Transient attachment

Other proteins are transiently attached to the cytosolic face of the membrane either by amide linkage of a myristate (Cl 4 0) molecule to an N-terminal Gly residue (myristoylated proteins Fig. 2b), or by thioether linkage of a 15-carbon farnesyl or a 20-carbon geranylgeranyl polyunsaturated hydrocarbon to a C-terminal Cys residue (prenylated proteins Fig. 2c). Farnesyl and geranylgeranyl are synthesized from isopentenyl pyrophosphate, the precursor of cholesterol (see Topic K5). Some proteins are also modified on Cys residues with covalently attached palmitate (Cl 6 0) (palmitoylated proteins). These include some with membrane-spanning polypeptides (Fig. 2d), some prenylated proteins and some myristoylated proteins. 

Optical excitation of metals with intense femtosecond laser pulses can create extreme non-equilibrium conditions in the solid where the electronic system reaches several thousand degrees Kelvin on a sub-picosecond timescale, while the lattice (phonon) bath, stays fairly cold. As illustrated in Figure 3.22, photoexcited hot electrons may transiently attach to unoccupied adsorbate levels and this change in the electronic structure may induce vibrational motions of the adsorbate-substrate bond. For high excitation densities with femtosecond pulses, multiple excitation/deexcitation cycles can occur and may eventually lead to desorption of adsorbate molecules or reactions with co-adsorbed species. After 1-2 ps, the hot electron... 

The reactions catalyzed by transketolase and transaldolase are distinct yet similar in many ways. One difference is that transketolase transfers a two-carbon unit, whereas transaldolase transfers a three-carbon unit. Each of these units is transiently attached to the enzyme in the course of the reaction. In transketolase, the site of addition of the unit is the... 

The biomembrane passage of a drug depends primarily on its physicochemical properties and especially on its partition coefficient (Chapters 22 and 34). Thus, the transient attachment of a lipophilic carrier group to an active principle can provide a better bioavailability, mostly by facifitating cell membrane crossing by passive diffusion. Peroral absorption, as well as rectal absorption, ocular drug delivery and dermal drug delivery, are dependent on passive diffusion. Finally, lipophilic carriers can sometimes be useful to reduce first-pass metabolism. ... 

Transient Attachment and Long-Lived Negative Ions. 99... 

Figure 3.15 Effect on a cycloaddition of the transient attachment of the dipolarophile to...

L. Barr, S. F. Lincoln, C. J. Easton, Reversal of regioselectivity and enhancement of rates of nitrile oxide cycloadditions through transient attachment of dipolarophiles to cyclodextrins, Chem. Eur. J., 2006, 12, 8571-8580. 

Peskin et al [1993] have proposed the Brownian ratchet theory to describe the active force production. The main component of that theory was the interaction between a rigid protein and a diffusing object in front of it. If the object undergoes a Brownian motion, and the fiber undergoes polymerization, there are rates at which the polymer can push the object and overcome the external resistance. The problem was formulated in terms of a system of reaction-diffusion equations for the probabilities of the polymer to have certain number of monomers. Two limiting cases, fast diffusion and fast polymerization, were treated analytically that resulted in explicit force/velocity relationships. This theory was subsequently extended to elastic objects and to the transient attachment of the filament to the object. The correspondence of these models to recent experimental data is discussed in the article by Mogilner and Oster [2003]. 

Nature s strategy of transient attachment of a stereodirecting enzyme via an enamine has been exploited by chemists using much simpler molecules, such as the natural amino acid proline (see also Section 26-1). Proline features the secondary amine function necessary for enamine formation, in addition to a carboxy substituent that renders the molecule chiral, the stereocenter exhibiting the S configuration. In the example shown below, proline catalyzes the enantioselective aldol addition of acetone to the — CHO group of 2-methylpropanal. As in the enzyme-catalyzed process, the sequence begins with enamine formation, now... 

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