Oxyanion binding

Serine proteinases such as chymotrypsin and subtilisin catalyze the cleavage of peptide bonds. Four features essential for catalysis are present in the three-dimensional structures of all serine proteinases a catalytic triad, an oxyanion binding site, a substrate specificity pocket, and a nonspecific binding site for polypeptide substrates. These four features, in a very similar arrangement, are present in both chymotrypsin and subtilisin even though they are achieved in the two enzymes in completely different ways by quite different three-dimensional structures. Chymotrypsin is built up from two p-barrel domains, whereas the subtilisin structure is of the a/p type. These two enzymes provide an example of convergent evolution where completely different loop regions, attached to different framework structures, form similar active sites. 

The oxyanion binding site stabilizes the transition state by forming two hydrogen bonds to a negatively charged oxygen atom of the substrate. Mutations that prevent formation of one of these bonds in subtilisin decrease the rate by a factor of about 10. ... 

Lawson, D. M., Williams, C. E., Mitchenall, L. A., and Pau, R. N. (1998). Ligand size is a major determinant of specificity in periplasmic oxyanion-binding proteins the 1.2 A resolution crystal structure of Azotobacter vinelandii ModA. Structure 6, 1529-1539. 

B. Asboth, E. Stokum, I. U. Khan, L. Polgar, Mechanism of Action of Cysteine Protei-nases Oxyanion Binding Site Is not Essential in the Hydrolysis of Specific Substrates ,... 

Tertiary alcohols are very unreactive toward hydrolases. There are, however, exceptions. Some enzymes have a certain amino acid motif located in the oxyanion binding pocket that allows the docking of space-demanding alcohols such as tertiary ones into the acylated enzyme [104]. One such hydrolase is Candida antarctica lipase A (CALA), which has been found to catalyze the acylation of the tertiary 2-phenyl-3-butyn-2-ol rac-110 by vinyl acetate in organic solvents. Thus efficient resolution of 110 was achieved in isooctane at room temperature (Scheme 4.34) [105]. 

Various kinetic studies of the bovine spleen acid phosphatase have been reported (Vincent etal., 1991). The results are consistent with a picture in which the oxyanions bind in a non-competitive fashion by bridging the two iron atoms in the PAP s dinuclear centre, with the smaller anions also able to bind in a competitive manner at a second site (Fig. 5-21). 

Fig. 5-21. Proposed model for oxyanion binding. A=non-competitive site, B=competitive site. Reproduced from Vincent et al. (1991).

B. Asboth and L. PoIgSr. Transition-state stabilization at the oxyanion binding sites of serine and thiol proteinases hydrolyses of thiono and oxygen esters. Biochemistry 22 117(1983). 

Table II. Oxyanion binding mechanisms and surface speciation...

Figure 14 1,3-Bis-aryl and 1,3-aryl-alkyl mono-urea monomers providing for strong binding to oxyanions. Binding constants for TBA (tetrabutyl ammonium) benzoate determined by H-NMR tilrations in DMSO-de have been indicated.

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