Polymerization nucleotide effects

Polymer growth J(c) showed nonlinear monomer concentration dependence in the presence of ATP (Carrier et al., 1984), while in the presence of ADP, the plot of J(c) versus monomer concentration for actin was a straight line, as expected for reversible polymerization. The data imply that newly incorporated subunits dissociate from the filament at a slower rate than internal ADP-subunits in other words, (a) the effect of nucleotide hydrolysis is to decrease the stability of the polymer by increasing k and (b) nucleotide hydrolysis is uncoupled from polymerization and occurs in a step that follows incorporation of a ATP-subunit in the polymer. Newly incorporated, slowly dissociating, terminal ATP-subunits form a stable cap at the ends of F-actin filaments. 

A modification of the monosaccharide units of polysaccharides may obviously be effected at different stages of the biosynthesis of a polymer (a) prior to formation of the activated form of a monosaccharide, (b) at the level of glycosyl nucleotides, (c) at the stage of formation of oligosaccharide intermediates, and (d) after the synthesis of a polymeric chain. 

Effect of various templates used in DNA polymerization reactions. A free 3 -OH on a hydrogen-bonded nucleotide at the strand terminus and a non-hydrogen-bonded nucleotide at the adjacent position on the template strand are needed for strand growth. Newly synthesized DNA is shown in color. 

The fj-nitrobenzyl and p-nitrobenzyl ethers can be prepared and cleaved by many of the methods described for benzyl ethers. In addition, the o-nitrobenzyl ether can be cleaved by irradiation (320 nm, 10 min, quant, yield of carbohydrate 280 nm, 95% yield of nucleotide ). This is one of the most important methods for cleavage of this ether. These ethers can also be cleaved oxidatively (DDQ or electrolysis) after reduction to the aniline derivative." Clean reduction to the aniline is accomplished with Zn(Cu) (acetylacetone, rt, >93% yield).Hydrogenolysis is also an effective means for cleavage. A polymeric version of the o-nitrobenzyl ether has been prepared for oligosaccharide synthesis that is also conveniently cleaved by photolysis. An unusual selective deprotection of a bis-o-nitrobenzyl ether has been observed. The photochemical reaction of o-nitrobenzyl derivatives has been reviewed. ... 

Replication or duplication of DNA is a semiconservative process which depends upon base pairing, that is, hydrogen bonding. The double helical DNA partially unwinds and cellular nucleotide triphosphates pair with the exposed bases. Enzymes effect the polymerization process with the result being two DNA helices, each with a parent strand and a daughter strand. 

CD spectra in the near- and far-ultraviolet regions of the muscle proteins G- and F-actin, in combination with either ADP or 6-mercaptopurine nucleotide, and the effect of polymerization on the bound nucleotide were measured (352). Far-ultraviolet ORD and CD spectra of G-actin, measured under various conditions, were reported (353). 

Study of the kinetics of bovine GDH has been complicated both by the polymerization-depolymerization phenomena mentioned earlier (Section II,B) and by the allosteric effects of the coenzymes. An additional difficulty is that the purine nucleotide allosteric effectors infiuence the degree of polymerization of the protein. Although these phenomena complicate the interpretation of data, they may not be significant in a discussion of enzymic properties since the specific activity is independent of the degree of polymerization (18). Moreover, as already noted, the rat... 

Some proteins specific for GTP will accept 8-N3-GTP, but not 8-N3-ATP, as a specific photoprobe. Tubulin provides an example in the dark, 8-N3-GTP substitutes for GTP in promoting tubulin polymerization, whereas photolysis leads to incorporation into the j8 subunit of the heterodimeric protein (224, 225). Protection against photoincorporation of 8-N3-GTP was provided specifically by GTP but not ATP, indicating that the decreased incorporation could not be attributed to the trivial effect of light absorption by the added nucleotides. Photoaffinity labeling of the G and G protein subunits of retinal rod outer segments was also effected by 8-N3-GTP (226). 

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