T, state

The Fe-N mode is at 222 in the R state and 207 cnY in the T state for the a subunits, but only shifted to 218 T state for the (3 subunits. This is consistent with the interpretation that the Fe-imidazole interations are weakened more in the T state of the a subunits than p subunits. Time-resolved resonance Raman studies have shown that the R T switch is complete on a 10 ps tuuescale [38]. Finally, UV excitation of the aromatic protein side chains yields... 

Vibrational stmcture is, at best, only partially resolved. The stmcture in the first and second systems is complex which could be due, in part, to there being a Jahn-Teller effect in each of them. Such an effect may arise when a molecule is in an orbitally degenerate E (or T) state. The Jahn-Teller effect involves a distortion of the molecule in order to destroy the... 

Pig. 4. Electron transitions in dye radicals, (a) A schematic spectral picture showing the ground (T ) state and two excited (T, 2) obtained after... 

Reactivity. Hemoglobin can exist ia either of two stmctural coaformatioas, corresponding to the oxy (R, relaxed) or deoxy (T, tense) states. The key differences between these two stmctures are that the constrained T state has a much lower oxygen affinity than the R state and the T state has a lower tendency to dissociate into subunits that can be filtered in the kidneys. Therefore, stabilization of the T conformation would be expected to solve both the oxygen affinity and renal excretion problems. 

CPR can be used to find continuous paths for complex transitions that might have hundreds of saddle points and need to be described by thousands of path points. Examples of such transitions include the quaternary transition between the R and T states of hemoglobin [57] and the reorganization of the retinoic acid receptor upon substrate entry [58]. Because CPR yields the exact saddle points as part of the path, it can also be used in conjunction with nonnal mode analysis to estimate the vibrational entropy of activation... 

Effector molecules switch allosteric proteins between R and T states... 

The theory predicts that such proteins are built up of several subunits which are symmetrically arranged and that the two states differ by the arrangements of the subunits and the number of bonds between them. In one state the subunits are constrained by strong bonds that would resist the structural changes needed for substrate binding, and this state would consequently bind substrates weakly they called it the tense or T state. In the other state, called the R state, these constraints are relaxed. 

For many years hemoglobin was the only allosteric protein whose stereochemical mechanism was understood in detail. However, more recently detailed structural information has been obtained for both the R and the T states of several enzymes as well as one genetic repressor system, the trp-repressor, described in Chapter 8. We will here examine the structural differences between the R and the T states of a key enzyme in the glycolytic pathway, phosphofructokinase. 

The two states have the same affinity for ATP but differ with respect to their affinity for the substrate F6P, the allosteric effector ADP and the inhibitor PEP. Because of these differences in affinity, ligand binding can shift the equilibrium between the R and T states to favor one or the other state depending on which ligand is bound. 

The group of Phil Evans, MRC Laboratory of Molecular Biology, Cambridge, UK, has determined x-ray structures of bacterial PFK both in the R and the T states. These studies have confirmed the above conclusions and given insight into how an allosteric enzyme accomplishes its complex behavior. 

The change in the quaternary structure and the structural change in the 6-F helix as the molecule moves from one state to the other are intimately related. The dimer interactions in the T state are not compatible with the presence of the 6-F helix, which would, if present, clash with the neighbouring dimer. The quaternary structure of the T state requires that the 6-F helix be unwound. Conversely the R state quaternary structure depends on the presence of the 6-F helix. 

The basic kinetic properties of this allosteric enzyme are clearly explained by combining Monod s theory and these structural results. The tetrameric enzyme exists in equilibrium between a catalytically active R state and an inactive T state. There is a difference in the tertiary structure of the subunits in these two states, which is closely linked to a difference in the quaternary structure of the molecule. The substrate F6P binds preferentially to the R state, thereby shifting the equilibrium to that state. Since the mechanism is concerted, binding of one F6P to the first subunit provides an additional three subunits in the R state, hence the cooperativity of F6P binding and catalysis. ATP binds to both states, so there is no shift in the equilibrium and hence there is no cooperativity of ATP binding. The inhibitor PEP preferentially binds to the effector binding site of molecules in the T state and as a result the equilibrium is shifted to the inactive state. By contrast the activator ADP preferentially binds to the effector site of molecules in the R state and as a result shifts the equilibrium to the R state with its four available, catalytically competent, active sites per molecule. 

Fi re 12.6 Schematic diagram Illustrating the proton movements in the photocycle of bacteriorhodopsin. The protein adopts two main conformational states, tense (T) and relaxed (R). The T state binds trans-tetinal tightly and the R state binds c/s-retinal. (a) Stmcture of bacteriorhodopsin in the T state with hflus-retinal bound to Lys 216 via a Schiff base, (b) A proton is transferred from the Schiff base to Asp 85 following isomerization of retinal and a conformational change of the protein. 

