All-or-none transition

All-or-none transitions occur if the chain length is relatively short (n < 15 tripeptide units) and if the cooperativity is high (a < 1) since in this special case, the concentration of intermediates is negligibly low. Besides, in the case of short chains we may conclude that back folding and oligomerization are negligibly small because of the shortness of the chain ends beyond the helical part. A further simplification is the assumption that only one helical sequence exists, which excludes the formation of loops within a helical part, because of reasons of stability. Under these circumstances, only two different products exist in a measurable concentration at equilibrium. 

To describe the all-or-none transition between distinct conformational states of enzymes or receptors — an allosteric transition. In keeping with this usage, the constant that describes the position of the equilibrium between the states (e.g., E0 in the schemes of Figures 1.11 and 1.28) is sometimes described as the allosteric constant. 

Note that negative cooperativity cannot occur in the Monod-Wyman-Changeux allosteric transition model, because the dissociation constant is equivalent for all sites. Thus, positive cooperativity can only result in this binding mechanism as a consequence of the recruitment of binding sites from the T-state in an all-or-none transition to the R-state. Any occurrence of negative cooperativity can be regarded as prima facie evidence... 

The equations so far apply to a simple two-state equilibrium of D N. That is, protein denaturation is a cooperative (or all-or-none) transition in which denatu-ration occurs in a single step without any intermediates accumulating. How can we tell that this is so ... 

Consider, for example, the kind of OOBEs that Miss Z and Robert Monroe have had. These are what I call full-scale or classical or discrete OOBEs (d-OOBEs). During the OOBEs, neither Miss Z nor Mr. Monroe felt any contact with the physical body. In terms of conscious experience, there is what I call a full-scale disconnect. During the OOBE, all perceptions and actions performed are relevant to the OOBE location. In order to reexperience sensations from the physical body or to control it, both Miss Z and Mr. Monroe must terminate the OOBE and go through a transition back to ordinary being. Because this is an all-or-none transition between two quite different patterns of experience, I call it a d-OOBE [169] just as I distinguish discrete states of consciousness [173]. 

All-or-none transition is highly cooperative with N being the cooperative length of a transition. 

All relaxation curves exhibited more than one phase at various degrees of conversion and at different temperatures. This clearly rules out the all-or-none mechanism (AON) although the AON model is able to fit easily to the measured equilibrium transition curve. However, a mechanism has been proposed which allows the existence of side... 

In the limit of vanishing a, the transition becomes of all-or-none type, and /0 is given by 1 + [(N/2)(a1/a0)2 — 1], . The dashed line in Fig. 12 represents this relation. The fact that all other curves in Fig. 12 appear below this straight line implies that helix-coil transition of polypeptide proceeds, not in all-or-none fashion, but through interrupted helices if a is nonzero. 

The gradual declines of x T/t]0 observed in Fig. 41 may be accounted for in part by departures of the actual polymer from the all-or-none model, and in part by the increase in the population of partially broken helical rods with the increase in helical fraction. At any rate, these experimental results on the system PCBL-ro-cresol are of great interest as the first affirmation of a system that exhibits a helix-coil transition close to the all-or-none type. 

Reports on the action of reduced glutathione give results quite different from the above. The bulky mercaptan does not reduce RNase at all at room temperature. As the thermal transition temperature is approached the reduction appears to be all or none with no evidence of partially reduced intermediates 186). In a study of the recovery of activity after denaturation in 8 M urea, Kim and Paik 187) reported... 

Both the denaturation process in proteins and the melting transition (also referred to as the helix-to-coil transition) in nucleic acids have been modeled as a two-state transition, often referred to as the all-or-none or cooperative model. That is, the protein exists either in a completely folded or completely unfolded state, and the nucleic acid exists either as a fully ordered duplex or a fully dissociated monoplex. In both systems, the conformational flexibility, particularly in the high-temperature form, is great, so that numerous microstates associated with different conformers of the biopolymer are expected. However, the distinctions between the microstates are ignored and only the macrostates described earlier are considered. For small globular proteins and for some nucleic acid dissociation processes,11 the equilibrium between the two states can be represented as... 

The experiments that we have described show that some spatial arrangements help and others hinder children s solution to various mathematical problems. Young children are adept at using simple, all-or-none spatial relations, and the principles of gestalt psychology provide a good account of the way that they do so. Add to this their evident ability to combine relationships in deductive and transitive inferences and you have already a formidable set of mechanisms for learning about the environment. 

---