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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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Conformational changes of the motor domain in the pre-power conformation

In the pre-power stroke conformation of the motor domain, switch II is bent and interacts with the nucleotide-binding pocket. Switch II also forms hydrogen bond interactions and a salt bridge with switch I stabilizing the pre-power stroke conformation. fig? Upon rotation of the 50-kDa upper subdomain away from the N-terminal subdomain, the actin-binding site is closed and the nucleotide-binding pocket is opened. Also, as switch I is pulled away from ADP it causes the protein-nucleotide interactions to break. As the 50-kDa upper domain rotates the conformation of switch II changes, opening the “back door” of the nucleotide-binding pocket leading to MgADP release.

Conclusions

Mg

Mg and its coordination with other atoms

ADP

Section 3

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Conclusions