Synthesis of a substrate of protein kinase C and its corresponding phosphopeptide

An Update on Protein Kinases as Therapeutic Targets-Part II: Peptides as Allosteric Protein Kinase C Modulators Targeting Protein-Protein Interactions

Human protein kinases are highly-sought-after drug targets, historically harnessed for treating cancer, cardiovascular disease, and an increasing number of autoimmune and inflammatory conditions. Most current treatments involve small molecule protein kinase inhibitors that interact orthosterically with the protein kinase ATP-binding pocket. As a result, these compounds are often poorly selective and highly toxic. Part I of this series reviews the role of PKC isoforms in various human diseases, featuring cancer and cardiovascular disease, as well as translational examples of PKC modulation applied to human health and disease. In the present Part II, we discuss alternative allosteric binding mechanisms for targeting PKC, as well as novel drug platforms, such as modified peptides. A major goal is to design protein kinase modulators with enhanced selectivity and improved pharmacological properties. To this end, we use molecular docking analysis to predict the mechanisms of action for inhibitor-kinase interactions that can facilitate the development of next-generation PKC modulators.

Cyclohexyl ketone inhibitors of Pin1 dock in a trans-diaxial cyclohexane conformation

Cyclohexyl ketone substrate analogue inhibitors (Ac–pSer-Ψ[C = OCH]-Pip–tryptamine) of Pin1, the cell cycle regulatory peptidyl-prolyl isomerase (PPIase), were designed and synthesized as potential electrophilic acceptors for the Pin1 active site Cys113 nucleophile to test a proposed nucleophilic addition-isomerization mechanism. Because they were weak inhibitors, models of all three stereoisomers were docked into the active site of Pin1. Each isomer consistently minimized to a trans-diaxial cyclohexane conformation. From this, we hypothesize that Pin1 stretches substrates into a trans-pyrrolidine conformation to lower the barrier to isomerization. Our reduced amide inhibitor of Pin1 adopted a similar trans-pyrrolidine conformation in the crystal structure. The molecular model of 1, which mimics the l-Ser-l-Pro stereochemistry, in the Pin1 active site showed a distance of 4.4 Å, and an angle of 31° between Cys113-S and the ketone carbon. The computational models suggest that the mechanism of Pin1 PPIase is not likely to proceed through nucleophilic addition.

Convergent synthesis of alpha-ketoamide inhibitors of Pin1

A convergent synthesis of alpha-ketoamide inhibitors of Pin1 is described. An alpha-hydroxyorthothioester derivative of Ser was reacted directly with an amine synthon. The reaction was catalyzed by HgO and HgCl(2) to form alpha-hydroxyamide. Thus, hydrolysis and coupling were combined in one step with 80% yield. Two diastereomers of a phospho-Ser-Pro alpha-ketoamide analogue were synthesized. The IC(50) values of 100 and 200 microM were surprisingly weak for Pin1 peptidyl prolyl isomerase.

A peptide substrate-based affinity label blocks protein kinase C-catalyzed ATP hydrolysis and peptide-substrate phosphorylation

Studies focused on the cAMP-dependent protein kinase (PKA) have led to the identification of conserved active-site residues involved in Ser/Thr protein kinase catalysis and have ruled out a role for Cys residues in the catalytic mechanism. Protein kinase C (PKC) is a Ser/Thr protein kinase isozyme family. We recently reported that the peptide-substrate analog N-biotinyl-Arg-Arg-Arg-Cys-Leu-Arg-Arg-Leu (N-biotinyl-RRRCLRRL) spontaneously forms intermolecular disulfide bridges with the active-site region of PKC isozymes concomitant with inactivation of histone kinase catalysis. Because Cys does not participate in PKC catalysis, one can analyze the active-site topology of PKC by examining which catalytic reactions are sterically hindered when the inactivator peptide is tethered to Cys in the active-site region of the enzyme. In this report, we show that N-biotinyl-RRRCLRRL inactivates the bulky PKC-catalyzed histone phosphorylation reaction, the comparatively less bulky PKC-catalyzed phosphorylation of a series of octapeptide, hexapeptide, and pentapeptide substrates, the intramolecular autophosphorylation reaction of PKC, and the least bulky PKC-catalyzed reaction, ATP hydrolysis, in a dithiothreitol-sensitive manner with comparable efficacy. Our results provide evidence that the covalent linkage of N-biotinyl-RRRCLRRL to the active-site region of PKC sterically hinders PKC catalysis, even in the absence of peptide and protein substrates.

