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UP MS Department of Biophysics
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BIOMECHANICS 3Origins and consequences of forces in biological systems
MOLECULAR MECHANISMS OF BIOLOGICAL MOVEMENT
AT THE LEVEL OF ORGANISMS MOLECULAR BASIS OF MUSCLE
CONTRACTION
DR. BEÁTA BUGYI - BIOPHYSICS LECTUREUNIVERSITY OF PÉCS – MEDICAL SCHOOL
DEPARTMENT OF BIOPHYSICS
MUSCLE; ORGAN BUILT FROM CONTRACTILE
TISSUE SPECILAIZED FOR MACROSCOPIC
BIOLOGICAL MOTION, WHICH RELIES ON
NANOSCOPIC MECHANOCHEMICAL SYSTEM
ASSEMBLED FROM PROTEINS.
(chemical energy mechanical work)
Muscle functioning
UP MS Department of Biophysics
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STRIATED MUSCLE SMOOTH MUSCLE
SKELETAL MUSCLE HEART MUSCLE(cardiac biophysics)
body location attached to bones or to
skin (some facial
muscles), 45 %
walls of the heart visceral organs, intrinsic eye
muscles, airways, large
arteries
regulation of
contractionvoluntary involuntary
(intrinsic regulatory system)
involuntary
striated pattern striated pattern no striated pattern
cell shape,
appearancevery long, cylindrical
multinucleate
branched chain of cells
uni-, binucleate
fusiform
uninucleate
Ca2+ source sarcoplasmic reticulum sarcoplasmic reticulum
extracellular fluid
sarcoplasmic reticulum
extracellular fluid
Ca2+ regulation troponin troponin calmodulin
Muscle types
v
MUSCLE FIBER
MYOFIBRILorganelle
http://bursaclab.bme.duke.edu/gallery.php?id=19
FASICLE
striated pattern
MUSCLE
dark (A) bright (I) band
nucleus Z disc
Striated muscle
UP MS Department of Biophysics
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v
dark (A) bright (I) band
nucleus Z disc
L~ 2.2 mm
THIN FILAMENT
TITIN
THICK FILAMENT
THE SMALLEST FUNCTIONAL (CONTRACTILE) UNIT OF MUSCLE
Z – Z DISTANCE
I band
isotrop/light
A band
anisotrop/dark
Sarcomere
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THICK FILAMENTSMYOSIN IITHIN FILAMENTS ACTIN REGULATORY PROTEINStropomyosin, troponin, tropomodulin
Miofilaments observed by transmission electron microscopy (TEM).
Sarcomere
Geeves and Holmes Advances in Protein Chemistry 2005.
3D structure of myosin II crossbridge
(head&neck)
head tailneck
myosin II filament – THICK FILAMENT
MYOSIN II
HEAD
NECK
TAILenergy source: ATPATPase activity (basal)
chemical energy
strucutral change
force generation, mechanical work
Thick filament – myosin II, the motor
UP MS Department of Biophysics
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ACTIN MONOMERGlobular-ACTIN
ACTIN FILAMENTFilamental-ACTIN
Thin filament – actin, the track
S3
S2
S1
S4
ATP Ca2+90o
Forrás: Dr. Bugyi Beáta Hungarians in Muscle Research 2018
polymerization
assembly
The loss of Pi is coupled to conformational changes that return myosin toward its basal state. The Pi dissociation step has the
largest negative free energy change, so it is presumed that energy derived from ATP binding and hydrolysis and stored in
conformational changes in the myosin head is used to do work or dissipated as heat at this point in the reaction pathway.
M: myozinII
Enzymatic cycle of skeletal muscle myosin IICross-bridge
A – M:ADP-PiCross-bridge: on
(weak)
A – M:ADPCross-bridge: on
(strong)
M:ATPCross-bridge: off
M:ADP-PiCross-bridge: off
RIGOR
rigor mortisON
A – MCross-bridge: on
(strong)
OFF
ATP hydrolysis
Pi dissociation ADP dissociation ATP binding
POWER STROKEFORCE
A: actin
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‚To see them contract for the first time, and to have reproduced in vitro one of the oldest
signs of life, motion, was perhaps the most thrilling moment of my life.’Szent-Györgyi Albert Lost in the 20th century. Annual Reviews in Biochemistry 1963
+ A
TP
1942. Albert Szent-Györgyi: ackto-myosin contractility, preparation of glycerinated muscle threadsSzent-Györgyi Albert The contraction of actomyosin threads. Studies 1942
‚muscle contraction was
essentially an interaction of
actomyosin and ATP’
Actin - myosin
In vitro motility of acto-myosin
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movement of the myosin II head in one crossbridge cycle:
����������~10 nN = 10 ∗ 10�� �
force generated in one crossbridge cycle:
����������~2 pN = 2 ∗ 10��� �
Number of cross-bridges: N
1 thick filament ~ 200 myosin → �~2 × 200 pN = 400 pN
1 myofibril ~ 10$ − 10&sarcomere
1 fibre ~ 2000 myofibril
1 mucle ~10$ − 10& fibre
� ~ 2 pN × 200 × 10& × 2000 × 10& = 8000 � ~ 800 /0
Total force: 123245= 6 ∗ 178399�:8;<=>
the number of acto-myosin crosbridges depends on:
overlap between thin and thick filaments
Frank-Starling law
ATPase activity of myosin II: ?@ABC��
crossbridge cycle time: D = 1/?@ABC��
Cross-bridge cycle – force generation
The acto-myosin filament system is a MECHANOCHEMICAL
MACHINERY, that converts CHEMICAL ENERGY through STRUCTURAL
CHANGES into FORCE GENERATION and MECHANICAL WORK.
! NO FORCE TRANSMISSION
! NO REGULATION
? WHAT ELSE DO WEE NEED?
SLIDING FILAMENT THEORYAnchors, Z and M lines / tendon
STERIC BLOCKING MODELCa2+ sensitive troponin – tropomyosin system
Force transmission
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Force transmission
M lineZ disk Z disk
Andrew.F. Huxley (1954), Hugh. E. Huxley
(1954)
Z-Z: sarcomere: shortens
I band: shortens
A band: constant
H band: shortens
Sliding filament theory
UP MS Department of Biophysics
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TROPONIN T (tropomyosin binding) 37 kDa
binds tropomyosin and the other troponin
subunits
stabilizes the troponin complex
TROPONIN I (inhibitory) 22 kDa
inhibits the myosin II – actin interaction
TROPONIN C (Ca2+ binding) 18 kDa
binds Ca2+
3D structure of troponinC with bound Ca2+.
Regulatory components - TROPONIN COMPLEX
PDB: 1TCF
PDB: 2TMANC
alpha helical coiled-coil dimer
forms a polimer along the actin filament (N-C overlap: head-to-tail overlap)
1 tropomyosin dimer binds 7 consecutive actin subunits
3D structure of tropomyosin and structural model of the
actin-tropomyosin filament.
Regulatory components - TROPOMYOSIN
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Steric blocking model
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MOLECULAR MECHANISM OF SKELETAL MUSCLE CONTRACTION
MOLECULES
myosin II
actin
tropomyosin
troponin
Ca2+
ATP
1. stimulus
2. [Ca2+]citoplasm↑
3. troponinC binds Ca2+
4. troponin-tropomyosin moves on
the actin filament: free myosin
II binding site
5. myosin II binds to actin
filaments
6. crossbridge cycle – ATPase
activity
7. contraction
