Threonine hydrolases

These three catalytic functionalities are similar in practically all hydrolytic enzymes, but the actual functional groups performing the reactions differ among hydrolases. Based on the structures of their catalytic sites, hydrolases can be divided into five classes, namely serine hydrolases, threonine hydrolases, cysteine hydrolases, aspartic hydrolases, and metallohydrolases, to which the similarly acting calcium-dependent hydrolases can be added. Hydrolases of yet unknown catalytic mechanism also exist. 

The serine hydrolases, threonine hydrolases, and cysteine hydrolases, the attacking nucleophile of which is a serine or threonine OH group or a cysteine thiolate group, respectively, and which form an intermediate covalent complex (i. e., the acylated enzyme). Here, an activated H20 molecule enters the catalytic cycle in the second step, i.e., hydrolysis of the covalent intermediate to regenerate the enzyme. 

The most recently discovered group of proteases are the N-terminal threonine hydrolases of the multi-catalytic protease complex (MPC) of proteasomes. The enzymes are arranged in a regular array inside proteasomal compartments as shown in Box 7-A. The active site is a catalytic dyad formed from the amino group at the N terminus of the P subunits.345-3463... 

Many enzymes have a specific requirement for potassium. In certain instances, potassium may act along with certain other ions in influencing enzyme activity besides ATPase examples include L-serine hydrolase, L-threonine hydrolase, S-adenosylme-thionine, L-tyrosine-RNA ligase, propionyl-CoA C02 ligase, and acetate CoA ligase (Oberleas etal. 1999). 

Threonine peptidases (and some cysteine and serine peptidases) have only one active site residue, which is the N-terminus of the mature protein. Such a peptidase is known as an N-terminal nucleophile hydrolase or Ntn-hydrolase. The amino group of the N-terminal residue performs the role of the general base. The catalytic subunits of the proteasome are examples of Ntn-hydrolases. 

Protein dephosphorylation is catalyzed by phospho-hydrolases called protein phosphatases. While the number of protein tyrosine kinases is roughly comparable to the number of protein tyrosine phosphatases, protein serine-threonine kinases vastly outnumber the protein serine-threonine phosphatases, of which about 25 different species are known to exist. This relative under-representation may be accounted for by the alternative diversification... 

Glycopeptides containing glycosyl L-aspara-gine, L-serine, and L-threonine, developments in, 50,277-310 Glycoside hydrolases, mechanistic information firm studies with reversible and irreversible inhibitors, 48, 319-384 Glycoside synthesis, anomeric-oxygen activation for (trichloroacetimidate method), 50,21 -123... 

The 20S proteasome is a protease complex comprising 28 subunits. It is a barrelshaped structure formed by the axial stacking of four rings made up of two outer a rings and two inner / rings arranged in / / order. Proteasomes are members of the N-terminal nucleophile- (Ntn-) hydrolase superfamily [19]. Their N-terminal threonine residues are exposed as the nucleophile in peptide bond hydrolysis [20, 21]. In eukaryotic cells, three of the fS-type subunits have N-terminal threonine residues, are active and have specificities determined largely by the nature of their... 

The structure of the 20S proteasome (Fig. 2.13) from Thermoplasma acidophilum displays four rings stacked upon each other surrounding a central cavity in which proteolysis takes place. An N-terminal threonine has been identified as an essential active site residue of the protease center. The OH-group of the threonine functions as a nucleophile during hydrolysis of the petide bond. A similar mechanism of hydrolysis has been shown for other hydrolases, which, because of this property, are now included in the family of N-terminal nucleophile hydrolases. For some /1-subunits of eucaryotes the N-terminal threonine is generated by autoproteolysis of an N-terminal prosequence. 

Figure 2.3 Serine protease and hydrolase ABPs. (A) Reaction of a general serine hydrolase probe containing a fluorophosphonate (FP) reactive electrophile. This class of probes has been used extensively to label various classes of serine hydrolases including proteases, esterases, lipases and others. (B) The peptide diphenyl phosphonate (DPP) reacts with the serine nucleophile in the active site of serine proteases. This probe is much less reactive than the FP class of probes but is more selective towards serine proteases over other types of serine hydrolases.(C) The natural product epoxomicin contains a keto-epoxide that selectively reacts with the catalytic N-terminal threonine of the proteasome P-subunit. This reaction results in the formation of a stable six-membered ring. This class of electrophile has been used in probes of the proteasome.

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... 

---