Solid-phase synthesis of a nucleopeptide from the linking site of adenovirus-2 nucleoprotein, -Ser(p5'CATCAT)-Gly-Asp-. Convergent versus stepwise strategy

The synthesis of a nucleopeptide with the sequence -Ser(p5'CATCAT)-Gly-Asp- has been undertaken by either convergent or stepwise solid-phase strategies, both of which use base-labile permanent protecting groups. The coupling of phosphitylated protected peptides onto oligonucleotide-resins did not afford the desired nucleopeptide, which was nevertheless obtained after oligonucleotide elongation at the hydroxyl group of the resin-bound peptide and deprotection under mild basic conditions. A preliminary study on the stability of different nucleopeptides to bases is also reported.

Irreversible inactivation of protein kinase C by a peptide-substrate analog

Protein kinase C (PKC) is a phospholipid-dependent isozyme family that plays a pivotal role in mammalian signal-transduction pathways that mediate cell growth and differentiation and pathological developments, such as the acquisition of drug resistance by cancer cells. Several peptide-substrate analogs have been shown to reversibly inhibit PKC with high potency and selectivity, but peptide-substrate analogs that antagonize PKC by forming a covalent complex with the enzyme have not been reported. The development of active site-directed irreversible inactivators of PKC could provide new insights into the catalytic mechanism and might ultimately lead to the design of novel therapeutics targeted at PKC. In this report, we show that the peptide-substrate analog Arg-Lys-Arg-Cys-Leu-Arg-Arg-Leu (RKRCLRRL) irreversibly inactivates PKC in a dithiothreitol-sensitive manner. The inactivation mechanism most consistent with our results is the formation of a covalent linkage between the inhibitor-peptide and the enzyme at its active-site. Limited proteolysis of PKC produces a catalytic-domain fragment that is independent of the phospholipid cofactor. RKRCLRRL antagonized the histone kinase activity of PKC and its catalytic-domain fragment with similar efficacies, achieving > 50% inactivation at an RKRCLRRL concentration of 10 microM. In contrast, RKRCLRRL analogs with single amino acid substitutions at Cys were non-inhibitory. The inactivated complex of the catalytic-domain fragment and RKRCLRRL was stable upon dilution, and the inactivation of PKC and the catalytic-domain fragment by RKRCLRRL was quenched by dithiothreitol, providing evidence that the enzyme and the synthetic peptide may be covalently linked in an inactivated complex by a disulfide bond. Substrates and substrate analogs protected the catalytic-domain fragment against inactivation by RKRCLRRL, providing evidence that inactivation entailed binding of RKRCLRRL at the active-site of the enzyme. S-Thiolation is the formation of mixed disulfides between proteins and low molecular weight thiols. PKC is thought to have a highly reactive Cys residue in its active-site, and Cys residues that are flanked by basic residues, as is the case in RKRCLRRL, display enhanced reactivity. Our results support an inactivation mechanism that entails S-thiolation of the active-site of PKC by RKRCLRRL. This is the first report of irreversible inactivation of PKC by an active site-directed peptide.

Solid-phase synthesis of a range of O-phosphorylated peptides by post-assembly phosphitylation and oxidation

A completely general method for the O-phosphorylation of peptides of any given composition using solid-phase methodology is described. Peptides were assembled using Fmoc amino acid active esters, with base used for Fmoc deprotection. Unprotected amino acid side chain hydroxyl groups were phosphitylated and oxidised at the end of the assembly using bis(benzyloxy)(diisopropylamino)phosphine and tert.-butylhydroperoxide respectively. TFA was used for final deprotection of the amino acid side chains and for simultaneous cleavage from the resin. The synthesis of O-phosphopeptides of up to 15 residues in length is described.

