three phosphoryl groups of dTTP are coordinated to Mn2+, but in the ternary complex with the DNA polymerase only the y-phosphoryl group remains coordinated, while Mn—/3-P and Mn-a-P distances are 4.9 and 4,2 A respectively. The cation has the role of linking the enzyme to the y-phosphoryl group of the substrate to assist the leaving of the pyrophosphate group. 

This enzyme is a non-specific phosphomonoesterase that shows maximum activity at pH values greater than 8.569 It also catalyzes the transfer of phosphoryl groups. These reactions involve the formation of a phosphoseryl intermediate and the hydrolyzed substrate. The phosphoenzyme may transfer the phosphoryl group to water or to an acceptor molecule to give a new phosphoester (equations 19 and 20, where E—P represents the covalently bound phosphoenzyme and E-P a non-covalent complex, in which phosphate is coordinated to the zinc). The phosphoenzyme may be formed from either direction. 

The system may be regarded as involving a Na+/Mg2+ co-catalysed phosphorylation step and a K+ catalysed dephosphorylation. Each phosphorylation/dephosphorylation step involves a pseudorotation of an Mg2+-stabilised 5-coordinate intermediate, resulting in transport of the alkali metal cations. The cation transport ability of the enzyme is a direct result of the enzymatic reactivity of the protein. There are three binding sites with high Na+ affinity and two with K+ affinity (occupied by Rb+ in the crystal structure determination). The structure (which is of the E2K state of the system) reveals that carboxy end of the a-subunit is held in a pocket in between transmembrane helices and acts as an unusual regulating element that controls sodium affinity and may be influenced by the membrane potential. 

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