Weak-binding state

The above mentioned experiments show that guests are preferentially bound by the aa conformation of 9a. In Sect. 3 we will describe how this concept can be used to construct a molecular system that switches between strongly and weakly binding states. 

Figure 12-5 (A) Stereoscopic view of the structure of the catalytic site of phosphorylase b in the inhibited T-state with the inhibitor nojirimycin tetra-zole bound into the active site. Inorganic phosphate (P ) as well as the coenzyme pyridoxal 5 -phosphate (PLP) are also shown. (B) Details of interactions of the inhibitor, P , and PLP with the protein and with water molecules (small circles). This is a weak-binding state but the P has displaced the negatively charged side chain carboxylate of Asp 283 (visible at the lower right in A).

Starting from the pre-powerstroke state myosin complex with ADP and Pi tightly bound (summarized structure in Table I), the M.D.Pi is in rapid equilibrium with an actin-bound state on the microsecond-millisecond time scale. This is very dependent on ionic strength (Furch et al., 2000 White and Taylor, 1976) and is therefore probably a non-stereo-specific weak binding state and is controlled by the ionic interactions between loop 2 and the N-terminus of actin. Other ionic interactions may also be involved. This loose association between actin and myosin probably does not alter the overall conformation of myosin. 

Squire, J. M., and Harford, J. J. (1988). Actin filament organisation and myosin head labelling patterns invertebrate skeletal muscles in the rigor and weak-binding states. / Mus. Res. Cell Motil. 9, 344-358. 

In the presence of actin, the scheme is necessarily more complex. In the absence of ATP, myosin s affinity for actin is very large and, under most conditions, myosin would be fully complexed with actin. Binding of ATP greatly weakens the affinity of myosin for actin and dissociation of the two proteins rapidly ensues at low protein concentrations. The extent of the ATP-induced dissociation of actin and myosin is a function of the ionic conditions and protein concentration and a rapid equilibrium exists between the two states. Hydrolysis of ATP can occur either by the dissociated myosin or by the actomyosin complex. The net result of ATP binding and hydrolysis is a rapid equilibrium between the four species shown in Fig. 8 in the box marked "weak binding states." The rate of hydrolysis of ATP to give the bound products is fast and is not affected by phosphorylation of HMM (J. R. Sellers, unpublished data). 

Such models are characterized by the existence of molecules with some subunits in the weak-binding state and some in the strong. 

The symbols "T" and "R" stand for "tense" and "relaxed" the significance of this is explained in the next section.) An equilibrium between the T and R states is presumed to exist, and partial oxygenation shifts that equilibrium toward the R state. The shift is a concerted one (different from the sequential model above), so that mixed molecules with some subunits in the weak-binding state and some in the strong-binding state are specifically excluded. 

The fact of a transfer of an electron from an absorbed particle to adsorbent [25] is widely considered as a criterion to differentiate between various forms of adsorption. Yet, as it has been already mentioned in previous section, there is a neutral form of chemisorption, i.e. weak binding formed without changing the surface charge state which only affects the dipole component of the work function. On the other hand, in several cases the physical adsorption can result in electron transitions in solids. Indeed, apart from formation of a double layer, changing the work function of adsorbent [26] the formation of surface dipoles accompanying physical adsorption can bring free charge carriers to substan-... 

The predicted state of the sorbing surface in the two calculations differs considerably. At pH 4, the surface carries a positive surface charge and potential. The electrical charge arises largely lfom the predominance of the protonated surface species > (w)FeOH, which occupies about two thirds of the weakly binding sites. At pH 8, however, the surface charge and potential nearly vanish because of the predominance of the uncomplexed species >(w)FeOH, which is electrically neutral. 

The weak binding of pl20GAP to Ras in the GTP-state was enhanced considerably when GTP (or GppNHp) was replaced by GDP and fluoroaluminate. In the a subunits of heterotrimeric G-proteins A1F, and GDP were shown to mimic a kind of transition state of GTP hydrolysis. For Ras this state could only be shown... 

The enzyme is in the weak-binding T-state but inorganic phosphate (P ) is bound below it and next to the phosphate group of PLP, as required by the mechanism of Eq. 12-15. 

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