In the lowest optieally excited state of the molecule, we have one eleetron (ti ) and one hole (/i ), each with spin 1/2 which couple through the Coulomb interaetion and can either form a singlet 5 state (5 = 0), or a triplet T state (S = 1). Since the electric dipole matrix element for optical transitions — ep A)/(me) does not depend on spin, there is a strong spin seleetion rule (AS = 0) for optical electric dipole transitions. This strong spin seleetion rule arises from the very weak spin-orbit interaction for carbon. Thus, to turn on electric dipole transitions, appropriate odd-parity vibrational modes must be admixed with the initial and (or) final electronic states, so that the w eak absorption below 2.5 eV involves optical transitions between appropriate vibronic levels. These vibronic levels are energetically favored by virtue... 

TTie photoreduction can be quenched by known triplet quenchers. The effecti e quenchers are those which have T] states less than 69kcal/moI above S,. Quenchers with higher triplet energies are ineffective because the benzophenone n-n triplet is then not sufficiently energetic to effect energy transfer. 

Don t state the issue status of reference documents in your procedures and specifications unless absolutely necessary. 

In 1965, Jacques Monod, Jeffries Wyman, and Jean-Pierre Changeux proposed a theoretical model of allosteric transitions based on the observation that allosteric proteins are oligomers. They suggested that allosteric proteins can exist in (at least) two conformational states, designated R, signifying relaxed, and T, or taut, and that, in each protein molecule, all of the subunits have the same conformation (either R or T). That is, molecular symmetry is conserved. Molecules of mixed conformation (having subunits of both R and T states) are not allowed by this model. 

FIGURE 15.9 Monod-Wyman-Changeux (MWC) model for allosteric transitions. Consider a dimeric protein that can exist in either of two conformational states, R or T. Each subunit in the dimer has a binding site for substrate S and an allosteric effector site, F. The promoters are symmetrically related to one another in the protein, and symmetry is conserved regardless of the conformational state of the protein. The different states of the protein, with or without bound ligand, are linked to one another through the various equilibria. Thus, the relative population of protein molecules in the R or T state is a function of these equilibria and the concentration of the various ligands, substrate (S), and effectors (which bind at f- or Fj ). As [S] is increased, the T/R equilibrium shifts in favor of an increased proportion of R-conformers in the total population (that is, more protein molecules in the R conformational state). 

FIGURE 15.12 71 versus [S] curves for an allosteric V system. The V system fits the model of Moiiod, Wyman, and Chaiigeux, given the following conditions (1) R and T have the affinity for the substrate, S. (2) The effectors A and I have different affinities for R and T and thus can shift the relative T/R distribution. (That is, A and I change the apparent value of L.) Assume as before that A binds only to the R state and I binds only to the T state. (3) R and T differ in their catalytic ability. Assume that R is the enzymatically active form, whereas T is inactive. Because A perturbs the T/R equilibrium in favor of more R, A increases the apparent Vmax- I favors transition to the inactive T state. 

Glycogen phosphorylase conforms to the Monod-Wyman-Changeux model of allosteric transitions, with the active form of the enzyme designated the R state and the inactive form denoted as the T state (Figure 15.17). Thus, AMP promotes the conversion to the active R state, whereas ATP, glucose-6-P, and caffeine favor conversion to the inactive T state. 

FIGURE 15.17 The mechanism of covalent modification and allosteric regnlation of glycogen phosphorylase. The T states are bine and the R states bine-green. 

A model for the allosteric behavior of hemoglobin is based on recent observations that oxygen is accessible only to the heme groups of the a-chains when hemoglobin is in the T conformational state. Perutz has pointed out that the heme environment of /3-chains in the T state is virtually inaccessible because of steric hindrance by amino acid residues in the E helix. This hindrance dis-... 

This reaction is driven to the right in tissues by the high COg concentration the equilibrium shifts the other way in the lungs where [COg] is low. Thus, car-bamylation of the N-termini converts them to anionic functions, which then form salt links with the cationic side chains of Arg al41 that stabilize the deoxy or T state of hemoglobin. 

Angeb., abbrev. (Angebot) offer, angeben, v.t. state, declare, tell specify accuse indicate devise. 

The reaction between Cell—O radical and vinyl monomers leads to the formation of grafted cellulose. In the presence of photosensitizers generally used as photoinitiators, such as benzophenone and phenylace-tophenone derivatives, the photoinitiator absorbs the UV radiation and transforms to its singlet (S ) and then triplet (T ) states. After that it may decompose into free radicals or transfer its energy to cellulose or any other molecules in the system. Take benzophenone as an example ... 

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