Terminal complex hypothesis

The Terminal Complex Hypothesis and Its Relationship to Other Contemporary Research Topics... 

The terminal complex hypothesis proposes that the cellulose synthesizing enzyme complex can be visualized with electron microscopy. Terminal complex is the name given to collections of plasma membrane particles thought to represent the cellulose synthase. While direct evidence is still not available to support this hypothesis, the amount of indirect supporting evidence has grown dramatically in the past few years. The relationship between terminal complexes, cellulose physical structure and the biochemical events of cellulose biosynthesis will be discussed. 

The terminal complex hypothesis proposes that structural manifestations of the cellulose synthase enzyme complex can be visualized with the freeze fracture specimen preparation technique for electron microscopy. These... 

Hotchkiss A.T., Jr. 1989. Cellulose biosynthesis the terminal complex hypothesis and its relationship to other contemporary research topics. In Lewis N.G. and Paice M.G. (eds.) Plant Cell Wall Polymers. Biogenesis and Biodegradation. American Chemical Society, Washington, DC, pp. 232 247. 

TCs (Delmer 1999 Roberts et al. 2002), these domains are obvious candidates for playing a role in particle association in rosettes. This hypothesis is supported by the results of in vitro assays that directly implicate the zinc-binding domain in rosette assembly (Kurek et al. 2002). Terminal complex dissociation in response to cellulose synthesis inhibitors (Mizuta and Brown, Jr. 1992 Peng et al. 2001 Kiedaisch et al. 2003) suggest that the particles that compose linear and rosette TCs are held together in different ways. One of these inhibitors (AE FI 50944) is effective in organisms with rosettes, but not linear TCs (Kiedaisch et al. 2003), whereas another (dichlorobenzonitrile) disrupts the linear TCs of Vaucheria hamata (Mizuta and Brown, Jr. 1992). 

It is well known that the rosette and linear terminal complexes (TCs) can be observed by the freeze-fracture replication technique. The structures revealed by this technique are known as putative cellulose-synthesizing TCs. Kimura et al. (1999) demonstrated that TCs in vascular plants contain cellulose synthases using a novel technique of sodium dodecyl sulfate (SDS)-solubilized freeze fracture replica labeling (SDS-FRL). The localization of the cellulose synthase to the TC was accomplished almost 40 years after the hypothesis of Roelofsen (1958) in which he stated that enzyme complexes could be involved in cellulose biosynthesis. It has been more than 30 years since the discovery of the first TC by Brown, Jr. and Montezinos (1976) and in particular, 26 years after the discovery of rosette TCs in plants by Mueller and Brown, Jr. (1980). 

More complex models for diffusion-controlled termination in copolymerization have appeared.1 tM7j Russo and Munari171 still assumed a terminal model for propagation but introduced a penultimate model to describe termination. There are ten termination reactions to consider (Scheme 7.1 1). The model was based on the hypothesis that the type of penultimate unit defined the segmental motion of the chain ends and their rate of diffusion. 

Pd(OAc)2 works well with strained double bonds as well as with styrene and its ring-substituted derivatives. Basic substituents cannot be tolerated, however, as the failures with 4-(dimethylamino)styrene, 4-vinylpyridine and 1 -vinylimidazole show. In contrast to Rh2(OAc)4, Pd(OAe)2 causes preferential cyclopropanation of the terminal or less hindered double bond in intermolecular competition experiments. These facts are in agreement with a mechanism in which olefin coordination to the metal is a determining factor but the reluctance or complete failure of Pd(II)-diene complexes to react with diazoesters sheds some doubt on the hypothesis of Pd-olefin-carbene complexes (see Sect. 11). 

A very successful example for the use of dendritic polymeric supports in asymmetric synthesis was recently described by Breinbauer and Jacobsen [76]. PA-MAM-dendrimers with [Co(salen)]complexes were used for the hydrolytic kinetic resolution (HKR) of terminal epoxides. For such asymmetric ring opening reactions catalyzed by [Co(salen)]complexes, the proposed mechanism involves cooperative, bimetallic catalysis. For the study of this hypothesis, PAMAM dendrimers of different generation [G1-G3] were derivatized with a covalent salen Hgand through an amide bond (Fig. 7.22). The separation was achieved by precipitation and SEC. The catalytically active [Co "(salen)]dendrimer was subsequently obtained by quantitative oxidation with elemental iodine (Fig. 7.22). 

Until recently, the hypothesis that the termination reaction of type II photooxygenation reactions occurs between the substrate and an excited light absorber-oxygen complex seemed to be well established. The typical products obtained from cyclic 1,3-dienes and olefins (see Fig. 1 and Sect. IV) could only be made by photochemical reactions. 

Following our hypothesis that bradykinin adopts a C-terminal (3 turn upon complexation with the receptor, the , / backbone dihedral angles in the tetrapeptide corresponding to the C-terminus of bradykinin (Ser-Pro-Phe-Agr) were constrained in a harmonic fashion (force constant = 15 Kcal A-1 mof ) to values that define a type If [3-turn [43]. This tetrapeptide probe was then systematically translated about the interior of a theoretical box inscribing the rat... 

A hypothesis which may explain the experimental observations can be developed as follows Transfer has been assumed to occur by proton transfer to monomer. Previous studies (18,19) indicate that propagation and transfer have similar transition states in cationic polymerizations. For this reason it is possible that these two processes may both occur within the ion-counterion-monomer complex. Termination has been assumed to occur by ion-counterion collapse (20), for example, for EtAlCl2 ... 

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