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PubMed1977

Reactivity of essential thiols of myosin. Chemical probes of the activated state.

14C-Labeled fluorodinitrobenzene and N-ethylmaleimide have been used as chemical probes of the conformational states of myosin induced by the binding of MgADP and MgATP. The results indicate that in the high-energy conformation, MMgADP-Pi, the essential thiols are protected from modification but their diminished reactivity does not result from depletion of the reagent by reaction at nonessential thiols. The binding of MgADP to myosin exposes the essential thiols as reflected by an increased rate of their modification. The influence of the divalent cations Mg2+ and Ca2+ on the conformation of the M species has also been investigated. By monitoring the incorporation of fluorodinitrobenzene, the conformations of the M state in the presence of these cations can be clearly discerned.

Adenosine Diphosphate↗
PubMed1975

Mode of inhibition of herpes simplex virus DNA polymerase by phosphonoacetate.

Phosphonoacetate is a highly specific inhibitor of herpes simplex virus-induced DNA polymerase. Sensitivity of herpesvirus type 1 or type 2 induced DNA polymerase to the drug was similar. However, DNA polymerases from other sources such as the host cells (Wi-38), Micrococcus luteus, and hepatitis B virus were highly resistant. In addition, Escherichia coli RNA polymerase and reverse transcriptase of Rous sarcoma virus were also insensitive to the drug. Enzyme kinetic studies showed that inhibition was noncompetitive with respect to deoxyribonucleotide triphosphates. The Ki value was about 0.45 muM. The apparent Km values for dTTP, dATP, dCTP, and dGTP were 0.71, 0.75, 0.42, and 0.39 muM, respectively. The base composition of template has no profound effect on the extent of inhibition. The drug caused uncompetititve inhibition with respect to template which indicated that phosphonoacetate did not bind directly to template DNA. Results are presented which suggest that phosphonoacetate did not affect the formation of the enzyme-DNA complex but probably inhibited the elongation step of DNA polymerase reaction.

Acetates↗
PubMed1997

Zn2+ promotes the self-association of human immunodeficiency virus type-1 integrase in vitro.

It has been recently demonstrated that the Mg(2+)-dependent 3'-processing activity of purified human immunodeficiency virus type-1 (HIV-1) integrase is stimulated by the addition of exogenous Zn2+ [Lee, S. P., & Han, M. K. (1996) Biochemistry 35, 3837-3844]. This activation was hypothesized to result from integrase self-association. In this report, we examine the Zn2+ content of purified HIV-1 integrase by atomic absorption spectroscopy and by application of a thiol modification reagent, p-(hydroxymercuri)benzenesulfonate, with a metallochromic indicator, 4-(2-pyridylazo)resorcinol. We find that the Zn2+ content of HIV-1 integrase varies from 0.1 to 0.92 equiv of Zn2+ per monomer depending on the conditions of protein purification. In vitro activity assays, time-resolved fluorescence emission anisotropy, and gel filtration chromatographic analyses all indicate that EDTA yields an apoprotein which is predominantly monomeric and less active with Mg2+. Further, sedimentation equilibrium studies reveal that reconstitution of the apoprotein with Zn2+ results in a monomer-tetramer-octamer transition. These results suggest that Zn2+ promotes a conformation with enhanced oligomerization and thereby stimulates Mg(2+)-dependent 3'-processing. This may also imply that multimers larger than dimers (tetramers and possibly octamers) are required for in vitro activity of integrase in the presence of Zn2+ and Mg2+. It should be noted, however, that the content of Zn2+ did not significantly affect the 3'-processing and strand transfer reactions with Mn2+ in vitro.

Chromatography, Gel↗
PubMed1997

Metal ion catalysis during splicing of premessenger RNA.

The removal of intervening sequences from premessenger RNA is essential for the expression of most eukaryotic genes. The spliceosome ribonucleoprotein complex catalyses intron removal by two sequential phosphotransesterification reactions, but the catalytic mechanisms are unknown. It has been proposed that two divalent metal ions may mediate catalysis of both reaction steps, activating the 2'- or 3'-hydroxyl groups for nucleophilic attack and stabilizing the 3'-oxyanion leaving groups by direct coordination. Here we show that in splicing reactions with a precursor RNA containing a 3'-sulphur substitution at the 5' splice site, interaction between metal ion and leaving group is essential for catalysis of the first reaction step. This establishes that the spliceosome is a metalloenzyme and demonstrates a direct parallel with the catalytic strategy used by the self-splicing group I intron from Tetrahymena. In contrast, 3'-sulphur substitution at the 3' splice site provides no evidence for a metal ion-leaving group interaction in the second reaction step, suggesting that the two steps of splicing proceed by different catalytic mechanisms and therefore in distinct active sites.

