BIOMECHANICS 3 Origins and consequences of forces in biological systems

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1 BIOMECHANICS 3 Origins and consequences of forces in biological systems MOLECULAR MECHANISMS OF BIOLOGICAL MOVEMENT AT THE LEVELOF ORGANISMS MOLECULAR BASIS OF MUSCLE CONTRACTION DR. BEÁTA BUGYI - BIOPHYSICS LECTURE UNIVERSITY OF PÉCS MEDICAL SCHOOL DEPARTMENT OF BIOPHYSICS Muscle functioning MUSCLE; ORGAN BUILT FROM CONTRACTILE TISSUE SPECILAIZED FOR MACROSCOPIC BIOLOGICAL MOTION, WHICH RELIES ON NANOSCOPIC MECHANOCHEMICAL SYSTEM ASSEMBLED FROM PROTEINS. (chemicalenergy mechanicalwork) 1

2 Muscle types SKELETAL MUSCLE body location attached to bones or to skin(some facial muscles), 45 % regulationof contraction cellshape, appearance voluntary STRIATED MUSCLE HEART MUSCLE (cardiac biophysics) walls of the heart involuntary (intrinsic regulatory system) SMOOTH MUSCLE visceral organs, intrinsic eye muscles, airways, large arteries involuntary striated pattern striated pattern no striated pattern very long, cylindrical multinucleate branched chain of cells uni-, binucleate Ca 2+ source sarcoplasmicreticulum sarcoplasmicreticulum extracellular fluid fusiform uninucleate Ca 2+ regulation troponin troponin calmodulin sarcoplasmic reticulum extracellular fluid Striated muscle MUSCLE FASICLE v MUSCLE FIBER nucleus Z disc dark(a) bright (I) band MYOFIBRIL organelle striated pattern 2

3 dark(a) bright (I) band v nucleus Z disc Sarcomere THESMALLESTFUNCTIONAL(CONTRACTILE)UNITOFMUSCLE Z ZDISTANCE I band isotrop/light A band anisotrop/dark THIN FILAMENT TITIN THICK FILAMENT L~ 2.2 mm 3

4 Sarcomere THICK FILAMENTS MYOSIN II THIN FILAMENTS ACTIN REGULATORY PROTEINS tropomyosin, troponin, tropomodulin Miofilaments observedbytransmission electronmicroscopy (TEM). Thick filament myosin II, the motor MYOSIN II 3D structure of myosin II crossbridge (head&neck) head neck tail HEAD NEC K myosin II filament THICK FILAMENT Geevesand Holmes Advancesin Protein Chemistry energy source: ATP ATPase activity(basal) chemicalenergy strucutralchange force generation, mechanical work TAIL 4

5 Thin filament actin, the track 90o ATP Ca 2+ S4 S2 S3 S1 ACTIN MONOMER Globular-ACTIN polymerization assembly ACTIN FILAMENT Filamental-ACTIN Forrás: Dr. Bugyi Beáta Hungarians in Muscle Research 2018 Enzymatic cycle of skeletal muscle myosin II Cross-bridge M: myozinii A: actin ON POWER STROKE FORCE RIGOR rigor mortis OFF M:ADP-Pi Cross-bridge: off A M:ADP-Pi Cross-bridge: on (weak) A M:ADP Cross-bridge: on (strong) A M Cross-bridge: on (strong) M:ATP Cross-bridge: off Pi dissociation ADP dissociation ATP binding ATP hydrolysis The loss of Piis coupled to conformational changes that return myosin toward its basal state. The Pidissociation 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. 5

6 Actin- myosin 1942.Albert Szent-Györgyi: ackto-myosincontractility, preparation of glycerinated muscle threads Szent-Györgyi Albert The contraction of actomyosin threads. Studies ATP muscle contraction was essentially an interaction of actomyosin and ATP 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 In vitro motility of acto-myosin 6

7 Cross-bridge cycle force generation 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 thickfilament~ 200 myosin ~2 200 pn=400 pn 1 myofibril~ sarcomere 1 fibre~ 2000 myofibril 1 mucle~10 10 fibre ~ 2 pn =8000 ~ 800 Total force: = the number of acto-myosin crosbridges depends on: overlap betweenthinand thickfilaments Frank-Starlinglaw ATPaseactivityof myosinii: crossbridgecycletime: =1/ Force transmission 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?WHATELSEDOWEENEED? SLIDING FILAMENT THEORY Anchors, Z and M lines/ tendon STERIC BLOCKING MODEL Ca 2+ sensitivetroponin tropomyosinsystem 7

8 Force transmission Z disk M line Z disk Sliding filament theory Andrew.F. Huxley (1954), Hugh. E. Huxley (1954) Z-Z: sarcomere: shortens I band: shortens A band: constant H band: shortens 8

9 Regulatory components- TROPONIN COMPLEX TROPONIN T (tropomyosin binding) 37 kda bindstropomyosinand theothertroponin subunits stabilizesthetroponincomplex TROPONIN I (inhibitory) 22 kda inhibitsthemyosinii actininteraction TROPONIN C (Ca2+ binding) 18 kda bindsca 2+ PDB: 1TCF 3D structure of troponinc with bound Ca2+. Regulatory components- TROPOMYOSIN alphahelicalcoiled-coildimer formsa polimer alongtheactinfilament(n-c overlap: head-to-tailoverlap) 1 tropomyosindimerbinds7 consecutiveactinsubunits PDB: 2TMA C N 3D structure of tropomyosin and structural model of the actin-tropomyosin filament. 9

10 Steric blocking model 10

11 MOLECULAR MECHANISM OF SKELETAL MUSCLE CONTRACTION MOLECULES myosinii actin tropomyosin troponin Ca 2+ ATP 1. stimulus 2. [Ca 2+ ] citoplasm 3. troponincbindsca troponin-tropomyosinmoveson the actin filament: free myosin II binding site 5. myosiniibindstoactin filaments 6. crossbridge cycle ATPase activity 7. contraction 11