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Various chemical, physical and geological observations indicate that smectite clays are probably the major components of the Martian soil. Satisfactory ground-based chemical simulation of the Viking biology experimental results was obtained with the smectite clays nontronite and montmorillonite when they contained iron and hydrogen as adsorbed ions. Radioactive gas was released from the medium solution used in the Viking Labeled Release (LR) experiment when interacted with the clays, at rates and quantities similar to those measured by Viking on Mars. Heating of the active clay (mixed with soluble salts) to 160 degrees C in CO2 atmosphere reduced the decomposition activity considerably, again, as was observed on Mars. The decomposition reaction in LR experiment is postulated to be iron-catalyzed formate decomposition on the clay surface. The main features of the Viking Pyrolytic Release (PR) experiment were also simulated recently (Hubbard, 1979) which the iron clays, including a relatively low '1st peak' and significant '2nd peak'. The accumulated observations on various Martian soil properties and the results of simulation experiments, thus indicate that smectite clays are major and active components of the Martian soil. It now appears that many of the results of the Viking biology experiments can be explained on the basis of their surface activity in catalysis and adsorption.
J Mol Evol 1979 Dec
PMID:Smectite clays in Mars soil: evidence for their presence and role in Viking biology experimental results. 4 7

Theoretical conformational analysis of the antibiotic gramicidin A HCO--L-Val--Gly--L-Ala--D-leu--L-Ala--D-Val--L-Val--D-Val--(L-Trp--D-Leu)3--L-Trp--NHCH2CH2OH has been carried out by stagewise computations of a serie of LD penta-decapeptide analogs, which approximated the structure of the natural compound at the final stage. The potential surface of the LD-peptide skeleton of the gramicidin molecule is shown to predetermine the existence of a set of pi4LD--Pi6LD structures. Low-energy helical structures with no hydrogen bonds have also been revealed, which are due to compensational relations between hydrogen bonding and nonbonded energies. Inclusion of D-Val into the amino acid sequence discriminate against alpha-helix, while Trp and Leu residues contribute to a formation of pi4LD and pi6LD helices and to a reduction of energy differences between them. Conformational properties and geometrical parameters of the lowest-energy helical structures of gramicidin provide transport of protones and of all alkali metal ions. A mechanism of cation transportation through the gramicidin trans-membrane channel is discussed.
Mol Biol (Mosk)
PMID:[Conformational state and mechanism of functioning of gramicidin A]. 8 46

Data on the kinetics of 1H greater than 3H exchange between water and C(8)H groups of guanylic residues in the poly(rG) and poly poly(rG)-poly(rC) are presented. Furthermore, optical properties (CD spectra and hyperchromism) of neutral solutions of these polymers from 20 to 100 degrees C are described. It is shown that the exchange in poly(rG) within the temperature range from 20 to 80 degrees C proceeds faster than in rGMP. Within the temperature range from 20 to 40 degrees C such an acceleration of the exchange is observed also in poly(rG)-poly(rC). According to the ylide mechanism of the exchange reaction the observed accleration of the exchanged in in C(8)H groups of guanylic residues is considered as a consequence of an increase of the positive charge at N(7) atoms. This effect is due to formation of additional hydrogen bonds in which N(7) atoms take part. The exchange in poly(rG)-poly(rG) at temperatures hihger than 75 degrees C, when these additional hydrogen bonds are absent, proceeds more slowly than in rGMP. Such picture is usual in other previously studied polynucleotides whose structure in solution is stabilized only by Watson - Crick hydrogen bonds and stacking interactions. The data obtained support a Guschelbauer's model of the four-stranded stranded poly(rG). They also indicate the posibility of associates formation in poly(rG)-poly(rC) solutions at temperature lower than 40 degrees C being stabilized by hydrogen bonds in which N(7) atoms of guanylic residues take part.
Mol Biol (Mosk)
PMID:[Reaction capabilities and structure of poly(rG) and poly(rG)-poly(rC) in solution by the method of the kinetics of hydrogen ion exchange]. 18 2

Polarity of double and ternary water-nonelectrolyte systems at the component ratio, corresponding to a half-transition point of DNA from B to A form was evaluated from ESR spectra of a spin-probe. In all cases examined the isotropic super-fine splitting constant (aN) is the same with an accuracy of 0.05 gauss. Small differences in aN are well correlated with the concentration of the groups which are able to form hydrogen bonds with the nitroxide fragment of the radical. Thus, media polarity is a factor which determines the A--B equilibrium of DNA in solution.
Mol Biol (Mosk)
PMID:[Polarity of the environment as a factor determining DNA conformation]. 20 79

The sensitivity of the molybdenum-iron(MoFe)-protein of Clostridium pasteurianum nitrogenase toward oxidation has been studied by determining the enzymatic activity of this component after incubating it anaerobically in ferricyanide solutions of various oxidizing strengths (as measured by their oxidation potentials). It was found that the MoFe-protein remains active at potentials up to +350 mV (vs. standard hydrogen electrode) but becomes readily inactivated at more oxidizing potentials, after a lag period, depending on the potential level and temperature. Oxidative inactivation by ferricyanide results in the release of most of the Mo, Fe and S atoms from the protein which causes the loss of the absorption bands in the visible region. The metals and sulfur could be re-incorporated by incubation in a mixture containing thiol, sulfide, molybdate, and ferric iron. The EPR spectrum of the oxidatively inactivated MoFe-protein showed that both the high- and low-field signals are readily affected. Re-incorporation of the metals and sulfur into the "bleached" protein produced an EPR spectrum similar to that of the air-inactivated protein. Incubation of the Mo-Fe-protein with mersalyl abolished its enzymic activity. The difference spectrum before and after mersalyl treatment resembles that of the soluble spinach ferredoxin.
Mol Cell Biochem 1979 Jul 31
PMID:Oxidative inactivation of the molybdenum-iron-protein component of nitrogenase from clostridium pasteurianum. 22 73

