Site switching mechanism

Therefore, according to Razavi et al. [127,143], chain migratory insertions and chain stationary insertions would follow each other alternately, probably involving a pen-penultimate mechanism dictated by the periodicity of the helices of the growing chain [413]. If such a coordination site switching mechanism intervenes periodically, the resulting polypropylene chain would expose a syndioisoblock microstructure. 

FIGURE 22. The active site of alkaline phosphatase (above) and an allosteric kinetic switch mechanism (below) for the regulatory function of the Mg + ions in controlling the conformation of the nonequivalent subunits (square and circle). Reprinted with permission from Reference 214. Copyright 2005 American Chemical Society... 

Interestingly, the arginine switch mechanism was first recognized when it was artificially induced in AAT. When AAT was mutated in six distinct positions, a substantial increase of activity with aromatic substrates was observed. The crystal structure of the engineered enzyme showed that the aromatic side chains could be accommodated at the active site as a result of R292 movement. A similar observation was made on an AAT mutant form whose substrate specificity was broadened using direct evolution techniques, in order to include branched chain and aromatic amino acids. 

The kinetics of nitrogenase was elucidated many years ago and comprises two cycles, the Fe-protein cycle and the actual conversion of Nj to NHj in the MoFe protein (Figure 6.2). The electrons needed for the reduction of N2 are provided by the Fe protein. To this end, the Fe-protein in its reduced state ([Fc4S4] ) temporarily associates with the MoFe-protein, and one electron is transferred from each Fe-protein to the corresponding catalytic unit. The relatively large distance between the nucleotide binding sites and the protein complex interface (>10 A) requires a structural switch mechanism within the protein matrix that initiates the concomitant hydrolysis of two MgATP... 

Taken together, these results indicate that similar to other proton-translocating membrane proteins, both types of Na /H exchangers contain critical sulfhydryl groups that are involved in the transport mechanism. These sulfhydryl groups do not appear to be present at the external transport site but may be involved in switching from an inactive to an activated state. 

In the case of LNT, different proposals have been advanced to explain the mechanisms governing the NO, release. Recent papers suggested that the NO release is provoked by the heat generated upon the reducing switch (thermal release) [40], by the decrease of the gas-phase oxygen concentration that destabilizes the stored nitrates [41], by spillover and reduction of N02 onto reduced Pt sites or by the establishment of a net reducing environment, which decreases the equilibrium stability of nitrates [12,42,43],... 

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