Muscle myosin II is studied by the same three measurements.

PHYSICS · Multi-part · Fermi University Physics · Biophysics & Medical Physics · Molecular machines, transport and bioenergetics · Molecular machines, transport and bioenergetics

Forces, Machines and Signals, Ch Enzymes and molecular motors

Problem

Muscle myosin II is studied by the same three measurements. ((a)) Its actin-activated ATPase rate is

v=5\,\mathrm{s^{-1}}

[\mathrm{ATP}]=50\,\mathrm{\mu M}

and

v=8\,\mathrm{s^{-1}}

[\mathrm{ATP}]=200\,\mathrm{\mu M}

. Find

k_{\rm cat}

and

K_M

. ((b)) The randomness parameter, computed from the waiting times between successive ATP turnovers of a single head, is

r=1.0

at all ATP concentrations tested. How many rate-limiting sub-steps does this indicate? ((c)) A single head stays attached to actin for

2\,\mathrm{ms}

out of a

50\,\mathrm{ms}

cycle. Compute the duty ratio, and use it to explain why a single myosin II head could not move a muscle fibre and why muscle instead uses large parallel arrays of heads.

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