CHEMOMECHANICAL TRANSDUCTION IN THE ACTOMYOSIN MOLECULAR MOTOR BY 2',3'-DIDEOXYDIDEHVDRO-ATP AND CHARACTERIZATION OF ITS INTERACTION WITH MYOSIN SUBFRAGMENT-1 IN THE PRESENCE AND ABSENCE OF ACTIN

The effect of torsional freedom about the N-glycoside bond of ATP in t he ability of the nucleoside triphosphate to support chemomechanical t ransduction (Takenaka et al., 1978) has been investigated by examining the ability of the nucleotide analogue 2',3'-dideoxy-2',3'-didehydro- ATP (Ib, enf-ATP) to act as a substrate for myosin subfragment 1 in th e presence and absence of actin and to support actin sliding in the st andard in vitro motility assay, By converting the ribosyl ring of the natural substrate to the rigid and almost planar enofuranosyl ring, ef fects on torsional freedom about the N-glycoside bond due to changes i n ribosyl ring pucker and/or by steric interferences of the protons at tached to the 2' and 3' carbons are eliminated allowing for increased torsional freedom about the N-glycoside tend. The data indicate that t his enofuranosyl analogue is an excellent substrate for subfragment 1 and actosubfragment 1 and produces actin sliding velocities which are twice as fast as those observed with ATP in the standard in vitro moti lity assay, The analogue diphosphate is trapped in S1 by the common P- i analogues, but the rate of formation of the ternary complex formed w ith V-i is very slow compared to that observed with MgADP, Similar con formations of S1 are formed with Mg enf-ATP and MgATP under steady-sta te conditions, but SI with bound Mg . enf-ADP differs significantly fr om that observed with MgADP.