Threonine residues

Arsenic competes with phosphate for incorporation into precursors of DNA or RNA such as dATP, rATP, dGTP, rGTP, etc. (28,37.38.39). If arsenic is incorporated into molecules such as ATP, then it may also be added to serine and threonine residues of cellular proteins, since protein kinase uses the terminal phosphate of ATP in the phosphorylation of a variety of cellular proteins. 

ACS Symposium Series American Chemical Society Washington, DC, 1980. 

Transformation of Cells in Tissue Culture by Carcinogenic Metals and Their Compound  

Mammalian cell cultures have been used as the basis of several systemsin detect ng the potential carcinogenic activity of chemicals. Basically, two general approaches have been utilized continuous cell lines and primary cell cultures. Cell lines have the advantage of ease of use, in that cultures do not have to be obtained fresh from animals prior to each test, but may be maintained for months to years by proper subculturing techniques. They have the disadvantage of possessing one or more transformed characteristics (e.g., immortality). In some cases cell lines may also lack certain enzyme systems required for metabolic activation of chemicals. Some of the cell lines used for transformation assays include the murine (BALB/3T3) A31 system ( ), and the baby kidney-21 (BHK-21) systems (43). 

Primary cell cultures have advantages over cell lines in that the cells are not initially immortal, and usually have none of the transformed characteristics which may be seen in some cell lines Additionally, the embryonic cells which are most 

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The last part of this account will be devoted to protein kinases and protein phosphatases and some recent results we have obtained for them. Protein kinases and phosphatases are signaling biomolecules that control the level of phosphorylation and dephosphorylation of tyrosine, serine or threonine residues in other proteins, and by this means regulate a variety of fundamental cellular processes including cell growth and proliferation, cell cycle and cytoskeletal integrity. 

In both structures the ion is coordinated to six ligands with octahedral geometry. Four water molecules as well as the side chain oxygen atom of a serine residue from the P-loop and one oxygen atom from the (3-phosphate bind to Mg + in the GDP structure. Two of the water molecules are replaced in the GTP structure by a threonine residue from switch I and an oxygen atom from the y phosphate (similar to the arrangement shown in... 

A variety of cellular and viral proteins contain fatty acids covalently bound via ester linkages to the side chains of cysteine and sometimes to serine or threonine residues within a polypeptide chain (Figure 9.18). This type of fatty acyl chain linkage has a broader fatty acid specificity than A myristoylation. Myristate, palmitate, stearate, and oleate can all be esterified in this way, with the Cjg and Cjg chain lengths being most commonly found. Proteins anchored to membranes via fatty acyl thioesters include G-protein-coupled receptors, the surface glycoproteins of several viruses, and the transferrin receptor protein. 

FIGURE 9.26 The carbohydrate tnoiedes of glycoproteins may be linked to the protein via (a) serine or threonine residues (in the O-linked saccharides) or (b) asparagine residues (in the N-linked saccharides), (c) N-Linked glycoproteins are of three types high mannose, complex, and hybrid, the latter of which combines structures found in the high mannose and complex saccharides. 

Lipase from Aspergillus niger, 0.2 M phosphate buffer, acetone, pH 7, 37°, 50-96% yield. This lipase was used in the cleavage of phosphopeptide heptyl esters. These conditions are sufficiently mild to prevent the elimination of phosphorylated serine and threonine residues." ... 

The a subunits, for which two isoforms exist in mammals (al, a2), contain conventional protein serine/threonine kinase domains at the N-terminus, with a threonine residue in the activation loop (Thr-172) that must be phosphorylated by upstream kinases (see below) before the kinase is active. The kinase domain is followed by an autoinhibitory domain, whose effect is somehow relieved by interaction with the other subunits. The C-terminal domain of the a subunit is required for the formation of a complex with the C-terminal domain of the (3 subunit, which in turn mediates binding to the y subunit. The al and a2 catalytic subunit isoforms are widely distributed, although a2 is most abundant in muscle and may be absent in cells of the endothelial/hemopoietic lineage. 

These enzymes are activated by the binding of cAMP or cGMP. When activated, cAKs and cGKs phosphorylate specific serine or threonine residues in target proteins control the activity of these proteins. 

Histone phosphorylation is a common posttranslational modification fond in histones, primarily on the N-terminal tails. Phosphorylation sites include serine and threonine residues, tyrosine phosphorylation has not been observed so far. Some phosphorylation events occur locally whereas others occur globally throughout all chromosomes during specific events like mitosis. Histone phosphorylation is catalyzed by kinases. Removal of the phosphoryl groups is catalyzed by phosphatases. 

