Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: EC:3.1.30.2 (endonuclease)
 18,621 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Enzymes of DNA synthesis, thymidine kinase (ATP-thymidine-5'-phospho-transferase, EC 2.7.1.21), DNA polymerase (EC 2.7.7.7) and nuclease activities were investigated in isolated purified nuclei of swine aorta. Thymidine kinase which is detectable in these nuclei can be stimulated by the addition of phospholipase C. DNA polymerase activity of isolated nuclei is strongly dependent on addition of an exogenous template; the preferred template is activated DNA. The activity in the absence of an added template is very low except when labelled dCTP is used as the precursor. This incorporation of labelled dCTP does not require the addition of the other three triphosphates, and under these conditions, dCTP seems to be incorporated into what may be a homopolymer. As with other tissues, solubilized preparations of aortic nuclei have two DNA polymerase activities which also prefer activated DNA template. There is no detectable endonuclease in aortic nuclei.
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PMID:Enzymes of DNA synthesis in isolated nuclei of swin aorta. 94 21

Alkaline phosphatase was the first zinc enzyme to be discovered in which three closely spaced metal ions (two Zn ions and one Mg ion) are present at the active center. Zn ions at all three sites also produce a maximally active enzyme. These metal ions have center-to-center distances of 3.9 A (Zn1-Zn2), 4.9 A (Zn2-Mg3), and 7.1 A (Zn1-Mg3). Despite the close packing of these metal centers, only one bridging ligand, the carboxyl of Asp51, bridges Zn2 and Mg3. A crystal structure at 2.0-A resolution of the noncovalent phosphate complex, E.P, formed with the active center shows that two phosphate oxygens form a phosphate bridge between Zn1 and Zn2, while the two other phosphate oxygens form hydrogen bonds with the guanidium group of Arg166. This places Ser102, the residue known to be phosphorylated during phosphate hydrolysis, in the required apical position to initiate a nucleophilic attack on the phosphorous. Extrapolation of the E.P structure to the enzyme-substrate complex, E.ROPO4(2-), leads to the conclusion that Zn1 must coordinate the ester oxygen, thus activating the leaving group in the phosphorylation of Ser102. Likewise, Zn2 appears to coordinate the ester oxygen of the seryl phosphate and activate the leaving group during the hydrolysis of the phosphoseryl intermediate. Both of these findings suggest that there may be a significant dissociative character to each of the two displacements at phosphorous catalyzed by alkaline phosphatase. A water molecule (or hydroxide) coordinated to Zn1 following formation of the phosphoseryl intermediate appears to be the nucleophile in the second step of the mechanism. Dissociation of the product phosphate from the E.P intermediate is the slowest, 35 s-1, and therefore the rate-limiting, step of the mechanism at alkaline pH. Since the determination of the initial crystal structure of alkaline phosphatase, two other crystal structures of enzymes involved in phosphate ester hydrolysis have been completed that show a triad of closely spaced zinc ions present at their active centers. These enzymes are phospholipase C from Bacillus cereus (structure at 1.5-A resolution) (43) and P1 nuclease from Penicillium citrinum (structure at 2.8-A resolution) (74). Both enzymes hydrolyze phosphodiesters. Substrates for phospholipase C are phosphatidylinositol and phosphatidylcholine, while P1 nuclease is an endonuclease hydrolyzing single stranded ribo- and deoxyribonucleotides. P1 nuclease also has activity as a phosphomonoesterase against 3'-terminal phosphates of nucleotides. The Zn ions in both enzymes form almost identical trinuclear sites.(ABSTRACT TRUNCATED AT 400 WORDS)
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PMID:Structure and mechanism of alkaline phosphatase. 152 73

Negative selection of self-reactive immature T cells is mediated by TCR engagement and is thought to occur via apoptosis (programmed cell death). The requirement for the co-receptors CD4 and CD8 in negative selection has been demonstrated, but the biochemical mechanisms underlying their involvement in this process remain undefined. Here we present evidence that co-receptor engagement dramatically enhances CD3-induced endonuclease activation and cell death characteristic of apoptosis in immature thymocytes. The responses are associated with increased tyrosine phosphorylation of a number of cellular substrates, including the gamma isoform of phospholipase C, and with increased association of tyrosine phosphoproteins, including the protein tyrosine kinase p56lck, with the TCR complex. Co-receptor engagement also potentiated CD3-mediated Ca2+ increases via a mechanism dependent upon tyrosine kinase activation. Sustained Ca2+ availability was found to be necessary for endonuclease activation and apoptosis to occur. We suggest that CD4 and CD8 may participate in negative selection by enhancing TCR/CD3-induced tyrosine kinase activation and sustained Ca2+ increases that lead to endonuclease activation and apoptosis in self-reactive CD4+ CD8+ thymocytes.
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PMID:Co-receptor (CD4/CD8) engagement enhances CD3-induced apoptosis in thymocytes. Implications for negative selection. 807 59