Zipper-type

In the absence of hormone, the three-dimensional configuration of the receptor favors binding to corepressors present in the cell nucleus. The interaction is produced at the level of zipper-type sequences of leucines (-L-X-X-L-), present in the corepressors, with the LBD of the receptor. This has a structure that is complementary to the leucine zipper, which remains accessible while the receptor itself does not bind to the hormone (Gruber et al. 2002 Nilsson et al. 2001). 

When Fos and Jun operate as partners to form a leucine zipper type of transcription factor, this can lead to the activation of genes to make anything you can think of, from enzymes to receptors to structural proteins (see Fig. 2—42). 

Diethynylbenzenes 126 react with two equivalents of 2-((trimethylsilyl)ethynyl)benzene-l,4-diol 127 via an alkyne-zipper type bicyclization to afford the complex indenopyran ring systems 128 (Equation 60) <1998TL8237>. 

Table XVI gives a partial list of native proteins that have been hydrolyzed with proteolytic enzymes. A discussion of the interpretation of each example listed is beyond the scope of this review, but a few comments concerning certain features of proteolysis are ivarranted. The mechanism of enzymatic hydrolysis of native proteins was studied in detail by Tiselius and Eriksson-Quensel (1939), who examined the action of pepsin on ovalbumin. Two mechanisms of proteolysis were considered by these workers. In the first mechanism the enzyme hydrolyzes all susceptible peptide bonds in one substrate molecule before hydrolysis of a second molecule begins. This type of mechanism has been described by Lmderstrpm-Lang (1952) as the all or none type. In the second mechanism, the enzyme hydrolyzes the single, most susceptible bond in all substrate molecules before hydrolysis of other bonds occurs. This mechanism is called the zipper type. Hydrolysis of a protein can proceed by either of the two mechanisms or by a mechanism which has features of both types. General aspects of the problem have been reviewed and theoretical equations which describe the kinetics of ea( h mechanism have been derived (Linderstr0m-Lang, 1952, 1953).

In other studies it appears that the kinetics of digestion of human serum albumin by pepsin and chymotrypsin follow the all or none mechanism, whereas tryptic action is of the zipper type (Kaminski and Tanner,... 

This one-pot procedure for titanium-induced reactions is also applicable to the synthesis of crowded products 5 to completely chemo- and regioselective "zipper-type" polycyclizations,7 to bimolecular reductions of alkynes,5 and to conventional McMurry reactions of aldehydes or ketones.5 Some representative examples are compiled in the Table. 

This reaction is believed to involve a number of complex processes, by which four alkynes coordinate to a metal center to undergo either a stepwise coupling or a concerted zipper-type cyclization. However, the Ni2(COT)2-catalyzed reaction involves the formation of a bis(cyclopentadienylnickel) complex, a homobimetallic center,as supported by the actual determination of the structure from X-ray crystallography by the fast initiation of the reaction without an induction period when used directly for alkynes compared to other types of catalysts,and by kinetic studies and labeling experiments. An illustrative mechanism catalyzed by Ni2(COT)2 is given here. 

Fig. 10.23 Three examples of zipper type junction zones double helix, hydrogen-bonded ladder (with small loops), and eggbox junction. (Reprinted with permission from Ref. [67].)...

Scheme 8.69 A palladium-driven zipper-type sequential reaction [404].

Figure 28 Building blocks used for construction of topologically matching zipper-type assemblies. (Reproduced from Ref. 124. Wiley-VCH, 2009.)...

The combination of carbonylation with other reactions is one strategy often used to make a novel cascade reaction such as a zipper-type reaction [16], In the 1990s Negishi s group has reported a series of palladium-catalyzed cascade reactions involving carbonylation [17],... 

Finally, the intramolecular codimerization of C=C and C=C bonds under the influence of palladium catalysts have frequently been used for cyclization reactions. Although there is little firm mechanistic evidence for the reaction sequence involved, suitable design of the starting material has now allowed the simultaneous formation of up to five cyclopentane rings in a zipper-type reaction shown in Eq. (5). 

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