ATPase cofactors

When H2O2 is a necessary component of a luminescence system, it can be removed by catalase. If a luminescence system involves superoxide anion, the light emission can be quenched by destroying O2 with superoxide dismutase (SOD). The ATP cofactor usually present in the fresh extracts of the fireflies and the millipede Luminodesmus can be used up by their spontaneous luminescence reactions, eventually resulting in dark (nonluminous) extracts containing a luciferase or photoprotein. The process is, however, accompanied by a corresponding loss in the amount of luciferin or photoprotein. The use of ATPase and the elimination of Mg2+ in the extract may prevent such a loss. 

The Li+-induced inhibition of the production of the HSV virus may be related to its actions upon viral DNA polymerase production and activity. Li+ reduces both the synthesis of DNA polymerase in tissue culture and the activity of DNA polymerase in vitro, each by about 50%. It has been proposed that Li+ reduces the biosynthesis of viral polypeptides and nucleic acids, and hence inhibits viral DNA replication by competition with Mg2+, a cofactor of many enzymes [243]. However, the inhibitory effect of Li+ on HSV replication in tissue culture is not affected by Mg2+ levels. A more likely hypothesis is the alteration of the intracellular K+ levels, possibly modifying levels of the high-energy phosphate compounds by replacement of either Na+ or K+ in Na+/K+-ATPase [244]. In tissue culture, HSV replication has been shown to be affected by the... 

Maruta, H. Korn, E.D. Acanthamoeba cofactor protein is a heavy chain kinase required for actin activation of the Mg -ATPase activity of Acanthamoeba myosin I. J. Biol. Chem., 252, 8329-8332 (1977)... 

An interesting point concerns the reversibility of all of the reaction steps shown in Scheme 12.6. This implies that under certain circumstances, the system might change its behaviour from an ATPase mimic to ATP synthase-type behaviour. This has been achieved through the use of a phosphorylating cofactor, acetyl phosphate (AcP, MeC02P032-). ar,d a divalent metal cation as a promoter (Mg2+ or Ca2+). Under... 

Figure 11. Proposed reaction cycles of the Na+-K+-ATPase (A) and the SR Ca2+-AT-Pase (B) involving transitions between different conformational states of the enzymes (see text for further explanation). The cytoplasmic side of the membrane is upward and the extracytoplasmic side downward. Brackets indicate that all the cation binding sites reside in an occluded state. A tentative H+-countertransport limb is shown for the SR Ca2+-ATPase (most likely n = 2). s indicates a relatively slow reaction step. ATP boxes indicate steps accelerated by ATP not being hydrolyzed. Mg2+ serving as a cofactor in phosphorylation and dephosphorylation is not shown.

Early life forms thriving near thermal vents in waters enriched in heavy metal ions would have had to have been endowed with mechanisms to deal with toxic metal ions and it is conceivable that efflux mechanisms for these metals evolved before or concomitandy with their use as cofactors. In line with such a hypothesis, the CPx-type ATPases encompass a wider spectrum of ion specificities than the non-heavy metal ATPases, now including Cu+, Ag+, Zn +, Cd +, and Pb. It is to be expected that other metal ions will be added to this list. ATPases transporting silver, zinc, cadmium, and lead are involved in bacterial resistance to these toxic metal ions, while copper-transporting ATPases have a role both in copper uptake to meet cellular demands and in copper extrusion when ambient... 

These enzymes are extraordinarily abundant over 1200 restriction endonucleases had been isolated and characterized by early 1990. Of three classes defined, type II restriction enzymes, which generally cut within their recognition sequences, have found uses in a host of biomedical research and diagnostic applications to be discussed below. Type 1 enzymes cut nonspecifically many nucleotides distal to specific recognition sequences and contain both restriction enzyme and DNA modification (see below) activities on different subunits of multienzyme complexes. Type III restriction enzymes share the multienzyme aspeas of type I enzymes but vary in other properties such as ATPase activity and cofactor requirements. 

Potassium is a cofactor and activates a large variety of enzymes, including glycerol dehydrogenase, pyruvate kinase, L-threonine dehydrase, and ATPase. Its acute toxicity is primarily due to its action as an electrolyte. Excessive or diminished potassium levels can disrupt membrane excitability and influence muscle cell contractility and neuronal excitability. 

Polypeptide binding and release by Hsp70 is coupled to its nucleotidebinding state. Furthermore, Hsp70 proteins often act in concert with cofactors involved in the regulation of its ATPase cycle. These cofactors thereby indirectly contribute to the process of polypeptide binding and release (see chapter by Mayer et al., in this volume). 

For systems that are naturally and reversibly photosensitive such as the carbon monoxide complexes of heme proteins [18,. 30, 31], initiation by a light pulse is possible. By preparation of inert but photoactivable reactant or cofactor precursors such as caged ATP [32-34], photosensitivity may be conferred on otherwise photoinert systems [21], thus extending the generality of this approach. This approach was first combined with x-ray monitoring by Blasie [35, 36], in studies of oriented multilayers containing the Ca2+-ATPase from sarcoplasmic reticulum. 

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