guanosine-triphosphate and formycin-triphosphate

Guanosine triphosphate and formycin triphosphate (FTP) in the presence of excess Mg2+ can bind to empty non-catalytic sites of spinach chloroplast ATPase (CF1). This results in a greatly reduced capacity for ATP hydrolysis compared to the enzyme with non-catalytic sites filled with ATP. With two GTP bound at non-catalytic sites the inhibition is about 90%; with two FTP bound about 80% inhibition is obtained. Binding and release of the nucleotides from the non-catalytic sites are relatively slow processes. Exposure of CF1 with one or two empty non-catalytic sites to 5-10 microM FTP or GTP for 15 min suffices for about 50% of the maximum inhibition. Reactivation of CF1 after exposure to higher FTP or GTP concentrations requires long exposure to 2 microM EDTA. The findings show that, contrary to previous assumptions, GTP can bind tightly to non-catalytic sites of CF1. They suggest that the presence of adenine nucleotides at non-catalytic sites might be essential for high catalytic capacity of the F1 ATPases.

Topics: Adenosine Diphosphate; Adenosine Triphosphate; Bicarbonates; Binding Sites; Calcium; Chloroplasts; Formycins; Guanosine Triphosphate; Hot Temperature; Kinetics; Magnesium; Plants; Proton-Translocating ATPases; Ribonucleotides

Myosin subfragment-1 (S1) has been prepared from the fibrillar flight muscles of the giant water bug Lethocerus by chymotryptic digestion of myofibrillar suspensions in the absence of magnesium ions. The S1 obtained has a single light chain and a heavy chain with molecular weights of about 18 kDa and 90 kDa respectively. The kinetics of the elementary steps of the magnesium-dependent ATPase of insect S1 and rabbit S1 are similar, both with ATP and with ATP analogues as substrates. However, the presence of variable amounts of inactive protein within our preparation means that several rate constants cannot be obtained with as much precision in the case of insect S1. The most striking differences between the rabbit and insect S1 are values for the Vmax and the Km of actin during actin-activation of the MgATPase activity, which are up to an order of magnitude lower and greater in the insect than in the rabbit, respectively. The mechanical properties of strain activation and of capacity to do extended oscillatory work are unique to insect fibrillar flight muscle and distinguish it from vertebrate striated muscle. It is likely that these properties reflect differences in the organization of actin and myosin within the respective filament lattices rather than intrinsic differences in the ATPase mechanisms of the isolated myosin molecules from the two types of muscle.

Topics: Actomyosin; Adenosine Diphosphate; Adenosine Triphosphatases; Adenosine Triphosphate; Animals; Fluorescence; Formycins; Guanosine Triphosphate; Insecta; Kinetics; Molecular Weight; Myofibrils; Myosin Subfragments; Myosins; Peptide Fragments; Rabbits; Ribonucleotides; Suspensions; Thionucleotides

Topics: Adenosine Triphosphate; Aspartate Carbamoyltransferase; Binding Sites; Crystallography; Cytidine Triphosphate; Diphosphates; Escherichia coli; Ethenoadenosine Triphosphate; Formycins; Guanosine Triphosphate; Ligands; Models, Molecular; Phosphates; Ribonucleotides