Latch bridges

For these reasons the Cooperative Hypothesis seems to be a plausible alternative to the Latch Bridge Hypothesis. Given that computer calculations of the behavior of various hypothetical schemes are now possible, an independent, noninvasive measure of distribution of myosin among the states would be of great use for further understanding of crossbridge kinetics. 

Cell Biol. 28, 1185-1189, 1996 Szymanski, P.T., Calponin (CaP) as a latch-bridge protein — a new concept in regulation of contractility in smooth muscle, J. Muscle Res. Cell Motil. 25,7-19,2004 Lehman, W., Craig, R., Kendrick-Jones, J., and Sulherland-Smith, A.J., An open or closed case for the conformation of calponin homology domains on F-actin J. Muscle Res. Cell Motil. 25, 351-358, 2004 Feijani, L, Fattoum, A., Maciver, S.K. et al., A direct interaction with calponin inhibits the actin-nucleating activity of gelsohn, Biochem. J. 396, 461 68, 2006. 

A model involving cooperativity has been proposed as an alternative to the Hai and Murphy formulation (Vyas et al., 1992, 1994). The models are similar in that in both models cross-bridges exist in the same four basic states. The novelty of this model is a finite attachment rate of unphosphorylated cross-bridges (kg in Fig. 2B), which is dependent on the number of attached, phosphorylated cross-bridges (AMp). In effect, some slowly cycling "latch-bridges" are postulated to form cooperatively as a result of the attach-... 

Fig. 4. Schematic diagram (modified from Rembold and Murphy 1993) for the crossbridge cycling in smooth muscle according to the latch bridge model (Hai and Murphy 1988a, 1988b). Actin is indicated with A M and Mp denotes the unphosphorylated and phosphorylated myosin, respectively. Note that the latch state (AM) refers to a myosin bound state which is not identical to the rigor A.M state shown in Figure 1, chapter 2.

An important problem with any latch theory is that the latch-bridge in many ways remains descriptive and has not been coupled to specific biochemical actomyosin states or mode of actomyosin turnover. Skinned fibres could provide a way to explore this problem but at present there is a striking lack of a latch state in skinned smooth muscle preparations with a few exceptions. The hyperbolic phosphorylation force relationship has been demonstrated in e.g. skinned smooth muscle strips from chicken... 

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