Membrane phosphorylation

E-IIs might be capable of phosphorylating free sugar located at the cytoplasmic side of the membrane (phosphorylation without transport). 

Some of the effects of insulin on the carbohydrate metabolism (right part of the illustration) are possible without protein synthesis. In addition to Grb-2, another dimeric adaptor protein can also bind to phosphorylated IRS. This adaptor protein thereby acquires phos-phatidylinositol-3-kinase activity (PI3K) and, in the membrane, phosphorylates phospholi-... 

In contrast to the hydrolysis and synthesis of ATP connected with proton translocation in mitochondria, chloroplasts and bacterial membranes, the energy linked movement of calcium ions gives rise to the appearance of an acid-stable phosphorylated intermediate in the membranes. A cation specific phosphorylation also occurs in the membranes of the sodium potassium transport system183. However, due to the inability to correlate phosphorylation and ion movement in the latter membranes, membrane phosphorylation has been questioned as being a step in the reaction sequence of ion translocation184,18s. Solely the sarcoplasmic calcium transport system allows to correlate directly and quantitatively ion translocation with the phosphoryl transfer reactions. 

Ca2+ ATPase (calcium pump) Microtubule disassembly Membrane phosphorylation... 

The analysis demonstrates that glucose uptake is very dynamic and complex. In the simple context, glucose is transported over the cell membrane, phosphorylated, and then oxidized, converted to lactate or fat, or stored as glycogen as shown in Fig. 6.6a. But these processes take place at very different rates in the different organs and they depend strongly on the metabolic state of the organism. The real challenge to the... 

J Bennett (1977) Phosphorylation of chloroplast membrane polypeptides. Nature 269 344-346 A Telfer, JF Allen, J Barber and J Bennett (1983) Thylakoid membrane phosphorylation during state 1- state 2 transitions In osmotically shocked pea chloroplasts. Biochim Biophys Acta 722 176-181 P Horton, JF Allen, MT Black and J Bennett (1981) Regulation of phosphorylation of chloroplast membrane polypeptides by the redox state of plastoquinone. FEBS Lett 125 193-196... 

Phosphorylation makes sugars anionic the negative charge prevents these sugars from spontaneously leaving the cell by crossing lipid-bilayer membranes. Phosphorylation also creates reactive intermediates that wil more readily form linkages to other molecules. For example, a multipk phosphorylated derivative of ribose plays key roles in the biosyntheses ol purine and pyrimidine nucleotides (p. 712). 

Cyclic AMP has been implicated in synaptic transmission due to its actions on a number of important synaptic and neuronal events, such as membrane permeability, synaptic membrane phosphorylation, neurotransmitter synthesis, and cell growth and differentiation. As pointed out earlier, neurotransmitter-receptor interactions can result in direct physical perturbations of the membrane with consequent alterations in membrane permeability to specific ions. This effect is particularly the case when the ionophore is located near the receptor. However, if the ionic channel is distant from the receptor, mechanisms such as phosphorylation can result in an alteration of channel permeability. Cyclic AMP is known to lead to a hyperpolarization of neurons in a number of brain regions such as the cerebral cortex, the caudate nucleus, the peripheral paravertebral sympathetic ganglia, the cerebellar cortex, and the hippocampus. Although it has been hypothesized that this hyperpolarization is the consequence of the phosphorylation of specific neuronal membrane proteins, the relatively short duration of hyperpolarization... 

As a matter of fact, the difference in understanding the role of protons in membrane phosphorylation reflects the existence of two different principal approaches to the problem of energy coupling in biomembranes that involve numerous concrete models. The first of them is most clearly expressed in the widely accepted Mitchell chemiosmotic concept [39, 40, 45, 46]. According to Mitchell s postulate, X represents a transmembrane difference in the electrochemical potential of hydrogen ions, A/Ih+- This value can be presented in the form of the sum of two separately measured quantities A/Ih+ = Aq> -h (RT/F) ln([H ]i /[H ], , t), where Aq> = — [Pg.117]

The main principles of membrane phosphorylation are the same in chloroplasts, mitochondria, and photosynthetic bacteria. In this section, in order to analyze the role of protonmotive force in the processes of energy transduction in biomembranes, we will focus our attention on the consideration of proton-transport processes in chloroplasts. In thylakoids the ApH is the main component of transmembrane difference in electrochemical potentials of hydrogen ions, AjuH+ = Acp — 2.3(RT/F)ApH. The conductivities of the thylakoid membrane for the majority of cations (Mg ", Na ), existing... 

As we discussed in Section 5.2, there is experimental evidence demonstrating that under certain conditions an efficient steady state ATP synthesis can proceed upon the transmembrane A/Ih+ difference between water phases lower than the threshold level that is assumed, according to the hemiosmotic concept of membrane phosphorylation, to be necessarily held for stoichiometric phosphorylation. All these results were usually interpreted as the indication of the existence of an alternative (nonhemiosmotic) pathway of proton transport coupled to ATP synthesis (for details, see Section 5.2.5). We can give a simple explanation of these results within the frame of the above-described model for the reiterative mode of ATPsynthase functioning. 

Other possible mechanisms for insulin action have been proposed. It has been proposed that membrane phosphorylation regulates transport activity across membranes. Thus, if membrane phosphorylation decreases transport, it would be expected that dephosphorylation would enhance transport (Randle and Smith, 1958). However, insulin does not appear to phosphorylate plasma membrane proteins (Avruch et al., 1976b,c). The insulin activation of glucose transport in fat cells has been postulated to involve the oxidation of certain membrane sulfhydryl groups to the disulfide form (Czech, 1977). A rat liver plasma membrane subfraction has been isolated containing cAMP phosphodiesterase activity that could be stimulated by nanomolar amounts of... 

The ionic conductivity in the wet state of phosphorylated chitosan membranes prepared from the reaction of orthophosphoric acid and urea on the surface of chitosan membranes in AA -dimethylformamide was investigated by Wan et alP The authors observed that similarly to unmodified chitosan membranes phosphorylated chitosan membranes are hardly conductive in their dry states with conductivities between 10" and 10 S/cm. The entire conduction process occurs after the water incorporation increases the ionic conductivity values up to 10 and 10 S/cm depending on the phosphorus content in the sample. The best result of 1.2 x 10" S/cm was obtained with the sample containing 87.31 mg/m of phosphorus content. They also observed that the increase in the phosphorus content promotes a decrease in the crystallinity of phosphorylated chitosan membranes, an increase in the swelling index and not a significant loss of their tensile strength and thermal stability in comparison with the unmodified chitosan membranes. 

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