Practice question

Myosin architecture dissected by limited proteolysis yields fragments S1 head, heavy meromyosin HMM containing two heads plus coiled tail portion, and light meromyosin LMM tail. Crystallographic structures of Dictyostelium and chicken myosin II S1 show N terminal globular domain 80 kilodalton comprising upper 50k lower 50k subdomains forming actin interface, central 25k P loop subdomain containing nucleotide binding site with Walker A motif GXXXXGKT, Switch I 200 to 205, Switch II 454 to 459 coordinating MgATP, converter domain linking to lever arm. ATPase activity measured by phosphate release assay, actin activated ATPase 10 to 50 per second resides entirely in head; neck alone helical IQ motifs binding light chains lacks ATPase, tail forms coiled coil dimerization and filament formation domain no catalysis. Mutations in head Switch II cause hypertrophic cardiomyopathy by altering chemomechanical cycle, while tail mutations affect filament assembly. Therefore ATPase catalytic center localized to myosin head domain responsible for converting chemical energy into mechanical force and movement.