Harpers

562 / CHAPTER 49123Thick filamentLMMS-2 S-1Thin filamentFigure 49–7. Representation of the active crossbridgesbetween thick and thin filaments. This diagramwas adapted by AF Huxley from HE Huxley: Themechanism of muscular contraction. Science1969;164:1356. The latter proposed that the force involvedin muscular contraction originates in a tendencyfor the myosin head (S-1) to rotate relative to the thinfilament and is transmitted to the thick filament by theS-2 portion of the myosin molecule acting as an inextensiblelink. Flexible points at each end of S-2 permitS-1 to rotate and allow for variations in the separationbetween filaments. The present figure is based on HEHuxley’s proposal but also incorporates elastic (the coilsin the S-2 portion) and stepwise-shortening elements(depicted here as four sites of interaction between theS-1 portion and the thin filament). (See Huxley AF, SimmonsRM: Proposed mechanism of force generation instriated muscle. Nature [Lond] 1971;233:533.) Thestrengths of binding of the attached sites are higher inposition 2 than in position 1 and higher in position 3than position 2. The myosin head can be detached fromposition 3 with the utilization of a molecule of ATP; thisis the predominant process during shortening. Themyosin head is seen to vary in its position from about90° to about 45°, as indicated in the text. (S-1, myosinhead; S-2, portion of the myosin molecule; LMM, lightmeromyosin) (see legend to Figure 49–4). (Reproducedfrom Huxley AF: Muscular contraction. J Physiol 1974;243:1. By kind permission of the author and the Journal ofPhysiology.)Another cycle then commences with the hydrolysisof ATP (step 1 of Figure 49–6), re-forming the highenergyconformation.Thus, hydrolysis of ATP is used to drive the cycle,with the actual power stroke being the conformationalchange in the S-1 head that occurs upon the release ofADP. The hinge regions of myosin (referred to as flexiblepoints at each end of S-2 in the legend to Figure49–7) permit the large range of movement of S-1 andalso allow S-1 to find actin filaments.If intracellular levels of ATP drop (eg, after death),ATP is not available to bind the S-1 head (step 4above), actin does not dissociate, and relaxation (step 5)does not occur. This is the explanation for rigor mortis,the stiffening of the body that occurs after death.Calculations have indicated that the efficiency ofcontraction is about 50%; that of the internal combustionengine is less than 20%.Tropomyosin & the Troponin ComplexPresent in Thin Filaments Perform KeyFunctions in Striated MuscleIn striated muscle, there are two other proteins that areminor in terms of their mass but important in terms oftheir function. Tropomyosin is a fibrous molecule thatconsists of two chains, alpha and beta, that attach toF-actin in the groove between its filaments (Figure 49–3).Tropomyosin is present in all muscular and muscle-likestructures. The troponin complex is unique to striatedmuscle and consists of three polypeptides. Troponin T(TpT) binds to tropomyosin as well as to the other twotroponin components. Troponin I (TpI) inhibits theF-actin-myosin interaction and also binds to the othercomponents of troponin. Troponin C (TpC) is a calcium-bindingpolypeptide that is structurally and functionallyanalogous to calmodulin, an important calcium-bindingprotein widely distributed in nature.Four molecules of calcium ion are bound per moleculeof troponin C or calmodulin, and both molecules havea molecular mass of 17 kDa.Ca 2+ Plays a Central Role in Regulationof Muscle ContractionThe contraction of muscles from all sources occurs bythe general mechanism described above. Muscles fromdifferent organisms and from different cells and tissueswithin the same organism may have different molecularmechanisms responsible for the regulation of their contractionand relaxation. In all systems, Ca 2+ plays a keyregulatory role. There are two general mechanisms ofregulation of muscle contraction: actin-based andmyosin-based. The former operates in skeletal and cardiacmuscle, the latter in smooth muscle.Actin-Based Regulation Occursin Striated MuscleActin-based regulation of muscle occurs in vertebrateskeletal and cardiac muscles, both striated. In the gen-