Adenoviridae↗
PubMed1972

D-myoinositol 1:2-cyclic phosphate 2-phosphohydrolase.

1. An enzyme in extracts of mammalian tissues catalyses the hydrolysis of d-myoinositol 1:2-cyclic phosphate (an intermediary in the enzymic degradation of phosphatidylinositol) to produce d-myoinositol 1-phosphate. 2. The enantiomorph of the substrate is not attacked. 3. The pH optimum is about 8.1-8.3 and the reaction is stimulated by Mg(2+) ions. 4. Extracts from rat kidney cortex and medulla are very rich sources of the enzyme; brain, testis and small intestine contain intermediary activities, and other tissues contain very small amounts.

Animals↗
PubMed1973

Calcium ion-dependent p-nitrophenyl phosphate phosphatase activity and calcium ion-dependent adenosine triphosphatase activity from human erythrocyte membranes.

In the presence of ATP and of Mg(2+), human erythrocyte membranes show a phosphatase activity towards p-nitrophenyl phosphate which is activated by low concentrations of Ca(2+). The effect of Ca(2+) is strongly enhanced if either K(+) or Na(+) is also present. Activation of the p-nitrophenyl phosphate phosphatase by Ca(2+) reaches a half-maximum at about 8mum-Ca(2+) and is apparent only when the ion has access to the inner surface of the cell membrane. Ca(2+)-dependent phosphatase activity can only be observed if ATP is at the inner surface of the cell membrane, and the presence of ATP seems to be absolutely necessary, since either its removal or its replacement by other nucleoside triphosphates abolishes the activating effect of Ca(2+). The properties of the (ATP+Ca(2+))-dependent phosphatase are very similar to those of the Ca(2+)-dependent ATPase (adenosine triphosphatase), also present in erythrocyte membranes, which probably is involved in Ca(2+) transport in erythrocytes. The similarities suggest that both activities may be properties of the same molecular system. This view is further supported by the fact that p-nitrophenyl phosphate inhibits to a similar extent Ca(2+)-dependent ATPase activity and ATP-dependent Ca(2+) extrusion from erythrocytes.

Adenosine Triphosphatases↗
PubMed1988

Metal ion binding to D-xylose isomerase from Streptomyces violaceoruber.

The binding of two activating cations, Co2+ and Mg2+, and of one inhibitory cation, Ca2+, to D-xylose isomerase from Streptomyces violaceoruber was investigated. Equilibrium-dialysis and spectrometric studies revealed that the enzyme binds 2 mol of Co2+/mol of monomer. Difference absorption spectrometry in the u.v. and visible regions indicated that the environment of the first Co2+ ion is markedly different from that of the second Co2+ ion. The first Co2+ appears to have a six-co-ordinate. The conformational change induced by binding of Co2+ to the first site is maximum after the addition of 1 equivalent of Co2+ and yields a binding constant greater than or equal to 3.3 x 10(6) M-1. Binding of Co2+ to the second, weaker-binding, site caused a visible difference spectrum. The association constant estimated from Co2+ titrations at 585 nm agrees satisfactorily with the value of 4 x 10(4) M-1 obtained from equilibrium dialysis. Similarly, the enzyme undergoes a conformational change on binding of Mg2+ or Ca2+, the binding constants being estimated as 1 x 10(5) M-1 and 5 x 10(5) M-1 respectively. Competition between the activating Mg2+ and Co2+ and the inhibitory Ca2+ ion for both sites was further evidenced by equilibrium dialysis and by spectral displacement studies.

Aldose-Ketose Isomerases↗
PubMed1988

Isolation of an active form of the ATP + Mg2+-dependent protein phosphatase stimulated by the deinhibitor protein and by p-nitrophenyl phosphate.

A 75 kDa + 37 kDa-subunit high-Mr form of the ATP + Mg2+-dependent phosphatase, containing no modulator or inhibitor-1 and specific towards the beta-subunit of phosphorylase kinase, was isolated from dog liver. The phosphorylase phosphatase activity is stimulated by the deinhibitor protein, by p-nitrophenyl phosphate or by trypsin treatment. The sensitivity to modulator or inhibitor-1 can be increased dramatically by partial proteolysis. The enzyme inactivation by modulator after trypsin treatment can be reversed by protein kinase FA. The effects of p-nitrophenyl phosphate and the deinhibitor protein are synergistic, and p-nitrophenyl phosphate does not influence the sensitivity of the phosphatase to the heat-stable inhibitor proteins. These observations confirm that p-nitrophenyl phosphate stimulation is a suitable criterion for the identification of the active enzyme forms of the ATP + Mg2+-dependent phosphatase.

Adenosine Triphosphate↗