Proteins, classes phospho

Protein phosphatases are several classes of enzymes that catalyze the hydrolysis of phospho-amino acids within a peptide or protein, thus resulting in dephosphorylation. 

Protein phosphatases are classified according to their activity toward phospho-amino acids they act on (Fig. 1). Nomenclature is independent of regulation simply because stimuli were unknown. Protein phosphatases hydrolyzing O-phospho-monoesters are currently subdivided into two major classes (i) phosphatases acting on phosphoserine (pSer) and phosphothreonine (pThr), and (ii) the second class... 

Hie first protein kinase obtained in a purified form was the Ser/Thr-specific phospho-rylase kinase of muscle, in 1959 (Krebs et al., 1959). With the discovery of the Tyr-specific protein kinases (Erikson et al., 1979), the Ser/Thr-specific protein kinases were joined by another extensive class of protein kinases of regulatory importance, to which a central function in growth and differentiation processes was soon attributed. At present, several hrmdred different protein kinases are known in mammals, most of which are Ser/Thr- or Tyr-specific. In addition, there are some protein kinases that phospho-rylate other amino acids (review Hrmter, 1991). 

Inhibition of protein phosphatases by OA class compounds leads to a rapid increase of phospho-rylated proteins in cells. This effect seems to be responsible for the diarrhea and the degenerative changes in absorptive epithelimn of small intestine induced by these toxins. In fact, the increased level of phosphorylated cytoskeletal proteins modifies the junctional elements between the intestinal epithelial cells controlling the permeability to solutes, which, in turn, results in passive loss of fluids. For more details about the diarrheic mechanism, see the in vitro activities section. 

Anthranilate phosphoribosyl transferase (EC 2.4.2.18) (AnPRT) belongs to the class of transferase enzyme, which enacts the transfer of specific functional group (e.g., methyl or glycosyl group) from one molecule (called the donor) to another (called the acceptor). It transfer a ribose group between the basic functional group of anthranilate and 5-phospho-alpha-D-ribose 1-diphosphate (PRPP). AnPRT has four domains and its quaternary structure consists of two identical protein structures [13]. Each domain of AnPRT contains a magnesium ion and a pyrophosphate molecule as the active site. The secondary structure of AnPRT consists mainly of alpha helices with a beta sheet within each domain (Fig. 14.6). 

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