Sequences that represent functional

To demonstrate the existence of functional elements responsible for pore properties of channel proteins, peptides with sequences that represent such functional segments are synthesized and their ability to mimic the targeted biological activity is tested by incorporation of the peptides into lipid bilayers. This approach allows rapid determination of which presumed transmembrane helices may form functional channels. The peptides self-assemble in the membrane to generate conductive oligomers, presumably with hydrophobic surfaces that face the phospholipid and hydrophilic residues that fine the pore. Channels of different sizes (oligomeric number) result (37, 48). 

Responsive peptides that are derived from naturally inspired motifs fall into two categories. First, responsive polypeptides can be prepared based on the structure of elas-tin, an extracellular matrix protein that is found in connective tissue such as skin and arteries (MacEwan Chilkoti, 2010). These materials typically respond to a stimulus through a change in conformation of the polymer. The second category of naturally inspired peptides consists of short peptide sequences that represent substrates for enzymes. In these materials, the change induced by the stimulus (the enzyme) is an alteration of the chemical functionality displayed on the material surface. Both response mechanisms are shown in Figure 3.4. 

Figure 6. The c-mos negative regulatory element (NRE). Nucleotide positions of the NRE are shown relative to the spermatocyte transcription start site, taken as 280 base pairs upstream of the c-mos ATG (see Fig. 4). The endpoints of the NRE are defined by deletions that allow c-mos expression in NIH 3T3 and other somatic cells. Mutations of the sequences designated by boxes 1,2, and 3 also allow c-mos transcription in NIH 3T3 cells, indicating that these sequences represent functional elements within the NRE. Boxes 1 and 2 are similar to sequences upstream of the protamine (Prot) promoter that inhibit in vitro transcription in HeLa cell extracts. A sequence just upstream of box 2 is also similar to a putative repressor-binding site in the regulatory region of Pgk2.

Although not all facets of the reactions in which complexes function as catalysts are fully understood, some of the processes are formulated in terms of a sequence of steps that represent well-known reactions. The actual process may not be identical with the collection of proposed steps, but the steps represent chemistry that is well understood. It is interesting to note that developing kinetic models for reactions of substances that are adsorbed on the surface of a solid catalyst leads to rate laws that have exactly the same form as those that describe reactions of substrates bound to enzymes. In a very general way, some of the catalytic processes involving coordination compounds require the reactant(s) to be bound to the metal by coordinate bonds, so there is some similarity in kinetic behavior of all of these processes. Before the catalytic processes are considered, we will describe some of the types of reactions that constitute the individual steps of the reaction sequences. 

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