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#troponin

6 public questions tagged with this topic.

Cytoskeleton

The interaction between myosin and actin filaments is regulated by:

Coordinated muscle contraction depends on interplay between chemical fuel and regulatory ion signals ensuring contraction only when needed. ATP provides both detach signal and energy currency: ATP binding to myosin head after power stroke dissociates rigor cross bridge, while its hydrolysis to ADP Pi stores elastic energy in lever arm for next stroke, and Pi plus ADP release execute mechanical work. Without ATP muscle enters rigor state as observed in cadaveric stiffness. Calcium provides temporal switch: at rest sarcoplasmic reticulum SERCA maintains cytosolic calcium about 100 nM insufficient for troponin C binding keeping tropomyosin blocked and myosin light chain kinase inactive. In striated excitation opens ryanodine receptors raising calcium tenfold unlocking thin filament. In smooth calcium calmodulin activates myosin light chain kinase phosphorylating regulatory light chains increasing actin activated ATPase. Dynein is microtubule minus end motor unrelated to actin myosin regulation. Hence availability of ATP determines whether cycles can turn while calcium determines whether they are permitted to start linking excitation to contraction.

Ref: Gordon et al., Physiol Rev 2000; Alberts Chapter 16 – Regulation of actin-myosin by ATP and Ca2+ availability.

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Cytoskeleton

Which protein prevents actin-myosin interaction in relaxed muscle?

In relaxed skeletal and cardiac muscle actomyosin interaction is prevented not by direct myosin inhibition but by steric blocking of thin filament through tropomyosin troponin complex evolutionarily conserved. Tropomyosin is alpha helical coiled coil dimer about 40 nm long that polymerizes head to tail along major groove of filamentous actin spanning seven actin monomers and forming continuous cable. In low calcium near 100 nM troponin I holds tropomyosin in blocked B state covering myosin binding sites on outer domain of actin. Elevation of calcium to micromolar via ryanodine receptor release allows calcium binding to troponin C N lobe causing structural shift of troponin complex moving tropomyosin azimuthally toward closed C state partially exposing sites then to open M state upon myosin strong binding. Only then can myosin heads attach release Pi and produce force. CapZ caps barbed ends gelsolin and cofilin sever or depolymerize actin but they do not regulate relaxation steric block essential for preventing wasteful ATP consumption at rest.

Ref: Alberts et al., Molecular Biology of the Cell 7th ed., Chapter 16: Tropomyosin blocks myosin binding in relaxed muscle.

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Cytoskeleton

Which troponin subunit is responsible for calcium binding?

Troponin heterotrimer every 38 nanometer along thin filament alongside tropomyosin provides calcium dependent switch. Subunits: TnT 30 to 35 kilodalton tropomyosin binding anchoring complex, TnI 21 to 24 kilodalton inhibitory binding actin blocking myosin in diastole, TnC 18 kilodalton calcium binding sensor homologous to calmodulin containing four EF hand helix loop helix motifs. N terminal low affinity regulatory sites I II bind calcium transiently during systole with Kd 5 micromolar triggering activation, affinity modulated by magnesium competition, C terminal high affinity structural sites III IV bind calcium magnesium constitutively with Kd nanomolar stabilizing complex. Calcium binding to N lobe induces opening exposing hydrophobic patch that binds switch peptide residues 148 to 158 of TnI removing C terminal inhibitory mobile domain from actin. This releases tropomyosin steric block exposing myosin binding sites permitting cross bridge cycling. Mutation TnC G159D reduces calcium affinity blunting contractility, TN-M nonexistent. Thus TnC uniquely senses calcium transient translating electrical signal into mechanical contraction and force generation.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 18: Troponin C as Calcium-Binding Subunit.

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