Adenosylcobalamin-dependent rearrangements are enzyme catalyzed reactions in which a hydrogen atom is transfered from one carbon atom to an adjacent one in exchange for a group X which migrates in the opposite direction. In the hydrogen transfer step, the mechanism of which is reasonably well understood, the cofactor serves as an intermediate hydrogen carrier. The transfer of hydrogen to the cofactor involves homolysis of the carbon-cobalt bond to generate cob(II) alamin and the 5'-deoxyadenos-5'-yl radical, followed by abstraction of a hydrogen atom from the substrate to form 5'-deoxyadenosine and the substrate radical. After migration of group X, the hydrogen atom is returned to the product radical by the reverse of the above reactions to generate the final product and reconstitute the cofactor. In contrast to the transfer of hydrogen, the mechanism of group X migration is poorly understood. Many reactions mechanisms have been proposed on chemical grounds, but there is insufficient biochemical evidence to permit a choice among these propsals. A quantity of negative evidence has accumulated suggesting that group X migration does not involve alkylation of the cobalt of cobalamin by the substrate, but in the absence of firm data supporting an alternative mechanism, even this weak conclusion must be regarded as provisional.
Mol Cell Biochem 1977 Apr 12
PMID:The mechanism of cobalamin-dependent rearrangements. 30 95

Two types of reactivities of thiophosphates have been demonstrated: one being nucleophilic displacement by the P-S moiety of nucleoside phosphorothioates and the other, phosphorylation via P-S cleavage as the driving force. We have designed a system where both displacement on carbon and P-S cleavage are possible. Adenosine derivatives have been synthesized with 5'-deoxy-5'-chloro and 5'-O-tosyl substitutions as leaving groups utilizing the 3'-O-phosphorothioate as the biphilic center. The main products of cyclization were 5'-O-tosyl and 5'-chloroadenosine 2':3'-cyclic phosphate. Formation of 3':5'-S-phosphorothioate was slow even using an excellent leaving group. This is possibly due to hydrogen bonding between the 2'-OH and the neighboring P-O.--KOH hydrolysis of the cyclic phosphorothioate yielded 2'(3') phosphorothioates in a 1:1 ratio. The 2' and 3' isomers were separated and used to study the relative rates of cyclization. The cyclization via P-S cleavage of 2'(3')-O-phosphorothioates showed that the 2' isomer was more reactive. This is the first report of superior reactivity of the 3'-OH of a ribonucleoside.
J Mol Evol 1978 May 12
PMID:Mechanistic possibilities in prebiotic thiophosphate chemistry. 30 64

The kinetics of hydrogen exchange of collagens from different animals was studied by the radioisotopic method (tritium) and infrared spectroscopy (deuterium). It has been shown that collagens from different animals (rat, pike, cod, carp, frogs) differ in amino acid composition and thermostability but are similar in the amount of slowly exchanged hydrogens. All the studied collagens have (1.00 +/- 0.05) very slowly exchanged hydrogens per triplet and (0.6 +/- 0.1) slowly exchanged hydrogens per triplet. Identifying the quantity of slowly exchanged hydrogens with the quantity of hydrogen bonds in the macromolecule, it can be concluded that collagens differing in stability do not differ by the quantity and composition of intramolecular hydrogen bonds.
Mol Biol (Mosk)
PMID:[Number of hydrogen bonds in the structure of collagen]. 30 13

Joint molecules of lambda DNA formed in the absence of DNA replication, which may be involved in the process of genetic recombination can be observed as branched DNA derived from different phage particles. These molecules are associated through base-pair hydrogen bonding in synaptic regions, usually with short single-stranded gaps. Furthermore, joint molecules could be accumulated up to ten fold when lambda was irradiated with ultraviolet light before infection of polI mutant of E. coli. Infection at low multiplicity did not give rise to joint molecules. These results suggest that single-strand breaks and gaps introduced in duplex lambda DNA facilitate the formation of joint molecules.
Mol Gen Genet 1977 Jan 07
PMID:Joint molecules of lambda DNA as an intermediate of genetic recombination. 31 43

Association of the apo-beta 2 and the holo-(beta-PLP)2 subunits of tryptophan synthase from Escherichia coli (L-serine hydro-lyase (adding indole) (EC 4.2.1.20)) with alpha subunits of the same enzyme has been studied by microcalorimetry. The results obtained from thermometric titrations clearly demonstrate that only the native complex alpha2beta 2 is formed, independent of an excess of alpha protein. The reaction of the holo-(beta-PLP)2 with alpha subunits at 25 degrees C is accompanied by a negative enthalpy change, which is almost twice as large as that for complex formation with the apo-beta 2 protein, thus indicating that the interaction enthalpy becomes more favorable in the presence of the coenzyme pyridoxal 5'-phosphate (PLP). Both reaction enthalpies show very large negative temperature coefficients, -3600 +/- 100 cal K-1 (Mol of beta 2)-1 being the value for the formation of the apoenzyme and -2300 +/- 100 cal K-1 (mol of beta 2)-1 pertaining to formation of the holoenzyme. The studies on the association of alpha and beta2 subunits in the two buffers revealed that at 25 degrees C approximately 0.75 proton are absorbed in the presence and absence of the coenzyme, whereas at 35 degrees C one proton is taken up from the solution when PLP is present, but two if the apo-beta 2 complex reacts. These results are a clear indication of energetic linkage between intersubunit interaction, hydrogen ion equilibria, and the binding of the coenzyme.
 ...
PMID:Subunit interaction in tryptophan synthase of Escherichia coli: calorimetric studies on association of alpha and beta 2 subunits. 37 96


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