(e.g. [60]). Even then, pili can still reach extreme specific tensions, with a median four times higher than that of other motors.4.2.2. Non-molecular motorsThe most striking result of this paper is that the formally defined tension of molecular motors turns out to be similar to the value f 200 kPa typical of muscle fibres. A hint to this uniformity stems from the basic arrangement of myosin motors in striated muscles (reviewed in e.g. [13,204]). Most of the space within muscle fibres is occupied by protein thick filaments along which groups of myosin globular motors (heads) are protruding with an axial spacing e = 14.6 nm. These motors are cyclically attaching to (and detaching from) adjacent thin filaments of actin to form the cross-bridges, and enable thin and thick filaments to slide past each other. Along each half thick filament (of total length 2l 1.6 , neglecting for this order-of-magnitude estimate a bare zone of smaller length free of motors) about 150 myosin molecules exert forces that add in parallel and only about one-third of the cross-bridges are attached during isometric contraction [47,205]. Therefore, the number of active individual myosin motors along each half thick filament is N 50. (Note that since l/e 50, this might imply that only one motor per group of three can attach simultaneously, a likely consequence of steric constraints brought about by the three-dimensional structure enabling transitory conformational changes.) With N motors acting in parallel each exerting a force Fmyosin the total force per thick filament is NFmyosin . Each thick filament and its associated lattice of thin filaments occupies an purchase HM61713, BI 1482694 equivalent cross-section s d2 , where d 40 nm is the lateral spacing of thick filaments, so the total tension in the structure is f fibre NFmyosin /s which acts (in series) along the length of the fibre. Tables 3 and 4 show that the myosin motor, of equivalent cross-sectional area A 36 nm2 , exerts a mean force Fmyosin f myosin A 7 pN. Substituting the values of Fmyosin , N and s in the above formula yields the tension in the structure f fibre 240 kPa. This rough estimate enables us to understand why the tension of muscles ( f fibre ) is of the same order of magnitude as the tension of the myosin motor f myosin 190 kPa. Indeed, the Y-27632 site tensions of muscle fibres and of myosin motors are in the ratio f fibre /f myosin NA/s, and the myosin motors are arranged so that the number N of them acting simultaneously in parallel is approximately equal to the ratio s/A ofthe equivalent cross-sectional area of each thick hin filament structure to that of an individual myosin motor head, which is not surprising because of steric constraints.rsos.royalsocietypublishing.org R. Soc. open sci. 3:…………………………………………4.3. Origins of variability of specific tension in various motorsOverall, tensions in most molecular and non-molecular motors are distributed around their means according to similar Gaussian functions with coefficients of variation s.d./mean 0.5. This variability may arise from methodological, experimental and biological factors.4.3.1. Methodological and experimental factorsThe cross-section A of molecular motors was estimated from their mass m using the formulae A = V 2/3 and V = m/ with protein density 10-3 pg nm-3 . This is admittedly rough, since the longer dimension of the motors considered can differ from the cross-diameter by nearly a factor of 2. The resulting error may not be negligi.(e.g. [60]). Even then, pili can still reach extreme specific tensions, with a median four times higher than that of other motors.4.2.2. Non-molecular motorsThe most striking result of this paper is that the formally defined tension of molecular motors turns out to be similar to the value f 200 kPa typical of muscle fibres. A hint to this uniformity stems from the basic arrangement of myosin motors in striated muscles (reviewed in e.g. [13,204]). Most of the space within muscle fibres is occupied by protein thick filaments along which groups of myosin globular motors (heads) are protruding with an axial spacing e = 14.6 nm. These motors are cyclically attaching to (and detaching from) adjacent thin filaments of actin to form the cross-bridges, and enable thin and thick filaments to slide past each other. Along each half thick filament (of total length 2l 1.6 , neglecting for this order-of-magnitude estimate a bare zone of smaller length free of motors) about 150 myosin molecules exert forces that add in parallel and only about one-third of the cross-bridges are attached during isometric contraction [47,205]. Therefore, the number of active individual myosin motors along each half thick filament is N 50. (Note that since l/e 50, this might imply that only one motor per group of three can attach simultaneously, a likely consequence of steric constraints brought about by the three-dimensional structure enabling transitory conformational changes.) With N motors acting in parallel each exerting a force Fmyosin the total force per thick filament is NFmyosin . Each thick filament and its associated lattice of thin filaments occupies an equivalent cross-section s d2 , where d 40 nm is the lateral spacing of thick filaments, so the total tension in the structure is f fibre NFmyosin /s which acts (in series) along the length of the fibre. Tables 3 and 4 show that the myosin motor, of equivalent cross-sectional area A 36 nm2 , exerts a mean force Fmyosin f myosin A 7 pN. Substituting the values of Fmyosin , N and s in the above formula yields the tension in the structure f fibre 240 kPa. This rough estimate enables us to understand why the tension of muscles ( f fibre ) is of the same order of magnitude as the tension of the myosin motor f myosin 190 kPa. Indeed, the tensions of muscle fibres and of myosin motors are in the ratio f fibre /f myosin NA/s, and the myosin motors are arranged so that the number N of them acting simultaneously in parallel is approximately equal to the ratio s/A ofthe equivalent cross-sectional area of each thick hin filament structure to that of an individual myosin motor head, which is not surprising because of steric constraints.rsos.royalsocietypublishing.org R. Soc. open sci. 3:…………………………………………4.3. Origins of variability of specific tension in various motorsOverall, tensions in most molecular and non-molecular motors are distributed around their means according to similar Gaussian functions with coefficients of variation s.d./mean 0.5. This variability may arise from methodological, experimental and biological factors.4.3.1. Methodological and experimental factorsThe cross-section A of molecular motors was estimated from their mass m using the formulae A = V 2/3 and V = m/ with protein density 10-3 pg nm-3 . This is admittedly rough, since the longer dimension of the motors considered can differ from the cross-diameter by nearly a factor of 2. The resulting error may not be negligi.
