Interaction of 6-thioguanylic acid homologues with ribonuclease T 1 . Spectrophotometric and gel filtration studies

Computer modeling studies on the subsite interactions of ribonuclease T1

The modes of binding of pGp,ApG,CpG and UpG to the enzyme ribonuclease T1 were determined by computer modeling. Essentially two binding modes are possible for all the four ligands--one with the 3'-phosphate group occupying the phosphate binding site (substrate mode of binding) and the second with the 5'-phosphate group occupying the phosphate binding site (inhibitor mode of binding). The latter binding mode is energetically favoured over the former and in this mode the base (G) and the 5'-phosphate moieties occupy the same sites on the enzyme as 5'-GMP when bound to RNase T1. The ribose moiety of pGp adopts a C3'-endo pucker form when bound to the enzyme and the glycosyl torsion angle will be in -syn range as 5'-GMP in the RNase T1-5'-GMP complex. Based on these results, a mechanism for the release of the product subsequent to cleavage of the substrate by the enzyme has been proposed. The amino acid residues Asn98 and Tyr45 are shown to form the subsites for the phosphate and the base respectively on the 5'-side of the guanine occupying the primary binding site. These studies also provide a stereochemical explanation for the specificity of the 1N subsite for adenine.

Circular dichroic spectra of 6-thioguanosine nucleotides and their complexes with myosin subfragment 1

The circular dichroic spectra of the thione form of 2-amino-6-mercapto-9 beta-ribofuranosylpurine 5'-triphosphate (thioGTP), thioGDP, thioGTP(gamma S), thioGMP-P(NH)P, and 6-mercapto-9 beta-ribofuranosylpurine 5'-diphosphate (thioIDP) and their complexes with myosin subfragment 1 and heavy meromyosin have been measured between 300 and 400 nm. The free nucleotides have a weak negative circular dichroic spectral peak at their absorption maxima, with a longer wavelength shoulder. On binding to the proteolytic fragments of myosin, the negative peak in enhanced approximately 10-fold and a longer wavelength positive peak appears. These effects are attributed to a change in the stereochemical structure of the nucleotide and specific interactions within the nucleotide binding site. All four thioguanosine nucleotides give these spectral changes although minor significant differences do occur. The CD spectra of the subfragment 1 steady-state complexes with thioGTP and thioGTP(gamma S) are similar to each other but different from the complexes of subfragment 1 with thioGDP and thioGMP-P(NH)P. Interactions between nucleotides in the complexes are excluded by a study of their spectra in 2-propanol when base stacking occurs. This is the first investigation where circular dichroism has been used to determine structural differences between different myosin--nucleotide states and provides evidence that the nucleotide in the gamma S bound state is similar to that of the triphosphate bound state.

The interaction of chromophoric nucleotides with subfragment 1 of myosin

The interaction of a series of chromophoric nucleotides derived from 6-mercapto-9-beta-ribofuranosylpurine (thioinosine, thiol) and 2-amino-6-mercapto-9-beta-ribofuranosyl-purine (thioguanosine, thioG) with myosin subfragment 1 isolated from rabbit skeletal muscle was investigated kinetically and spectroscopically. The Mg2+-dependent hydrolyses of thioITP and thioGTP are catalysed by subfragment 1 and probably proceed by a similar mechanism as for ATP hydrolysis, although with different rate constants. For example, the binary thioGDP-protein complex only comprises 8% of the steady-state intermediate of the thioGTPase at 5 degrees C and pH 6.5. Long-lived analogues of intermediates of the thioGTPase were generated by using thioGTP(gammaS) [thioguanosine 5'-(3-thio)-triphosphate], thioGMP-P(NH)P (5'-thioguanylylimidodiphosphate) and thioGDP. The near-u.v. spectra of the thioguanosine nucleotides bound to subfragment 1 were measured and showed that in all cases the purine ring is bound to the protein in a hydrophobic environment, although the pK of the purine thiol group only increases by 0.2-0.3. ThioGTP caused glycerinated rabbit psoas muscle to contract, but in contrast with thioITP was not able to relax muscle. The applications of these chromophoric nucleotides for investigating the mechanism of muscle contraction and other biological systems, particularly those involving guanosine nucleotide regulation, are discussed.