Threonine sidechains

It should be noted that this mechanism involves a final oxidation. The need for this is avoided when amino acids with an a-hydroxy group in the sidechain (serine, threonine) are involved, as pointed out by Shibamoto and Bernhard234 (see Scheme 5.9). Baltes and Bochmann235 obtained as many as 123 pyrazines, both mono- and bicyclic, by interacting sucrose with serine and threonine under coffee-roasting conditions. 

On the other hand, a particular protein function can be realized with different protein folds, and an example of this are protein phosphatases. Protein phosphatases feature two distinctively different catalytic mechanisms for hydrolytically cleaving phosphorylated amino acid residues. The active sites of serine/threonine protein phosphatases (PPs) contain two metal centers that directly activate a water molecule for nucleophilic attack of the phosphate ester bond. In contrast, protein tyrosine phosphatases (PTPs) [105] possess a Cys residue present in the active site loop containing the conserved PTP signature motif HCXXXXXRS. The Cys sidechain acts as the attacking nucleophile in the formation of a phosphocysteine intermediate, which is eventually hydrolyzed by a water molecule [106], The same catalytic mechanism is also shared by dual-specificity phosphatases (see below). 

The functions of 2 conserved residues, threonine (T4) and lysine (Kll) of A, nidulans (Table 1 " ), were investigated by site-directed mutagenesis- Changes were planned to determine whether the contribution of these residues was steric or due to their functional sidechains. 

Diversity of protein structure and function is enhanced by the different chemical functional groups seen in the 20 common amino acids. This variety, however, complicates the development of empirical force field parameters for proteins. For simplicity we will simply list a number of the model compounds used for the different amino acids. This is presented in Table 1. The selection of appropriate model compounds is based on a balance between the size of the compound and the available target data. For example, a large number of gas and condensed phase data are available for methanol however, sole use of that compound for the sidechains of serine or threonine avoids accurate tests of parameters associated with the covalent connection of the sidechain to the backbone. This is overcome by the use of larger compounds such as ethanol and isopropanol. Increases in computational resources will allow for ab initio calculations on larger model compounds. However, as discussed in the previous section, care... 

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