Post-rigor state

Fig. 3. Details of the seven-stranded /1-sheet and associated structures (A and B) in the post-rigor conformation and (C and D) in the pre-powerstroke conformation. The orientation of A and C is at right angles to that shown in Fig. 2. When attached to actin, it corresponds to that shown in Fig. 5B. The colors are as in Fig. 2. The views shown in B and D are at right angles to A and C looking out radially from the axis of the actin helix. Note the kink in the relay helix shown in C and D that leads to a 60° rotation of the converter domain. This in turn rotates the lever arm 60°. The P-loop (which constitutes the ATP-binding site) and the adjoining a-helix are shown in yellow. The flanking switch sequences (1 and 2) are also shown. The strands of the /1-sheet are numbered from the N-terminal (distal) end of the sheet. The lower part of strand 5 (light blue) constitutes switch 2. In the post-rigor state, switch 2 lies out of the plane of the /1-sheet (open) and in the pre-powerstroke state switch 2 is in the plane of the /1-sheet (closed).

The strongly bound pre-powerstroke state or top-of-powerstroke state is the transitory state labeled 4 in Fig. 1. It is experimentally difficult to characterize this either kinetically or structurally. At present, the structure can only be guessed at by an extrapolation of the properties of the adjoining structures. It seems very likely that the actin-binding cleft closes on strong binding in the top-of-powerstroke state. Comparison of the structures of the pre-powerstroke and post-rigor states with the nucleotide-free... 

Fig. 31. Implications about the contractile mechanism in insect flight muscle. Blue is insect flight muscle SI shape in pre-powerstroke state (Al-Khayat et al., 2003), and green is chicken skeletal muscle SI in the rigor state with no nucleotide bound (Rayment et al., 1993a). The actin filament (right) is shown with the Z-band at the bottom and M-band at the top. A transition from the pre-powerstroke/resting SI shape to the rigor/end of post-powerstroke shape would involve an axial swing of the lever arm by 100 A, resulting in the sliding of the actin filaments past the myosin filaments and toward the M-band.

To date, three primary conformations of the myosin crossbridge that can be associated with states in the Lymn-Taylor cycle have been identified. These have been named the post-rigor structure (Fig. 2 and state 2 in Fig. 1), the pre-powerstroke structure (corresponding to the myosin products complex, M.D.P , state 3 in Fig. 1), and the rigor Iihe (or rigor structure if it is associated with actin) state (shown as state 1 in Fig. 1). A comparison of these structures leads to the identification the following important conformationally flexible elements ... 

When compared with post-rigor or pre-powerstroke states the structural effects of cleft closure appear to include the movement of SW1, which opens the nucleotide-binding pocket, together with a twist of the central /Lsheet, which is associated with a large movement of the P-loop that considerably modifies the nucleotide binding site. Partial closure of the actin-binding cleft and a very similar twisting of the /3-sheet were also seen in the nucleotide-free structure of Dictyostelium myosin II reported by Reubold et al. (2003). The myosin V atomic model can be fitted without deformation into the electron microscope three-dimensional (3D) reconstruction of decorated actin (Holmes et al., 2004). For this and other... 

The relay/converter conformation is the readout of the result of these inputs. In going from the post-rigor to pre-power states, the only signal is SW2 going from open to closed, which produces the relay kink and the converter up. To go from the pre-power to rigor states requires the /i-twist, which is triggered by actin binding and cleft closure. 

How does a rigorously calculated electrostatic potential depend upon the computational level at which was obtained p(r) Most ab initio calculations of V(r) for reasonably sized molecules are based on self-consistent field (SCF) or near Hartree-Fock wavefunctions and therefore do not reflect electron correlation in the computation of p(r). It is true that the availability of supercomputers and high-powered work stations has made post-Hartree-Fock calculations of V(r) (which include electron correlation) a realistic possibility even for molecules with 5 to 10 first-row atoms however, there is reason to believe that such computational levels are usually not necessary and not warranted. The Mpller-Plesset theorem states that properties computed from Hartree-Fock wave functions using one-electron operators, as is T(r), are correct through first order (Mpller and Plesset 1934) any errors are no more than second-order effects. 

Tenderness. The contractile state of the muscle after rigor mortis is a major factor in meat tenderness, which is affected by post-mortem conditions creating differences in tenderness. Ageing of fresh pork can be used to improve tenderness. The process is based on a continuous weakening of the structural elements by different endogenous muscle peptidases along with an improved palatability (Taylor et al., 1995). 

The apparent positions in the Lymn-Taylor cycle of the structural states that we refer to as the pre-powerstroke, rigor-like, and post-powerstroke conformations are shown in Fig. 10. Thus it appears that three of the four... 

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