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Query: EC:3.6.4.4 (kinesin)
 5,033 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The Acanthamoeba myosin-IA and myosin-IB molecular motors bind to membranes, so they may produce the force to move organelles and membranes along actin filaments. We have determined the rate constants for the actin-activated myosin-I ATPase by pre-steady state kinetic analysis. ATP binds rapidly to myosin-I and dissociates the enzyme from actin filaments at a rate > 500 s-1. Myosin-I hydrolyzes ATP to ADP and inorganic phosphate (Pi) at 20-50 s-1. Phosphate dissociation is the rate limiting step in the ATPase cycle, 0.01 s-1 for myosin-I alone and at 10 s-1 when myosin-I is bound to actin filaments. ADP dissociation is rapid. Phosphorylation controls the ATPase cycle by increasing the rate of phosphate release from myosin-I bound to actin. At steady state the major species are myosin-ATP and myosin-ADP-Pi, which rapidly bind to and dissociate from actin filaments. During the ATPase cycle myosin-I binds so weakly to actin filaments that it cannot support processive movement like kinesin, unless several motors cluster together on a membrane or actin filament. These properties of the enzyme emphasize the importance of characterizing mechanisms that promote the self-association of myosin-I isoforms at specific binding sites in cells.
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PMID:The chemical mechanism of myosin-I: implications for actin-based motility and the evolution of the myosin family of motor proteins. 911 40

Ncd is a kinesin-related motor protein which drives movement to the minus-end of microtubules. The kinetics of Ncd were investigated using the dimeric construct MC1 (Leu(209)-Lys(700)) expressed in Escherichia coli strain BL21(DE) as a nonfusion protein [Chandra, R., Salmon, E. D., Erickson, H. P., Lockhart, A., and Endow, S. A. (1993) J. Biol. Chem. 268, 9005-9013]. Acid chemical quench flow methods were used to measure directly the rate of ATP hydrolysis, and stopped-flow kinetic methods were used to determine the kinetics of mantATP binding, mantADP release, dissociation of MC1 from the microtubule, and binding of MC1 to the microtubule. The results define a minimal kinetic mechanism, M.N + ATP M.N.ATP M.N.ADP.P N. ADP.P N.ADP + P M.N.ADP M.N + ADP, where N, M, and P represent Ncd, microtubules, and inorganic phosphate respectively, with k(+1) = 2.3 microM(-1) s(-1), k(+2) =23 s(-1), k(+3) =13 s(-1), k(+5)= 0.7 microM(-)(1) s(-)(1), and k(+6) = 3.7 s(-)(1). Phosphate release (k(+4)) was not measured directly although it is assumed to be fast relative to ADP release because Ncd is purified with ADP tightly bound at the active site. ATP hydrolysis occurs at 23 s(-)(1) prior to Ncd dissociation at 13 s(-)(1). The pathway for ATP-promoted detachment (steps 1-3) of Ncd from the microtubule is comparable to kinesin's. However, there are two major differences between the mechanisms of Ncd and kinesin. In contrast to kinesin, mantADP release for Ncd at 3.7 s(-)(1) is the slowest step in the pathway and is believed to limit steady-state turnover. Additionally, the burst amplitude observed in the pre-steady-state acid quench experiments is stoichiometric, indicating that Ncd, in contrast to kinesin, is not processive for ATP hydrolysis.
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PMID:Kinetic studies of dimeric Ncd: evidence that Ncd is not processive. 1067 28

The chemical kinetic mechanism of kinesin (K) is considered by using a consensus scheme incorporating biochemically defined open, closed and trapped states. In the absence of microtubules, the dominant species is a trapped K*ADP state, which is defined by its ultra-slow release of ADP (off rate, k(off) approximately 0.002 s(-1)) and weak microtubule binding (dissociation constant, K(d) approximately 10-20 microM). Once bound, this trapped state equilibrates with a strongly binding open state that rapidly releases ADP (k(off) approximately 300 s(-1)). After ADP release, Mg*ATP binds (on rate, k(on) approximately 2 microM(-1)s(-1)) driving formation of a closed state that is defined by hydrolysis competence and by strong binding to microtubules. Hydrolysis (k(hyd) approximately 100-300 s(-1)) and phosphate release (k(off)>100 s(-1)) both occur in this microtubule-bound closed state. Phosphate release acts as a gate that controls reversion to the trapped K*ADP state, which detaches from the microtubule, completing the cycle.
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PMID:The kinetic mechanism of kinesin. 1527 84