BIOCHEM2027 Specialised Subject · 4.4
2027 · High School Specialised Subject · Biochemistry

5.3 Actin, Myosin and Molecular Motors

Protein interactions driven by chemical energy

Actin and myosin II, the banded sarcomere, the cross-bridge cycle and Ca²⁺ control, conversion of chemical energy into mechanical work, kinesin and dynein.

5 SECTIONS19 FIGURESEnglish edition
01

Actin and myosin are the main motor proteins of muscle

  • G-actin is a globular monomer of about 42 kDa; polymerisation gives F-actin
  • F-actin is a two-stranded helical microfilament about 7 nm thick, with structurally and kinetically distinct (+) and (−) ends
  • G-actin binds ATP or ADP, and polymerisation and depolymerisation underlie cytoskeletal dynamics
  • Myosin II has two heavy chains and four light chains; the skeletal isoform is roughly 500 kDa
  • The head carries the actin-binding site and the ATPase site; the neck acts as a lever that amplifies small rotations
  • The two coiled tails build the backbone of the thick filament
Table 5.3-1Thin and thick filaments compared
Figure 5.3-46Polymerisation from G-actin to F-actin and filament polarity; monomer counts are not to scale.
Figure 5.3-47Head, neck and tail of skeletal muscle myosin II and its ATPase and actin-binding regions.
02

Other proteins keep the filaments in ordered arrays

  • The thin filament of striated muscle carries tropomyosin and troponin in addition to actin
  • Tropomyosin lies along actin and restricts access to myosin-binding sites
  • Troponin has three parts: TnC binds Ca²⁺, TnI inhibits and TnT binds tropomyosin
  • Thick filaments bundle tails so heads point in both directions, leaving a bare zone at the centre
  • Opposing heads pull thin filaments from both Z discs toward the M line
  • The sarcomere is the unit between adjacent Z discs; the A band is the thick-filament region and keeps its length
  • The I band contains only thin filaments, the H zone only thick filaments, and the M line aligns and links thick filaments
Table 5.3-2Accessory proteins of the sarcomere
Figure 5.3-48Simplified arrangement of actin, tropomyosin and troponin on the thin filament.
Figure 5.3-49Myosin heads pointing in opposite directions on the thick filament, with the central bare zone.
Figure 5.3-50Z disc, M line, A band, I band and H zone of the sarcomere; not drawn to a fixed scale.
Figure 5.3-53Model of the relative positions and stabilising roles of the Z disc, M line and structural proteins.
03

Relative sliding of myosin and actin shortens the muscle

  • On contraction the Z discs approach, the A band stays nearly constant and the I band and H zone shorten
  • The band change reflects increased overlap, not shortening of the filaments themselves
  • A cross-bridge is a myosin head bound to actin
  • Cross-bridge cycle: ATP binding, detachment, re-cocking after hydrolysis, actin binding, Pi release and force, then ADP release
  • Repeated cycles produce relative filament motion; without ATP the heads cannot detach from the strong-binding state
  • Nerve stimulation releases Ca²⁺ from the sarcoplasmic reticulum, TnC binds it, tropomyosin moves and sites are exposed
  • When stimulation ends, SERCA Ca²⁺-ATPase pumps Ca²⁺ back at the cost of ATP and relaxation follows
  • Length–tension relation: over-stretching reduces available bridges and over-shortening causes interference, both lowering active tension
  • Passive tension from titin and connective tissue is considered separately
Table 5.3-3States of the cross-bridge cycle
Figure 5.3-51Relaxed and contracted states compared; filament lengths are fixed and only the overlap changes.
Figure 5.3-52Teaching geometry model of A, I and H band lengths against sarcomere length; not measurements.
Figure 5.3-54Representative states of the cross-bridge cycle with bound nucleotides; the real cycle has further intermediates.
Figure 5.3-56Pathway from the nerve signal through Ca²⁺ release and troponin binding to actin–myosin access.
Figure 5.3-57Hill-type calculation of contractile activity against Ca²⁺ concentration (n = 2.2, K = 0.65 μM); not measured data.
Figure 5.3-58Classroom simulated time course showing that the Ca²⁺ change precedes tension development.
04

Chemical energy is converted into mechanical work

  • The standard free-energy change of ATP + H2O → ADP + Pi is about −30.5 kJ/mol (25 °C, pH 7)
  • The cellular value is usually more negative, and ΔG = ΔG°′ + RT ln([ADP][Pi]/[ATP])
  • A negative ΔG means the overall reaction is favourable; bond cleavage does not create energy
  • Myosin couples ATP binding, hydrolysis and product release to structural change
  • Work W = Fd, with forces in pN and distances in nm, so energies are of order 10⁻²¹ J
  • Efficiency η = W/(|ΔG_ATP|/N_A) × 100 depends on conditions and on how work is defined
  • Example: 5 pN over 8 nm gives 4.0×10⁻²⁰ J, about 48% efficiency if ΔG is taken as −50 kJ/mol
  • Rigor mortis reflects ATP depletion together with loss of ion homeostasis, not simply stronger contraction
Figure 5.3-55Model of the free-energy difference of ATP hydrolysis; the profile is not a measured transition-state profile.
Figure 5.3-62Work for the same displacement rises as force increases (teaching calculation data).
05

The principle of the molecular motor extends beyond muscle

  • Myosin V carries intracellular cargo along actin
  • Kinesin and dynein mostly use microtubules as tracks
  • Many conventional kinesins move toward the (+) end, while cytoplasmic dynein moves mainly toward (−)
  • Not every kinesin or myosin moves in the same direction or walks the same way; exceptional isoforms exist
  • Contraction velocity falls as load rises, and mechanical work can be computed from ATP use and distance moved
  • The shared principle is a directional motor cycle that couples ATP turnover to binding, detachment and re-cocking
Table 5.3-4Molecular motors and their tracks
Figure 5.3-59Representative tracks and directions of myosin, kinesin and dynein; exceptional isoforms exist.
Figure 5.3-60Alternating-head walking of typical kinesin-1; a concept model illustrating the roughly 8 nm step.
Figure 5.3-61Contraction velocity tends to fall as load rises; a simple classroom function, not a measurement for a particular muscle.
Figure 5.3-63Sliding-filament assay concept: fluorescent actin moving over myosin fixed to a glass surface.
Figure 5.3-64Conceptual relationship between chemical energy, Ca²⁺ control and relative sliding.
2027 High School Specialised Subject · Biochemistry · Sangil Park (Asan High School) · Reference material is printed as an appendix.
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