Phosphorylation inhibitors

The unique properties and actions of an inhibitory substance can often help to identify aspects of an enzyme mechanism. Many details of electron transport and oxidative phosphorylation mechanisms have been gained from studying the effects of particular inhibitors. Figure 21.29 presents the structures of some electron transport and oxidative phosphorylation inhibitors. The sites of inhibition by these agents are indicated in Figure 21.30. 

Motlik, J., and Rimkevicova, Z. (1990). Combined effects of protein synthesis and phosphorylation inhibitors on maturation of mouse oocytes in vitro. Mol. Reprod. Dev. 27 230-234. 

Design and Synthesis of Protein Tyrosine Phosphorylation Inhibitors... 

If there is reason to suspect a phosphorylating inhibitor was ingested or the patient is descending further into severe respiratory distress, treatment with an oxime might be warranted. However, if the patient s condition appears to be stable and adequate ventilatory support is available, it might be better to treat the patient symptomatically. 

As outlined above, protein phosphorylation is a key process involved in many signal transduction pathways and reversal of this process is catalyzed by a multiplicity of phosphoprotein phosphatases (PPs). Major PPs catalyzing dephosphorylation of phosphoserine or phosphothreonine residues on proteins include PP1 (inhibited by phosphorylated inhibitor protein I-1 and by okadaic acid and microsystins), PP2 (also inhibited by okadaic acid and microcystins), PP2B or calcineurin (CaM-activated and having a CaM-like regulatory subunit) and PP2C (Mg2+-dependent) [18]. These PPs have been found in all eukaryotes so far examined [18, 19]. In addition, a variety of protein phosphotyrosine phosphatases can reverse the consequences of RTK or JAK/STAT receptor activation [20]. 

Another class of phosphorylation inhibitors functions by blocking the ATP-synthase enzyme directly. Inhibitors of this type include the antibiotic oligomycin. If mitochondria are treated with oligomycin, ADP no longer is able to increase the rate of respiration (fig. 14.16b). Oligomycin does not block the stimulation of respiration caused by an uncoupler, demonstrating that the two types of inhibitors of phosphorylation act by different mechanisms. 

Figure 18.14 Glycogen biosynthesis and degradation regulation. cAMP activates cAMP-dependent protein kinases. They cause the phosphorylation of glycogen synthase (inactivation), phosphorylase kinase (activation), and the inhibitory protein. The last inhibits phosphoprotein phosphatase. Activated phosphorylase kinase causes the phosphorylation of phosphorylase b, thus activating it to phosphorylase a. Phosphoprotein phosphatase is inhibited by the phosphorylated inhibitor protein. Such inhibition is released when the inhibitor protein is dephosphorylated. The phosphatase then reactivates glycogen synthase and inactivates phosphorylase kinase and phosphorylase a.

A major signalling pathway involves activation of a protein kinase that phosphorylates inhibitor kB proteins (IkBs) that normally inhibit the function of the nuclear transcription factor NFkB. Phosphorylation of IkB by the serine/threonine-specific IkB kinases (IKKs) leads to NFkB de-inhibition, nuclear translocation and expression of pro-inflammatory proteins such as inducible cyclooxygenase (iCOX) (which generates prostaglandins), inducible nitric oxide synthase (iNOS) (which generates vasodilatory and toxic free radicalgenerating NO) and pro-inflammatory cytokines. 

Morpholin-4-yl-piperizyl) quinazoline derivatives, (I), prepared by Kanter (2) were effective as kinase phosphorylation inhibitors and used in the treatment of proliferative diseases such as arteriosclerosis, vascular reobstruction after percutaneous coronary angioplasty, and bypass operation cell-proliferative. Quinazoline derivatives, (n), prepared by Lambert (3) were effective against the Src family of nonreceptor tyrosine kinases inhibitors and used in the treatment of uncontrolled cellular proliferation. 

Mitochondrial electron transport system Sulfuramid, chlorfenapyr Azocyclotin, cyhexatin, Fenbutatin-oxide, propargite, Tetradifon, diafenthiuron Uncouplers of oxidative phosphorylation Inhibitors of ATP synthase... 

A second novel mechanism of rubisco regulation involves a phosphorylated inhibitor of catalysis which can occupy the catalytic site of the enzyme. The discovery of this inhibitor [30,31] followed the observation that rubisco extracted from... 

Inhibitor of NF-Kb kinase, which phosphorylates inhibitor of NF-Kb for the proteasomal degradation to release NF-Kb dimers to translocate to the nucleus and activate transcription of target genes... 

Glycogen synthase kinase-3 (GSK-3 sites C30, C34, C38 and C42 also phosphorylates inhibitor-2 (see below), thereby activating protein phosphatase-1). 

Protein phosphatase 1 An enzyme that will hydrolyze phosphate groups from target proteins snch as glycogen synthase, phosphorylase, phos-phorylase kinase, and the phosphorylated form of inhibitor 1. The phos-phorylated inhibitor 1 is a substrate that binds well but is hydrolyzed slowly. While bound to protein phosphatase 1, the phosphorylated inhibitor 1 serves as an inhibitor of the enzyme. 

PP). Binding of the phosphorylated inhibitor to PP prevents this phosphatase from dephosphorylating the activated enzymes in the kinase cascade or the inactive glycogen synthase. 

The following is a summary of the effects of oxidation and phosphorylation inhibitors in addition to those which have been cited above. In resting E. the incorporation of S -methionine was inhibited... 

The following is a summary of the effects of oxidation and phosphorylation inhibitors in addition to those which have been cited above. In resting E. coli (86) the incorporation of S -methionine was inhibited by azide (0.005Af) 100%, 2,4-dinitrophenol (O.OOIM) 96%, and fluoride (0.02M) 100%. Malonate, at concentrations of 0.02, 0.01, and O.OOhiW respectively, inhibited the incorporation of glycine in fetal rat liver homogenate 75, 65, and 40% (137), and the incorporation of alanine. 

---