Phosphorylation, effect proteins

Ginsburg, A. Szczepanowski, R.H. Ruvinov, S.B. Nosworthy, N.J. Sondej, M. Umland, T.C. Peterkofsky, A. Conformational stability changes of the amino terminal domain of enzyme I of the Escherichia coli phosphoenolpyruvate sugar phosphotransferase system produced by substituting alanine or glutamate for the active-site histidine 189 implications for phosphorylation effects. Protein Sci., 9, 1085-1094 (2000)... 

One main line of future research could be in the inhibitory/activating effect on key enzymes involved in the pathogenesis of arteriosclerosis. In particular, enzymes regulating signal transduction involved in phosphorylation of proteins, such as PKC and tyrosine protein kinase, seems to be somehow modulated by different polyphenols and may represent a possible target for polyphenol activity. 

The functioning of G proteins may be influenced by phosphorylation. G proteins, as well as their associated receptors and RGS proteins, have been reported to undergo phosphorylation by a host of protein serine/ threonine kinases and protein tyrosine kinases. While the ramifications of receptor phosphorylation are becoming increasingly well understood (see Chs 23 and 24), the effect of phosphorylation of G proteins and RGS proteins, and its role in the regulation of physiological processes, have been more difficult to establish with certainty. This remains an important area of future investigation. 

Phospholamban is a protein that inhibits the SERCA pumps by decreasing their affinity for Ca2+. It can be phosphorylated by protein kinase A, for instance in response to /1-adrenoceptor activation, resulting in inhibition of its effects and enhanced SERCA activity. The effects of phospholamban on contraction depend on the relative importance of Ca2+ uptake or release in the smooth muscle in... 

A completely different type of effect is observed in metabotropic receptors (bottom right). After binding of the transmitter, these interact on the inside of the postsynaptic membrane with Gproteins (see p. 384), which in turn activate or inhibit the synthesis of second messengers. Finally, second messengers activate or inhibit protein kinases, which phosphorylate cellular proteins and thereby alter the behavior of the postsynaptic cells (signal transduction see p.386). 

The diverse effects of insulin (see p. 160) are mediated by protein kinases that mutually activate each other in the form of enzyme cascades. At the end of this chain there are kinases that influence gene transcription in the nucleus by phosphorylating target proteins, or promote the uptake of glucose and its conversion into glycogen. The signal transduction pathways involved have not yet been fully explained. They are presented here in a simplified form. 

Interferons are cellular glycoproteins produced by the host cells which exert complex antiviral, immunoregulatory and antiproliferative activities. After binding to interferon receptors it acts through cellular metabolic processes which involves synthesis of viral RNA and proteins. Interferon receptors are tyrosine protein kinase receptors which on activation phosphorylate cellular proteins. These then induce transcription of interferon induced proteins which exert antiviral effects. There are three type of interferons - alpha, beta and gamma. 

The effect of phosphorylation at 13 /xmoles/g protein on functional properties was minimal. Both solubility and EAI of the phosphorylated protein were slightly higher, compared to those of intact protein (not shown). Phosphorylation with protein kinase from bovine cardiac muscle is restricted by the limited number of potential phosphorylation sites in soy proteins. Experiments are... 

Fig. 7.16. The dual function of protein kinases and protein phosphatases. Phosphorylation of proteins (PI, P2) can fix the latter into an active or inactive state. In the case of PI, protein kinases have an activating effect and protein phosphatases are inactivating the reverse is trne for P2.

Fig. 9.11. Model of regulation and activation of Raf kinase. The active Ras.GTP complex binds and activates Raf kinase, which passes the signal on to the MAP kinase pathway. Various proteins including the 14-3-3 proteins and the molecular chaperons hsp 90 and p50 are thought to be involved in the regulation of the Raf kinase signahng function. In addition, Raf kinase is regulated by phosphorylation. Tyr phosphorylation (possibly via Src kinase) and Ser phosphorylation via protein kinase C have a stimulatory effect. In contrast, Ser phosphorylation via protein kinase A has an inhibitory effect. RTK receptor tyrosine kinase.

The /3-adrenergic receptor binds epinephrine, then through a stimulatory G protein, Gs, activates adenylyl cyclase in the plasma membrane. The cAMP produced by adenylyl cyclase is an intracellular second messenger that stimulates cAMP-dependent protein kinase, which mediates the effects of epinephrine by phosphorylating key proteins, changing their enzymatic activities or structural features. 

The control of glycogen phosphorylase by the phosphorylation-dephosphorylation cycle was discovered in 1955 by Edmond Fischer and Edwin Krebs50 and was at first regarded as peculiar to glycogen breakdown. However, it is now abundantly clear that similar reactions control most aspects of metabolism.51 Phosphorylation of proteins is involved in control of carbohydrate, lipid, and amino acid metabolism in control of muscular contraction, regulation of photosynthesis in plants,52 transcription of genes,51 protein syntheses,53 and cell division and in mediating most effects of hormones. 

The MAPK cascade also has direct effects upon protein synthesis, i.e., on the translation of mRNA messages. For example, insulin stimulates phosphorylation of proteins that regulate a translation initiation factor, a protein called eIF-4E (see Chapter 29). Phosphorylation of inhibitory proteins allows them to dissociate from the initiation factor so that protein synthesis can proceed 485/486... 

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