Catalytic mechanism of ATP hydrolysis in the myosin molecular motor.

Sonja Schwarzl, Jeremy C. Smith & Stefan Fischer*

Biochemistry 45, p.5830-5847, 2006. (full PDF)

Abstract

The acto/myosin motor converts chemical energy from ATP hydrolysis into mechanical work. After myosin binds ATP, it releases the actin fibril and swings its lever-arm from the post-power-stroke to the pre-hydrolysis conformation (this pathway has recently been described, Fischer et al. PNAS).  ATP hydrolysis then triggers the increase of affinity of nucleotide-bound myosin for actin, thus permitting the binding of myosin to actin before the next power stroke.  Here, the mechanism of ATPase catalysis by myosin II and its chemomechanical coupling to actin-binding were examined, using quantum mechanical/molecular mechanical (QM/MM) reaction path calculations with the Conjugate Peak Refinement (CPR) method.
The resulting reaction pathways are all associative with a pentavalent bipyramidal phosphorane transition state.  They differ in the activation mechanism of the attacking water molecule.  The bond between the Mg(2+) cation coordination sphere and the Switch-1 loop breaks in the product state.  This prepares for the opening of the Switch-1 loop that is believed to occur upon binding of actin and is necessary for the release of the hydrolysis products during the power-stroke.
 

The ATP-binding pocket in myosin is formed by three loops: the phosphate-binding loop (P-loop), the Switch-1 loop, and the Switch-2 loop .  Only the closed Switch-1 / closed Switch-2 (C/C) conformation (shown here) is catalytically active.  The arrow shows the attack of the water onto the gamma-phosphate.  The connection between Ser237 and the Mg(2+) (green) coordination sphere keeps Switch-1 closed in presence of ATP.  The paths show that immediately after hydrolysis, this connection is broken, preparing for the subsequent opening of Switch-1. Possible reaction pathways for ATP hydrolysis.  Three different mechanisms of water activation are shown: the purple arrow corresponds to the direct path, green arrows to the Ser236 path and red arrows to the Ser181 path.  The three water activation pathways are found to be equally likely.  The dashed arrow shows the attack of the 'activated water' (a) on the gamma-phosphate of ATP.  All the atoms treated quantum-mechanically are shown here explicitly.

"Direct path" :
Activation of the attacking water occurs by direct proton transfer of its proton to the gamma-phosphate of ATP.
At the transition state, the gamma-phospahate has trigonal bipyramidal geometry.
In the product state, the Mg(2+) moves between the beta and gamma phosphates, and away from Ser237, thereby breaking its coordination bond to the Switch-1 loop.  This weakens one of the strong interactions that maintain Switch-1 closed over the nucleotide.
Download the movie  (1.2 Mb)

"Ser236 path" :
Ser236 transfers its proton onto the gamma-phosphate, and the 'attacking water' (a) reorients into a position optimal for the transfer of its proton to Ser236.  The transition and product states are similar to the ones descibed for the "direct" path.
Download the movie  (1.3 Mb)

"Ser181 path" :
Ser181 transfers its proton to the gamma-phosphate, the 'helper water' (h) transfers a proton onto the sidechain of Ser181, and the 'attacking water' (a) transfers a proton onto the helper water.  The transition and product states are similar to the ones descibed for the "direct" path.
Download the movie  (2.1 Mb)

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