8.1 Muscle Contactions & locomotion Flashcards
How does myosin motor protein attach to actin filament?
myosin motor protein only attaches to an actin filament that is oriented in the proper direction.
What are the steps of the myosin cross bridge cycle or the myosin ATPase cycle?
Myosin has no ATP, ADP or Pi associated w/it and it is bound tightly to an actin monomer w/in actin filament (rigor state)
1) The first event in the cycle is the binding of ATP to myosin. This causes myosin to be released from the actin filament. No shape change at this point only release of actin filament due to ATP binding.
2) The second step is the hydrolysis of ATP to ADP and inorganic phosphate. It’s this step that releases energy and the energy is used to change the shape of the myosin motor protein. Myosin protein moves forward but does not reattach to actin filament.
3) The third step is the release of inorganic phosphate, which allows myosin to reattach to the actin monomers. ADP is still held by the myosin protein at this point
4) The final step in the cycle is the release of ADP. When this happens, the myosin motor protein closes down and returns to its resting shape, however, there’s an actin filament attached to the myosin motor protein and so the actin filament moves as the myosin motor protein moves. So ultimately, it’s the release of ADP that actually causes the movement of an actin filament.
What is the state of the myosin motor protein before binding to ATP in the ATPase cycle?
In this state, the myosin motor protein is bound tightly to an actin monomer within an actin filament. This is sometimes called a rigor state because when muscles run out of ATP, they display a condition known as rigor (rigamortis aka true crime)
What is rigamortis?
the stiffening of muscle tissue soon after death when ATP production has stopped
In the myosin ATPase cycle what causes actin filament to move?
Release of ADP by myosin motor protein causes force generation and movement
Does myosin move toward the (+) or (-) side of active filament? What is the exception?
All myosins except myosin VI (6) move toward the plus end of actin filaments
Which part of the myosin protein is conserved and which part of myosin is variable? How is myosin variable?
The portion of the protein that hydrolyzes ATP and interacts with actin is highly conserved, but the rest of the proteins are variable.
Some of the variability involves:
- changes in the length of the myosin arm
- changes in the speed of myosin movement
- Changes in magnitude of force that a myosin motor protein can apply to an actin filament
- variations that allow tail domain to dimerize or interact w/specific targets that need to be moved in a cell.
What are the different specializations/variabilities of myosin motor protein?
Length of myosin arms - changes speed of myosin movement and force it applies to filament
variations that allow the tail domain of myosins to dimerize or to interact with specific targets that need to be moved around in a cell, such as membrane-bound organelles.
How is Contractile (contraction of muscle) force generated in all types of cells?
By the action of bipolar myosin II thick filaments.
The class of myosin involved in contraction is always myosin type II, which has a long tail domain used for dimerization and thick filament assembly.
Contraction is achieved by moving two points in a cell closer to each other. if you had two myosin molecule proteins oriented in opposite directions, they would pull two actin filaments toward each other, along with anything that was attached to the actin filaments. Such an organization would allow myosin motor proteins to conduct contraction.
How are myosin thick filaments organized?
1) first, single myosin motor proteins get together and form a dimer.
2) The dimers are then bundled together in thick cables, with some heads pointing in one direction and others pointing in the opposite direction.
3) Since the proteins are polarized in one of two directions, the filaments they form are called bipolar filaments
4) there’s also a bare zone with no myosin heads (bare zone)
How do smooth muscles assemble myosin think filaments as needed?
by regulating myosin light chains (phosphorylating these chains leads to muscle contraction_
What are different examples of striated muscle?
Striated muscle includes skeletal or voluntary muscle as well as cardiac muscle.
What is smooth muscle?
Smooth muscle tissues are the tissues that are involved in involuntary movements of our bodies, such as movement of the food through our digestive system, or the opening and closing of the iris of our eyes and the dilation or narrowing of our blood vessels. Smooth muscle also plays a major role in many parts of the male and female reproductive systems.
What are dense plaques in smooth muscle?
dense plaques serve as strong anchor sites for actin filaments in smooth muscle
How is contraction accomplished/regulated in smooth muscle?
1) in relaxed smooth muscle cells, the actin filaments are present and assembled and held in place by dense plaques, but myosin thick filaments are not.
2) instead, the myosin dimers are present in an inactive state, formed by the interaction of the myosin tail region with light chains near the motor portion of the molecule.
3) when the myosin light chains are not phosphorylated, they are sticky and attached to the myosin tail domain.
In this configuration, myosin cannot form thick filaments and is inactivated.
4) When smooth muscle needs to contract, the myosin light chains are phosphorylated by protein Myosin Light Chain Kinase.
5) this phosphorylation releases the tail regions of the myosin molecules which can then be used to form bipolar thick filaments.
Once this happens, contraction will occur. Thus contraction of smooth muscle is regulated by phosphorylation of myosin light chains by myosin light chain kinase.
Thick filaments pull on the actin filaments and shorten cell.
what would happen If you had two myosin molecule proteins oriented in opposite directions?
they would pull two actin filaments toward each other along with anything attached to actin filament. This would allow contraction to happen.
In smooth muscle, how are actin filaments held together?
Held by dense plaques, which serve as strong anchor sites. These dense plaques are formed by alpha actinin.
that actin filaments are already present in relaxed smooth muscle cells and they’re held in position by those dense plaques, but the myosin is present in an inactive, monomeric form, not in thick filaments. Once the thick filaments form, they pull on the actin filaments and shorten the cell.
What are the striations in striated muscle caused by?
The striations are caused by highly ordered arrangements of actin and myosin filaments
What’s the difference between skeletal muscles and cardiac muscle?
Skeletal muscle is multi nucleated, while cardiac is not
multinucleated skeletal muscle cells are formed by the fusion of individual muscle stem cells into a single giant cell and such a fusion of cells is known as a syncytium
What is the organization of striated muscle?
The cells are packed with long chains of structures that display a characteristic pattern of light (I bands) and dark bands (A-bands).
What are I bands?
I bands are isotropic and are light under the electron microscope
Isotropic bands look the same under polarized light microscopy no matter which way the muscle fibers are oriented.
What are A bands?
“A” bands are anisotropic and are dark under the electron microscope.
Anisotropic bands interact with polarized light because of the density of protein filaments in the region and so these bands look different under polarized light microscopy depending on which way the muscle fiber is oriented.
