title: Magnesium ion dependent rabbit skeletal muscle myosin guanosine and thioguanosine triphosphatase mechanism and a novel guanosine diphosphatase reaction.
abstract: The mechanism of the Mg2+-dependent myosin subfragment 1 catalyzed hydrolysis of GTP and 2-amino-6-mercapto-9-beta-ribofuranosylpurine 5'-triphosphate (thioGTP) has been investigated by rapid-reaction techniques. The myosin was isolated from rabbit skeletal muscle. The steady-state intermediate of these reactions consists pre-dominantly of a protein-substrate complex unlike the myosin subfragment 1 ATPase reaction which has a protein-products complex as the principal steady-state component. The mechanism of GTP hydrolysis catalyzed by subfragment 1 has other marked differences from the ATPase mechanism. The second-order rate constant of binding of GTP to subfragment 1 is tenfold greater than that for GDP binding. The dissociation rate constant of GDP from subfragment 1 is 0.06 s-1 compared with the subfragment 1 catalytic center activity for GTP hydrolysis of 0.5 s-1 at pH 8.0 and 20 degrees C. This shows that GDP bound to subfragment 1 forms a complex which is not kinetically competent to be an intermediate of the GTPase mechanism. GDP is hydrolyzed in the presence of subfragment 1 to GMP and Pi. The subfragment 1 GTPase mechanism has a nuber if features in common with that of the elongation factor Tu GTPase of the protein biosynthetic system of Escherichia coli.
mesh_terms: D000818:Animals; D020558:GTP Phosphohydrolases / Q000378:metabolism*; D006153:Guanosine Diphosphate; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology*; D009132:Muscles / Q000201:enzymology*; D009218:Myosins / Q000378:metabolism*; D010744:Phosphoric Monoester Hydrolases / Q000378:metabolism*; D011755:Pyrophosphatases; D011817:Rabbits; D013873:Thionucleotides
chemical_list: D013873:Thionucleotides; D006153:Guanosine Diphosphate; D010744:Phosphoric Monoester Hydrolases; D020558:GTP Phosphohydrolases; D011755:Pyrophosphatases; C010357:guanosine-diphosphatase; D009218:Myosins; D008274:Magnesium