MAPK cascades are composed of three cytoplasmic kinases, the MAPKKK, MAPKK, and MAPK, that are regulated by phosphorylation (Fig. 1) [1, 2]. The MAPKKK, also called MEKK for MEK kinase, is a serine/threonine kinase. Selective activation of MAPKKKs by upstream cellular stimuli results in the phosphorylation of MAPKK, also called MEK for MAP/ERK kinase by the MAPKKK. MAPKKK members are structurally diverse and are differentially regulated by specific upstream stimuli. The MAPKK is phosphorylated by the MAPKKK on two specific serine/ threonine residues in its activation loop. The MAPKK family members are dual specificity kinases capable of phosphorylating critical threonine and tyrosine residues in the activation loop of the MAPKs. MAPKKs have the fewest members in the MAPK signaling module. MAPKs are a family of serine/threonine kinases that upon activation by their respective MAPKKs, are capable of phosphorylating cytoplasmic substrates as well as... 

The R2C2 complex has no enTymatic activity, but the binding of cAMP by R dissociates R from C, thereby activating the latter (Figure 43-5). The active C subunit catalyzes the transfet of the y phosphate of ATP to a serine or threonine residue in a variety of proteins. The consensus phosphotylation sites are -Arg-Arg/Lys-X-Ser/Thr- and -Arg-Lys-X-X-Ser-, where X can be any amino acid. 

Single protein kinases such as PKA, PKC, and Ca +-calmodulin (CaM)-kinases, which result in the phosphorylation of serine and threonine residues in target proteins, play a very important role in hormone action. The discovery that the EGF receptor contains an intrinsic tyrosine kinase activity that is activated by the binding of the hgand EGF was an important breakthrough. The insuhn and IGF-I receptors also contain intrinsic... 

Figure 52-6. Diagrammatic representation of the structures of the H, A,and B blood group substances. R represents a long complex oligosaccharide chain, joined either to ceramide where the substances are glycosphingolipids, or to the polypeptide backbone of a protein via a serine or threonine residue where the substances are glycoproteins. Note that the blood group substances are biantenna ry ie, they have two arms, formed at a branch point (not indicated) between the GIcNAc—R, and only one arm of the branch is shown. Thus, the H, A,and B substances each contain two of their respective short oligosaccharide chains shown above. The AB substance contains one type A chain and one type B chain.

Human interleukin 2, a 133-residue protein, has been separated into multiple molecular forms by selective immunoaffinity chromatography and chromatofocusing. Most of the heterogeneity has been attributed to variations in glycosylation of the threonine residue in position 3 of the polypep-... 

Townsend-Nicholson A, Schofield PR. A threonine residue in the seventh transmembrane domain of the human A, adenosine receptor mediates specific agonist binding. J Biol Chem 1994 269 2373-2376. 

Cytoplasmic serine/threonine protein kinases catalyze the transfer of phosphate groups to serine and threonine residues of target proteins. Serine/threonine kinases have been recognized as the products of protooncogenes (e.g., c-mos, c-raj) or as kinases intimately involved with the regulation of serine/threonine kinase activity by cAMP. Some of these kinases specifically phosphorylate cellular structural proteins, such as histone, laminins, etc. Others phosphorylate still more kinases, resulting in either the activation or deactivation of downstream protein kinases. Specific examples in which serine/threonine kinases elicit specific cellular responses are discussed in this chapter. 

This intermediate MAPK activator (MAPK kinase, MAPKK) is a 45 kDa phosphoprotein capable of phosphorylating MAPK on serine/threonine and tyrosine residues (Matsuda et al., 1992 Nakielny et al., 1992a Kosako et al., 1993). Like MAPK, the activity of MAPKK is regulated by phosphorylation. During oocyte maturation MAPKK is phosphorylated on threonine residues (Kosako et al., 1992), and this phosphorylation is required for its activity (Ahn et al., 1991 Gomez and Cohen, 1991 Kosako et al., 1992 Matsuda et al., 1992). MPF can activate both MAPKK and MAPK in vitro, with the activation of MAPK lagging behind that of MAPKK however, MPF cannot activate either purified MAPKK or MAPK that has been dephosphorylated by phosphatases (Matsuda et al., 1992). MAPKK and MAPK are therefore believed to function downstream of MPF (Fig. 3). 

Seger, R., Ahn, N. G., Boulton, T. G., Yanopoulos, G., Panayotatos, N., Radzziejewska, E., Ericsson, L., Bratlien, R. L., Cobb, M. H., and Krebs, E. G. (1991). Microtubule-associated protein 2 kinases, ERK1 and ERK2, undergo autophosphorylation on both tyrosine and threonine residues implications for their mechanism of activation. Proc. Natl. Acad. Sci. USASS 6142-6146. 

While in the uncatalyzed reaction of l-ethyleneimino-2-hydroxy-3-butene in THF, refluxing for four hours was necessary to produce 70% of the ester, in the presence of NaNH2 a 90% yield was achieved at room temperature after only five minutes.[13 14] An especially interesting example of the use of the imidazolide method for ester synthesis is illustrated by the total synthesis of actinomycin C3.[15],[16] Working with N-protected L-TV-methylvaline and CDI, esterification of the hydroxyl group on the threonine residue proved successful whereas this could not be accomplished by any of the conventional methods. 

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