Phosphorylation cycling

Edgecombe, M, Patel, R., and Whitaker, M. (1991). Acyclin-abundance cycle-independent p34 fc2 tyrosjne phosphorylation cycle in early sea urchin embryos. EMBO J. 10 3769-3775. 

Hence, the phosphorylation cycles represent a poised system for the reversible transfer of electrons from oxy anions [(HO)RO -> (HO)RO- + e ] to hydronium ions (H3O+ + e- —> H- + H2O), which is facilitated by (1) the coupling of the respective products to form H2O [(HO)RO- + H- —> H2O + R(O) -AGgp, 111 kcal mol ] and (2) the nucleophilic condensation reaction [ADP3- + R(O)]. Biological systems such as cytochrome-c oxidase and Photosystem 11 of green-plant photosynthesis produce net proton fluxes during turnover and thereby drive oxidative phosphorylation to store 5 kcal per mole of ATP produced from one mole of hydronium ions. 

Figure 13-1. Model of the KaiC phosphorylation cycle. Schematic diagrams illustrate enhancement of KaiC phosphorylation (or inhibition of dephosphorylation) by KaiA dimer (left) and inactivation of KaiA by KaiB (right adapted from Kageyama et al 2006). Only the KaiCII domains harbor phosphorylation...

Kageyama, H., Nishiwaki, T., Nakajima, M., Iwasaki, H., Oyama, T., and Kondo, T. (2006). Cyanobacterial circadian pacemaker Kai protein complex dynamics in the KaiC phosphorylation cycle in vitro. Mol. Cell 23, 161-171. 

Thus, oscillations become much more abrupt in the presence of autocatalysis by cdc2 kinase, but it appears that such an autocatalysis is not required for sustained oscillations in the mitotic cascade. In the presence of self-activation by cdc2 kinase, however, oscillations can occur in the absence of zero-order ultrasensitivity in the cdc2 kinase phosphorylation cycle. Thresholds due to zero-order ultrasensitivity in any cycle of the cascade nevertheless continue to favour oscillatory behaviour. The threshold due to self-amplification in cdc2 kinase activation (Hoffmann et al., 1993) may, however, substitute for the threshold due to zero-order ultrasensitivity in the first cycle of the cascade. This is what might... 

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. 

Two and twelve moles of ATP are produced, respectively, per mole of glucose consumed in the glycolytic pathway and each turn of the Krebs (citrate) cycle. In fat metaboHsm, many high energy bonds are produced per mole of fatty ester oxidized. Eor example, 129 high energy phosphate bonds are produced per mole of palmitate. Oxidative phosphorylation has a remarkable 75% efficiency. Three moles of ATP are utilized per transfer of two electrons, compared to the theoretical four. The process occurs via a series of reactions involving flavoproteins, quinones such as coenzyme Q, and cytochromes. 

Basically, AZT is anabohcaHy phosphorylated to AZT mono-, di-, and tri-phosphates by various enzymes (kinases) of a target ceU (159). AZT-triphosphate competes with other phosphorylated pyrimidine nucleosides for incorporation into HIV DNA by the viral reverse transcriptase. Incorporation of the AZT-triphosphate into reverse transcriptase results in viral DNA chain termination. Reverse transcriptase is essential in the repHcative cycle of HIV. 

A minimal mechanism for Na, K -ATPase postulates that the enzyme cycles between two principal conformations, denoted Ej and Eg (Figure 10.11). El has a high affinity for Na and ATP and is rapidly phosphorylated in the presence of Mg to form Ei-P, a state which contains three oeeluded Na ions (occluded in the sense that they are tightly bound and not easily dissociated from the enzyme in this conformation). A conformation change yields Eg-P, a form of the enzyme with relatively low affinity for Na, but a high affinity for K. This state presumably releases 3 Na ions and binds 2 ions on the outside of the cell. Dephosphorylation leaves EgKg, a form of the enzyme with two... 

FIGURE 10.11 A mechanism for Na, K -ATPase. The model assumes two principal conformations, Ei and E9. Binding of Na ions to Ei is followed by phosphorylation and release of ADP. Na ions are transported and released and ions are bound before dephosphorylation of the enzyme. Transport and release of ions complete the cycle. 

The combustion of the acetyl groups of acetyl-CoA by the citric acid cycle and oxidative phosphorylation to produce COg and HgO represents stage 3 of catabolism. The end products of the citric acid cycle, COg and HgO, are the ultimate waste products of aerobic catabolism. As we shall see in Chapter 20, the oxidation of acetyl-CoA during stage 3 metabolism generates most of the energy produced by the cell. 

FIGURE 18.16 Compartmentalization of glycolysis, the citric acid cycle, and oxidative phosphorylation. 

Glycolysis and the citric acid cycle (to be discussed in Chapter 20) are coupled via phosphofructokinase, because citrate, an intermediate in the citric acid cycle, is an allosteric inhibitor of phosphofructokinase. When the citric acid cycle reaches saturation, glycolysis (which feeds the citric acid cycle under aerobic conditions) slows down. The citric acid cycle directs electrons into the electron transport chain (for the purpose of ATP synthesis in oxidative phosphorylation) and also provides precursor molecules for biosynthetic pathways. Inhibition of glycolysis by citrate ensures that glucose will not be committed to these activities if the citric acid cycle is already saturated. 

FIGURE 20.1 Pyruvate produced hi glycolysis is oxidized in the tricarboxylic acid (TCA) cycle. Electrons liberated in this oxidation flow through the electron transport chain and drive the synthesis of ATP in oxidative phosphorylation. In eukaryotic cells, this overall process occurs in mitochondria. 

Whereas ATP made in glycolysis and the TCA cycle is the result of substrate-level phosphorylation, NADH-dependent ATP synthesis is the result of oxidative phosphorylation. Electrons stored in the form of the reduced coenzymes, NADH or [FADHa], are passed through an elaborate and highly orga-... 

The processes of electron transport and oxidative phosphorylation are membrane-associated. Bacteria are the simplest life form, and bacterial cells typically consist of a single cellular compartment surrounded by a plasma membrane and a more rigid cell wall. In such a system, the conversion of energy from NADH and [FADHg] to the energy of ATP via electron transport and oxidative phosphorylation is carried out at (and across) the plasma membrane. In eukaryotic cells, electron transport and oxidative phosphorylation are localized in mitochondria, which are also the sites of TCA cycle activity and (as we shall see in Chapter 24) fatty acid oxidation. Mammalian cells contain from 800 to 2500 mitochondria other types of cells may have as few as one or two or as many as half a million mitochondria. Human erythrocytes, whose purpose is simply to transport oxygen to tissues, contain no mitochondria at all. The typical mitochondrion is about 0.5 0.3 microns in diameter and from 0.5 micron to several microns long its overall shape is sensitive to metabolic conditions in the cell. 

Because the 2 NADH formed in glycolysis are transported by the glycerol phosphate shuttle in this case, they each yield only 1.5 ATP, as already described. On the other hand, if these 2 NADH take part in the malate-aspartate shuttle, each yields 2.5 ATP, giving a total (in this case) of 32 ATP formed per glucose oxidized. Most of the ATP—26 out of 30 or 28 out of 32—is produced by oxidative phosphorylation only 4 ATP molecules result from direct synthesis during glycolysis and the TCA cycle. 

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