ATP analogs and muscle contraction: Mechanics and kinetics of nucleoside triphosphate binding and hydrolysis

The mechanical behavior of skinned rabbit psoas muscle fiber contractions and in vitro motility of F-actin (V(f)) have been examined using ATP, CTP, UTP, or their 2-deoxy forms (collectively designated as nucleotide triphosphates or NTPs) as contractile substrates. Measurements of actin-activated heavy meromyosin (HMM) NTPase, the rates of NTP binding to myosin and actomyosin, NTP- mediated acto-HMM dissociation, and NTP hydrolysis by acto-HMM were made for comparison to the mechanical results. The data suggest a very similar mechanism of acto-HMM NTP hydrolysis. Whereas all NTPs studied support force production and stiffness that vary by a factor 2 or less, the unloaded shortening velocity (V(u)) of muscle fibers varies by almost 10-fold. 2-Deoxy ATP (dATP) was unique in that V(u) was 30% greater than with ATP. Parallel behavior was observed between V(f) and the steady-state maximum actin-activated HMM ATPase rate. Further comparisons suggest that the variation in force correlates with the rate and equilibrium constant for NTP cleavage; the variations in V(u) or V(f) are related to the rate of cross-bridge dissociation caused by NTP binding or to the rate(s) of product release.