Subunits, of proteins

The antisense oligonucleotide LErafAON against the serine/threonine kinase c-Raf has been tested in phase I clinical trials. The antisense oligonucleotides ISIS-5132, which also inhibits c-Raf, and ISIS-3521, which inhibits PKC, went through different phase clinical trials with solid tumour patients. Unfortunately, no objective responses occurred with these PKI. GEM-231, an oligonucleotide targeting the RIa subunit of protein kinase A is currently undergoing phase I/II clinical trials alone or in combination with traditional therapy for the treatment of solid cancers [3]. 

How do a wide variety of neurotransmitters and hormones produce tissue- and cell-specific biological responses, if many such responses are mediated by the same intracellular messengers, cAMP and cAMP-depen-dent protein kinase Specificity is achieved at two levels at the level of tissue-specific receptors for the neurotransmitter or hormone and at the level of tissue-specific substrate proteins for the protein kinase. Only tissues that possess specific receptors will respond to a certain neurotransmitter or hormone. Moreover, since all cells contain very similar catalytic subunits of protein kinase A (see Ch. 23), the nature of the proteins that are phosphor-ylated in a given tissue depends on the types and amounts of proteins expressed in that tissue and on their accessibility to the protein kinase. 

Goldberg, J., Huang, H. B., Kwon,Y. G., Greengard, P., Nairn, A. C. and Kuriyan, J. Three-dimensional structure of the catalytic subunit of protein serine/threonine phosphatase-1. Nature 376 745-753,1995. 

The critical discovery that acetyl phosphate is generated and the information gained from several studies of each of the components of GR allowed an enzyme mechanism to be proposed (Arkowitz and Abeles 1991). However, with the current knowledge that one of the subunits of protein B also contains selenium, further work is needed to characterize the intermediates of the reaction and to explain the role of an additional selenocysteine residue. Whether this additional selenocysteine residue in protein B might serve as a direct reductant of the postulated thioselenide derivative of selenoprotein A and possibly serve as a link to the Trx-TrxR system is unknown. It should also be noted that the selenium-limited cultures that were initially studied during analysis of selenoprotein A (Turner and Stadtman 1973) apparently contained active fractions of proteins B and C, suggesting the role for selenium in protein B may not prove to be absolutely necessary for enzyme catalysis. 

Anand G.S., Hughes C.A., Jones J.M., Taylor S.S., Komives E.A. Amide H/ 2H exchange reveals communication between the cAMP and catalytic subunit-binding sites in the Rla subunit of protein kinase A. J. Mol. Biol. 2002, 323, 377-386. 

Cyclin B is the positive regulatory subunit of protein kinase Cdkl. It was the first cychn discovered, as a protein that is destroyed at the end of each cell cycle in early embryos of marine invertebrates, such as sea urchins... 

Fig. 7.3. Contact points of protein kinase A with inhibitor and peptide substrates. The contact points are shown between the catalytic subunit of protein kinase A, the inhibitor PKI (5 — 24) and a peptide substrate (kemptide). The inhibitor binds, via two Arg residues (P3, P-2), to the same Glu residue of protein kinase A that also binds to the substrate. The phosphorylation site is labelled as P, the A-terminal and C-terminal situated amino acids are labelled as (-) and (+) and are numbered starting from the phosphorylation site. According to Kemp et ah, (1994).

Fig. 7.5. Functional domains of protein kinase A. The functional domains of the catalytic (C) and regulatory (R) subunits of protein kinase A (bovine) are shown in a hnear configuration. M myristoylation.

A crystal structime is available of the catalytic subunit of protein phosphatase I (Goldberg et al., 1995). The enzyme contains two metal ions in the active center, which are probably Mn 7 Both metal ions are attributed a function in catalysis and also in substrate binding. 

Fig. 7.17. Regulation of protein phosphatases by inhibitor proteins. The substrates of protein kinase A include protein phosphatase inhibitors that are phosphorylated by the C subunit of protein kinase A. In the phosphorylated state, the protein phosphatase inhibitors bind to the protein phosphatase and inhibit its enzyme activity...

The G subimit of protein phosphatase I, occurring in the glycogen-boimd form, is considered as its localization subimit. The G subunit enables targeted localization of the catalytic subunit of protein phosphatase I to glycogen so that a close spatial orientation of protein phosphatase and its substrates, the enzymes of glycogen metabolism, is created (see 7.6.2). 

The cyclins are regulatory subunits of protein kinases of the cell cycle (see Chapter 14). 

Organizing glucose disposal emerging roles of the glycogen targeting subunits of protein phosphatase-1. Diabetes 49, 1967-1977. 

This toxin subunit is an enzyme, an ADP-ribo-syltransferase which catalyzes transfer of ADP-ribosyl units from the coenzyme NAD+ to specific arginine side chains to form N-ADP-ribosyl derivatives of various proteins. Of the proteins modified by cholera toxin, the most significant is the guanyl nucleotide regulatory protein Gs of the adenylate cyclase system.C/f/h ADP ribosylation of arginine 201 of the a subunit of protein Gs inhibits the GTP hydrolysis that normally allows the protein to relax to an unactivated form.e The ADP-ribosylated Gs keeps adenylate cyclase activated continuously and... 

The mechanisms by which these cations activate enzymes are often uncertain. It appears unlikely in general that they bind substrate, and more probable that they bind to the enzyme and either stabilize a conformation, or bring about conformational change. In this case, they are probably bound some distance from the substrate and exert long range effects. In some instances dissociation of subunits of proteins is prevented by binding of these cations. 

Wang, H., Cai, Q., Zeng, X., Yu, D., Agrawal, S. and Zhang, R. (1999a) Anti-tumor activity and pharmacokinetics of a mixed-backbone antisense oligonucleotide targeted to RIa subunit of protein kinase A after oral administration. Proc. Natl. Acad. Sci. USA, 96, 13989-13994. 

Metabolic Effects of Mutant Enzymes Predict and explain the effect on glycogen metabolism of each of the following defects caused by mutation (a) loss of the cAMP-binding site on the regulatory subunit of protein kinase A (PKA) (b) loss of the protein phosphatase inhibitor (inhibitor 1 in Fig. 15-40) (c) overexpression of phosphorylase b kinase in liver (d) defective glucagon receptors in liver. 

Egloff MP, Johnson DF, Moorhead G, Cohen PT, Cohen P, Barford D (1997) Structural basis for the recognition of regulatory subunits by the catalytic subunit of protein phosphatase 1. Embo J 76 1876-1887. 

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