Nucleotide hydrolysis, coupling with

The ATP Synthase from chloroplasts catalyzes ATP synthesis / hydrolysis coupled with a transmembrane proton transport. The enzyme contains several nucleotide binding sites and it is assumed that three of them (located on the [3 subunits) have catalytic properties (1,2). The presence of several catalytic sites raises immediately the question whether they work independently or cooperatively. 

Wegner also treated the case wherein assembly is coupled to nucleotide hydrolysis. Here, we consider a slight modification of his model to deal with the microtubule process. Normally, the concentration of GTP is maintained by use of a GTP-regenerating system (MacNeal et ai, 1977), and the system at the steady-state plateau of assembly can be described as in Scheme II. Under these conditions, the assembly-disassembly reactions are no longer reversible, and the primed rate constants are used to emphasize that we are dealing with a different case. The rate equations for the two ends are now given as ... 

A substantial number of observations have demonstrated that the stress-70 proteins bind denatured proteins, as well as some short peptides, and that ATP hydrolysis (possibly with concomitant release of Pj or ADP) results in the release of bound peptides (de Silva et ai, 1990 Hendershot, 1990 Hurtley 1989 Kassenbrock 1988 Palleros etai, 1991). Conversely, bindingof peptides to stress-70 proteins induces ATPase activity above basal levels peptide-induced ATPase activity is often used as a facile in vitro assay for stress-70 protein activity. Schematically, this activity can be subdivided into (1) ATP binding and hydrolysis, followed by product release (P and ADP), (2) peptide binding and release, and (3) a mechanism of coupling peptide binding/release and nucleotide hydrolysis/product release. 

EC 6 Ligases catalyze synthetic reactions These enzymes catalyze the joining together of two molecules commonly coupled with the hydrolysis of a pyrophosphate bond in nucleotide triphosphates. The systematic names are formed on the systan, X Y ligase. Use of the term, synthetase is recommended. 

The ATPase complex of chloroplasts (H -ATPase, ATP-synthase) carries out synthesis (hydrolysis) of ATP coupled with transmembrane transport of hydrogen ions. This complex consists of a hydrophylic catalytic part called coupling factor CF, and a hydrophobic part, CFq, the function of which is to translocate protons towards CF. CFj can bind as many as six nucleotide molecules / /. After CF precipitation by ammonium sulfate with subsequent gel filtration, the enzyme retains about 1 mol of tightly bound nucleotides consisting mainly of ADP /2/. 

The described above properties of the tight nucleotide-binding site are of particular interest when considered together with data on its ability to catalyze hydrolysis (Fig.lb) and synthesis 73,8/ of ATP. A probable mechanism of ATP synthesis including Interchanging affinities of the enzyme catalytic site for ADP and ATP coupled with protonation--deprotonation of specific acid-base groups, has been proposed earlier 79/. 

Both the 26S proteasome and the RC hydrolyze all four nucleotide triphosphates, with ATP and CTP preferred over GTP and UTP [58]. Although ATP hydrolysis is required for conjugate degradation, the two processes are not strictly coupled. Complete inhibition of the peptidase activity of the 26S proteasome by calpain inhibitor I has little effect on the ATPase activity of the enzyme. The nucleotidase activities of the RC and the 26S proteasome closely resemble those of E. coli Lon protease, which is composed of identical subunits that possess both proteolytic and nucleotidase activities in the same polypeptide chain. Like the regulatory complex and 26S proteasome, Lon hydrolyzes all four ribonucleotide triphosphates, but not ADP or AMP [18]. 

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