Binding site identical

As in the coronands, binding site identity plays an important role in the complexation preferences of cryptands. Replacement of oxygens by sulfur or nitrogen in the bridges (45b-d) results in decreased stability and selectivity for alkali metal and alkaline earth cations (as was also noted for the coronands).66,164 Selectivity is then shifted toward softer metal ions such as Ag+, Tl+, Cd2+ and Pb2+. 

Suggestions concerning the structural element to which the enzyme binds vary. Razzell and Khorana (51), on the basis of minimal requirements for activity, considered the binding site identical with the hydrolytic site, which is limited to one doubly esterified 5 -nucleotide. Bjork (43) believed that besides a hydrolytic site additional binding sites exist. In this connection it should be mentioned that whereas a 5 -mono-... 

Protein inhibitors are often active against a variety of en2ymes, although each molecule may possess a separate and very distinct binding site for each en2yme. For example, many trypsin and chymotrypsin inhibitors are identical compounds (12). 

In the middle of the accuracy spectrum are the models based on approximately 35% sequence identity, corresponding to 85% of the Ca atoms modeled within 3.5 A of their correct positions. Eortunately, the active and binding sites are frequently more conserved... 

The enzyme is made of four identical subunits each having a single binding site for each ligand. 

Figure 8.14 Overall view of the complex between 434 repressor fragment and a palindromic synthetic 14mer of DNA (see Table 8.2). The two binding sites of the repressor dimer to the DNA are identical.

FIGURE 10.18 A model for the structure of the a-factor transport protein in the yeast plasma membrane. Gene duplication has yielded a protein with two identical halves, each half containing six transmembrane helical segments and an ATP-binding site. Like the yeast a-factor transporter, the multidrug transporter is postulated to have 12 transmembrane helices and 2 ATP-binding sites. 

Muscle glycogen phosphorylase is a dimer of two identical subunits (842 residues, 97.44 kD). Each subunit contains a pyridoxal phosphate cofactor, covalently linked as a Schiff base to Lys °. Each subunit contains an active site (at the center of the subunit) and an allosteric effector site near the subunit interface (Eigure 15.15). In addition, a regulatory phosphorylation site is located at Ser on each subunit. A glycogen-binding site on each subunit facilitates prior association of glycogen phosphorylase with its substrate and also exerts regulatory control on the enzymatic reaction. 

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