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Skeletal Muscle Contraction
Skeletal Muscle Contraction (Cross-Bridge Cycle)
Summary
Key Steps
- ATP binds myosin, which causes its release from actin.
- ATP hydrolysis causes the myosin head to rotate.
- High energy myosin binds ATP (forms the cross-bridge).
- Phosphate release initiates the power stroke.
Myofibril internal histology
See: myofibril
Thick filaments
- Form from myosin
- The A band refers to the length of the thick filaments, "think "A" for d-a-rk – they are aniosotropic (or birefringent) in polarized light.
- H Zone is a zone of only thick filaments.
- M line bisects the A band.
Thin filaments
- Form from actin
- The I band is the region along the thin filaments (between the thick filaments).
- Think "I" for L-i-ght – they are "isotropic" (do not alter polarized light).
Z disks
- Transverse bands at the ends of the thin filaments.
Sarcomere
- The contractile unit of the myofibril.
- Comprises the area between the Z-disks.
Thin myofilament: Details
See: myofilament
- The thin filament slides towards the H zone.
- The (+) end attaches to the Z-disc sarcomere; the (-) end of the filament points toward the H zone.
Actin
- Spherical molecules joined in pairs of strands (like beads on a string). It is referred to as F-actin for filamentous actin, and comprises a polymer of G-actin monomers that are arranged in a double helix.
- There are myosin binding sites on actin and an ATPase site, an ATP-splitting site.
Tropomyosin
- Threadlike strands
Troponin
- Protein complexes that bind tropomyosin, actin, and also calcium (show their calcium-binding sites).
thick filaments: details
- Comprise myosin molecules (technically myosin II), which form a golfclub shape, and comprise two heavy chains and two light chains.
- The head forms from the heavy chain and contains the actin-binding site.
The "cross-bridge" is the bond between actin and myosin.
The Huxley Sliding-Filament Model
The rigor state.
- The myosin head is bound to the thin filament.
- Calcium is bound to troponin.
- Calcium binding to troponin allows myosin access to its binding site on actin.
ATP induces release of actin.
- Myosin has ATP bound to its head.
- The actin molecules are separated from (no longer bound to) the myosin.
- ATP is required to move out of the rigor state.
- If ATP is absent, which occurs after death, rigor will persist, called rigor mortis.
ATP is hydrolyzed to ADP and Inorganic phosphate (Pi).
- The myosin head rotates on the neck: it is now "cocked": it's in its high-energy state.
- The "cocked" state causes the thin and thick filaments to again bind via their cross-bridge.
- ADP and inorganic phosphate (Pi) are still bound to the myosin head.
Pi release initiates the power stroke for the myosin head to release its energy.
- Accordingly, the thin filament begins its slide.
The myosin returns to its uncocked, low energy state.
- At some point after the power stroke, ADP is released.
- Note that this is an area of intertextual variation, some authors instead write that ADP is released at the same time as phosphate to initiated the power stroke.
ATP generation: 3 Key Means
Direct phosphorylation:
- Creatine phosphate (a high-energy molecule stored in muscle cells) couples with ADP to make ATP and creatine.
- Creatine kinase catalyzes the reaction.
- As a corollary, creatine kinase is the best marker for muscle disease; it lies within muscle cells, for instance on the inner mitochondrial membrane, myofibrils, and sarcoplasm, and muscle injury releases it into the serum. Thus, muscle diseases are commonly associated with elevated creatine kinase levels.
Anaerobic glycolysis
See: glycolysis
- Nets 2 ATP molecules.
- Each glucose molecule is broken down into pyruvic acid molecules and ATP, but the pyruvic acid converts to lactic acid.
- Strenuous anaerobic exercise results in lactate build-up, which causes pain.
Aerobic respiration (which occurs in the presence of oxygen)
- Glucose is entirely broken down within the mitochondria to yield carbon dioxide, water, and large amounts of ATP.
- As a corollary, we see that in the presence of oxygen, cells can maximize ATP production.
Reference Note:
- We closely follow these steps as they are described in the 6th edition of Molecular Cell Biology, Chapter 17 – as numerous intertextual variations exist on this process.