Threonine metal catalysis

PP2 C) coordinated by Asp, Asn, and His residnes to form a bin-nclear metal center in the active site. This metal center is bridged by a well-ordered water molecnle. In the proposed hydrolysis mechanism, the metal-bonnd water acts as the nncleophUe to attack the phosphorons center of phosphoserine/threonine. The metal centers contribnte by Lewis acid catalysis to lower the pKa of the bonnd water and to enhance its nncleophiUcity. 

Catalysis of the hydrolysis of O-phosphoryl threonine in the presence of pyridoxal is observed with Cu and but not with other transition-metal ions there is further rate enhancement when an organic base is added.It seems clear that these two metal ions form a 1 1 complex (21) with the... 

Peptidases including keratinases are hydrolases able to hydrolyze peptide bonds in proteins and peptides. They are classified using three different approaches (1) the chemical mechanism of catalysis (based on the catalytic amino acid or metal ion at then-active site, represented by serine, cysteine, threonine, aspartic, asparagine, glutamic and metallocatalytic type), (2) the catalytic reaction (this type of classification depends on the selectivity for the bonds that the peptidases will hydrolyze), and (3) the molecular structure and homology. In this latter approach, amino acid... 

The importance of secondary interactions in modulating the chemistry of a metal-bound water or hydroxide moiety has been discussed for metaUoenzymes of varying function [42, 43). For example, in zinc-containing carbonic anhydrase II, the zinc-bound hydroxide acts as a H-bond donor to a threonine residue (Thrl99 Fig. 8.7) [37]. This interaction orients the lone pair of the hydroxide and reduces the entropic barrier for catalysis [44]. Notably, perturbation of this H-bonding interaction is reported to increase the Zn-OH2 pJCa value by around 2 units [45]. The threonine residue also stabilizes the transition state via H-bonding and destabilizes the product (bicarbonate-bound) form of the enzyme. It is also interesting that an X-ray diffraction study of crystals of carbonic anhydrase II isolated at pH 7.8 revealed two water molecules that donate H-bonds to the zinc-bound hydroxide (Fig. 8.7) [46]. 

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