The myosin head comprises of a motor domain (DM) and a lever arm. fig!. The MD consists of 4 subdomains: the converter, the N-terminal subdomain, and upper and lower 50-kDa subdomains. They are linked together by 3 single-stranded joints termed the switch II, the relay, and SH1 helix. Conformational changes in these flexible joints coordinate rearrangements of the MD subdomains enabling the acto-myosin contractile cycle. The relatively small rearrangements of the MD domains cause large movement of the lever arm and thus enables the power stroke. The MD undergoes many conformational changes as it traduces ATP hydrolysis to mechanical work. The different conformational states of myosin are termed strong or weak actin-binding states.

Nucleotide binding pocket: ADP + Mg

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Role of the subdomains and joints in the mechanism of the contractile cycle

The MD has different conformational states in each step of the contractile cycle. The conformation of the MD in each state depends on which nucleotide is bound to the active site (if any). In each structural state the conformation of the MD changes relatively little, but these changes are enough to cause a substantial difference in the position of the lever arm.

The 50-kDa upper and lower subdomains as well as the converter control the motor function of the myosin head by rotating around the N-terminal subdomain. The rotations depend on the conformational changes of the 3 joints; switch II, SH1 helix region, and relay. The joints work together in the transition between the different conformational states of MD to control the overall organization of the myosin head. They also allow communication between the nucleotide-bonding pocket, acting-binding interface and the lever arm.

Switch II, a catalytic loop of the nucleotide-binding pocket, moves in and out of the nucleotide-binding pocket during enzymatic activity. It is responsible for the unwinding of SH1 helix, along with the conformational changes caused by nucleotide binding. Upon unwinding helix SH1 uncouples the converter/lever module from the MD. Movement of the converter is controlled by the relay joint. The converter/relay module attains different conformations changing the position of the lever arm and thus giving rise to the different states of the actomyosin cycle.

Taking part in the organization of the different conformations of the contractile cycle is also the so called switch I, which is a second catalytic loop of the nucleotide-binding pocket. Switch II forms a specific salt bridge and hydrogen bond interactions with switch I that stabilize the pre-power stroke state.

In the pre-power stroke conformation of the MD, switch II interacts with the nucleotide-binding pocket and forms the stabilizing hydrogen bond interactions and a salt bridge with switch I. fig? Rotation of the 50-kDa upper subdomain away from the N-terminal subdomain pulls switches I and II apart breaking the protein- nucleotide interactions between switch I and ADP, as well as changing the conformation of switch II. These changes result in closing of the actin-binding site and opening of the nucleotide-binding pocket, leading to MgADP release. At the same timeSH1 helix is unwound and the lever arm is able to change its position enabling sliding of myosin through the actin filament.

Conclusions

Mg

Mg and its coordination with other atoms

ADP

Section 3

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