Phosphoseryl residues

This enzyme [EC 2.7.1.70], also referred to as protamine kinase, catalyzes the reaction of ATP with protamine to produce ADP and an O-phosphoseryl residue in protamine. The enzyme will also phosphorylate histones and it requires cAMP. 

Milk acid phosphatase has been purified to homogeneity by various forms of chromaotgraphy, including affinity chromatography purification up to 40 000-fold has been claimed. The enzyme shows broad specificity on phosphate esters, including the phosphoseryl residues of casein. It has a molecular mass of about 42 kDa and an isoelectric point of 7.9. Many forms of inorganic phosphate are competitive inhibitors, while fluoride is a powerful non-competitive inhibitor. The enzyme is a glycoprotein and its amino acid composition is known. Milk acid phosphatase shows some similarity to the phosphoprotein phosphatase of spleen but differs from it in a number of characteristics. 

An additional example of how enzymes can affect the physical properties of purified proteins relates to the enzymic dephosphorylation of /3-casein (4). Bovine /3-casein contains five phosphate groups per monomer as phosphoseryl residues. Purified /3-casein from bovine milk was dephosphorylated by a phosphoprotein phosphatase. Both the native /3-casein and 65% dephosphorylated /3-casein self-associated to form polymers at 35°C. However, the dephosphorylated /3-casein had a larger sedimentation coefficient (S35 22.5) than that of the native /3-casein (S35 18.2). Also the sedimentation pattern of the dephosphorylated /3-casein was more polydisperse than the hypersharp pattern of the native /3-casein. These properties were accentuated with 95% dephosphorylated /3-casein. The decrease in the negative charge caused by the loss of phosphate apparently favors the self-association of /3-casein resulting from hydrophobic bonding. 

This involves the exchange of the divalent Ca (attached to casein via carboxyl and phosphoseryl residues) of the calcium para-caseinate net-... 

Based on a new proposed model, each CCP nanocluster is assumed as a core and si-> o s2- ]S-caseins are linked to this core. Since / -casein just contains 1 hydrophobic site, it links to only 1 CCP nanocluster in essence, as soon as ]S-casein links to CCP (core) the growth of micelle in that direction ceases. Contrary, si- and as2-caseins are multi-functional (bi-functional) caseins and own 2 hydrophobic sites. These caseins develop the network via cross-linking since each as-casein that is linked to a CCP is able to interact with the next CCP and in this way nanoclusters link to each other. The tendency of Ca-sensitive caseins to interact with CCP is directly related to their phosphoseryl residues. The multifunctional caseins continually link CCP nanoclusters to each other and this process continues till the end nanocluster links to the first one, and a loop is formed. Bacause multi-functional caseins link randomly to each other a variety range of micelle size is obtained. The location of K-casein and its role in the stability of micelle appear to be unclear in this model [15]. 

Their calculation takes the premise that the casein proteins do not change their titration behavior when bound to the colloidal calcium phosphate, an assumption that appears unreasonable if the phos-phoseryl residues are involved in the interaction. An alternative calculation which allows the casein phosphoseryl groups to titrate when the calcium phosphate is removed predicts, from the same titration data, that over half the orthophosphate groups in the micelles are protonated (Holt et al., 1989a). 

In intact cells insulin has been shown to stimulate receptor autophosphorylation. However, much smaller amounts of phosphotyrosine were found compared to that seen using purified, solubilized receptor preparations and, indeed, the predominant phosphorylation actually occurred on serine residues [69,71]. It has been suggested that there may be an insulin-stimulated, phosphoseryl-specific kinase which is loosely associated with the receptor and is activated by the receptor tyrosyl kinase [61]. This might account for the insulin-stimulated phosphoseryl kinase activity observed in both intact cells and when using crude, solubilized receptor preparations. Such an activity might also provide a mechanism for insulin s ability to enhance serine phosphorylation on target proteins in plasma membranes and elsewhere in the cell [25,78]. Nevertheless, (auto)-phosphorylation of the insulin receptor on tyrosine residues has been shown to occur immediately upon receptor occupancy by insulin and to precede any phosphorylation on serine residues [69]. Indeed, it remains to be seen as to whether the seryl phosphorylation occurs as a direct insulin-stimulation event or mediated by other kinases (cAMP/C-kinase) as a consequence of insulin-stimulated autophosphorylation on tyrosyl residues. 

With regard to selenocysteine biosynthesis in archaea and eukarya, a tRNA has been known for some time, which accepts L-serine and possesses an anticodon complementary to UGA. It shares a number of structural featmes with tRNA from E. coli, such as extended aminoacyl acceptor and D-arms. In eukarya, the serine residue attached to this tRNA can be phosphorylated by a specific kinase and it was first assumed that this tRNA inserts phosphoserine into proteins. However, closer examination revealed that this tRNA carries selenocysteine in vivo and certainly is the pendant to tRNA from E. coli. It is still elusive whether (9-phosphoseryl-tRNA is the biosynthetic intermediate for selenocysteyl-tRNA formation in eukaryotes also, there is no evidence yet of an eukaryal and archaeal enzyme, equivalent in its function to selenocysteine synthase from bacteria. 

Also, it is clearly established that the process occurs by first phosphorylating a serine residue of the enzyme and then by hydrolysis of the phosphoseryl enzyme (33). The enzyme will also transfer the phosphoryl group to another alcohol as well as to water, and it has been shown by using chiral... 

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