motor proteins
Summary
Motor proteins (myosin, kinesin, dynein) use energy from ATP hydrolysis to move along cytoskeletal filaments. Myosin works on actin; kinesin and dynein work on microtubules, with most kinesins moving toward the plus end and dynein toward the minus end.
Detail
Meaning: Motor proteins are ATPases that turn the energy of ATP hydrolysis into directed movement along cytoskeletal filaments. Myosin moves on actin (muscle contraction, cytokinesis). Kinesin and dynein move along microtubules, carrying vesicles and organelles. Most kinesins move toward the plus end and dynein toward the minus end. In myosin, ATP binding releases the head from actin, hydrolysis recocks it, and release of Pi drives the power stroke.
Equation: ATP + H₂O → ADP + Pi, with ΔG < 0 (J/mol). ΔG is the free energy change, ADP is adenosine diphosphate, and Pi is inorganic phosphate.
On the MCAT: Passages may describe a mutation or a nonhydrolyzable ATP analog and ask what stalls. Without ATP, myosin stays attached to actin (rigor), which explains rigor mortis. A common question is direction in a neuron: kinesin carries cargo along the axon toward the terminal (anterograde), and dynein carries it back toward the cell body (retrograde). Dynein also drives bending of cilia and flagella.
Common confusion: The filaments are the tracks, not the motors. Actin and microtubules provide the path, while myosin, kinesin and dynein do the stepping. Also, ATP is needed to release myosin from actin, not to attach it.
Sources
- Lehninger Principles of Biochemistry
- Molecular Biology of the Cell
- Biochemistry (Berg, Tymoczko, Stryer)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.