Troponin pointed ends

The myofibril also contains thin filaments of F-actin with tropomyosin dimers bound in the grooves and about one troponin complex per tropomyosin dimer. The thin filaments are also arranged in a bipolar fashion. All the pointed ends point away from each side of the Z-disk which contains a number of proteins including n-actinin. o-Actinin serves to bundle the actin filaments very close to their barbed ends and anchor them very firmly in the disk. 

In the light of the available crystallographic studies, the amino acid sequences and NMR studies, certain general points have been noted for parvalbumin, Wasserman protein, calmodulin, troponin C and SIOO. (1) Each calcium site is formed from residues in a hand which includes a /3-strand. The sites contain backbone carbonyl and side-chain carboxylate. (2) Each binding site is linked to two helices. (3) Each -strand backs on to another strand to form a Ca-/3-sheet-Ca unit, which then involves four helices. (4) The helices interact with each other through largely hydrophobic surfaces. (5) The connections between remote ends of helices are relatively mobile strands, and differ from protein to protein. (6) The four-hand proteins, the calmodulins and troponins, are simileir to the sum of their two-handed fragments. 

The time of peak force corresponds to the end of bond attachment. Physiologically, this point is associated with a lack of available bond attachment sites. This may be dne to lack of calcium ion for the release of steric inhibition of actin myosin interaction by troponin/tropomyosin, lack of myosin heads in a state necessary for attachment, or some other mechanism. Force relaxation then results simply from bond detachment due to the same internal myofilament motion mechanism. Net force smoothly recovers to the resting force level, in contrast to the dynamic motions of individual bonds and myofilaments. Surprisingly, this smooth relaxation occurs without the added feature of crossbridge recychng, that is, bond reattachment after detachment. 

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