Phosphorylation channels

In fungi, xylose is reduced to xylitol by NADH- or NADPH-dependent xylose reductase (XR) and thereafter is oxidized to xylulose by NAD -dependent xylitol dehydrogenase (XDH). The xylulose is phosphorylated, channeled into the pentose phosphate pathway [3]. XR of most fungi, including most yeasts, prefers NADPH to NADH. Because of the cofactor preference of XR (NADPH) and XDH (NAD, redox imbalance occurs under anaerobic condition [4]. Therefore, the oxygen-limited rather than anaerobic condition is ideal for bioconversion of xylose to ethanol, so that the accumulated reduced cofactor can be oxidized to reach redox balance. A critical level of oxygen should exist for the highest ethanol yield and productivity. 

FIGURE 24.12 The PIP2 second-messenger scheme. When a hormone binds to a receptor, it activates phospholipase C, in a process mediated by a G protein. Phospholipase C hydrolyzes PIPj to IP3 and DAG. IP3 stimulates the release of Ca from intracellular reservoirs in the ER. A complex formed between Ca and the calcium-binding protein calmodulin activates a cytosolic protein kinase for phosphorylation of a target enzyme. DAG remains bound to the plasma membrane, where it activates the membrane-bound protein kinase C (PKC). PKC is involved in the phosphorylation-channel proteins that control the flow of Ca + in and out of the cell. Ca from extracellular sources can produce sustained responses even when the supply of Ca + in intracellular reservoirs is exhausted. 

Phosphodiesterase Inhibitors. Because of the complexity of the biochemical processes involved in cardiac muscle contraction, investigators have looked at these pathways for other means of dmg intervention for CHF. One of the areas of investigation involves increased cycHc adenosine monophosphate [60-92-4] (cAMP) through inhibition of phosphodiesterase [9025-82-5] (PDE). This class of compounds includes amrinone, considered beneficial for CHF because of positive inotropic and vasodilator activity. The mechanism of inotropic action involves the inhibition of PDE, which in turn inhibits the intracellular hydrolysis of cAMP (130). In cascade fashion, cAMP-catalyzed phosphorylation of sarcolemmal calcium-channels follows, activating the calcium pump (131). A series of synthetic moieties including the bipyridines, amrinone and milrinone, piroximone and enoximone, [77671-31-9], C22H22N2O2S, all of which have been shown to improve cardiac contractiUty in short-term studies, were developed (132,133). These dmgs... 

FIGURE 10.26 Glucose transport in E. coli is mediated by the PEP-dependent phosphotransferase system. Enzyme I is phosphorylated in the first step by PEP. Successive phosphoryl transfers to HPr and Enzyme III in Steps 2 and 3 are followed by transport and phosphorylation of glucose. Enzyme II is the sugar transport channel. 

The possibility that acute ethanol directly activates PKC would seem to be ruled out by the lack of such effect occurring in various in vitro systems that have been studied. One possibility is the activation of a phosphatase, others are the modulation of the availability and type of activator. It is also possible that ethanol could modify the sensitivity of the ion channel to the effect of PKC phosphorylation or its proteolytic downregulation. 

Ion channels are often multimeric and are regulated by a wide variety of mechanisms (e.g., ligand binding, voltage changes, phosphorylation). 

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