Solid-phase synthesis of peptides containing phosphoserine using phosphate tert.-butyl protecting group

In this paper, we report the solid-phase synthesis of peptides containing O-phosphonoserine using BOP as coupling reagent. Commercially available Fmoc amino-acids linked to p-alkoxybenzyl resin were used in the first step and Alloc amino acids in the following. Alloc group was removed by catalytic hydrostannolytic cleavage. Acid-labile side-chain protecting groups (including phosphate residue) were used. Thus, both removal of side-chain protecting groups and cleavage of the phosphopeptide from the resin were achieved in one step by treatment with TFA. Alloc serine was phosphorylated by the phosphoramidite method. This strategy enables the preparation of peptides with selectively phosphorylated residue and overcomes problems due to repetitive treatments with TFA and final cleavage with HF.

N-myristyl-Lys-Arg-Thr-Leu-Arg: a novel protein kinase C inhibitor

In view of the critical role that the Ca2+- and phospholipid-dependent enzyme protein kinase C (PKC) plays in mediating proliferative responses to a number of growth factors, hormones, and tumor promoters, it is thought that selective PKC inhibitors may provide a new class of antiproliferative drugs. Established PKC inhibitors include three major classes of agents: agents that compete with the substrate ATP, agents that compete with the protein substrate, and agents that both compete with ATP and interact with the cofactor phosphatidylserine (PS). In this report, we have characterized the interactions between PKC and N-myristyl-Lys-Arg-Thr-Leu-Arg, a myristylated analogue of a synthetic peptide substrate of PKC. We determined that the myristylated peptide was a novel PKC inhibitor that interacted with PS as well as competed with the protein substrate of PKC. The inhibitory activity of the peptide was conferred by myristylation. We found that the myristylated peptide antagonized Ca2+- and PS-activated PKC with an IC50 of 75 microns, whereas the nonmyristylated peptide lacked this inhibitory activity. A fully active, Ca2+- and PS-independent catalytic fragment of PKC can be generated by limited proteolysis. Although the myristylated peptide was a very poor PKC substrate, this peptide inhibited the catalytic fragment of PKC by apparent competition with the phosphoacceptor substrate histone IIIS with an IC50 of 200 microM, whereas the nonmyristylated peptide showed no inhibitory activity against the catalytic fragment. Thus, the myristylated peptide may serve as a model for the development of selective PKC inhibitors, because its inhibitory mechanism exploits the substrate specificity of PKC, as well as the novel regulation of the enzyme. Furthermore, since endogenous PKC substrates include acylated proteins, the observations that we report here concerning a myristylated synthetic peptide suggest that acylation of proteins may be important in the regulation of PKC activity in vivo.

Biology of the protein kinase C family

Protein kinase C (PKC) is composed of a family of isozymes that transduce signals of certain hormones, growth factors, lectins, and neurotransmitters. This review addresses the role of PKC in the regulation of cellular proliferation and its disorders. PKC is directly activated in vivo by the second messenger diacylglycerol, a lipid produced by phospholipase C-catalyzed hydrolysis of phosphatidylinositol and polyphosphoinositides. Diacylglycerol activates PKC by reducing the enzyme's requirement for Ca2+. Phorbol ester tumor promoters and related agents potently activate PKC by a mechanism analogous to that of diacylglycerol, providing evidence that PKC activation is a critical event in tumor promotion. However, the role of PKC activation in tumor promotion is not entirely clear. For example, bryostatin is a potent PKC activator that antagonizes phorbol ester-mediated tumor promotion, and mezerein is a second-stage tumor promoter that potently activates PKC. In addition to studies concerned with tumor promotion, studies of oncogene action also indicate a role for PKC in carcinogenesis. A number of plasma membrane-associated oncogene products and related proteins are PKC substrates, and PKC activation leads to induction of the expression of oncogenes that code for nuclear proteins. PKC is implicated in human breast and colon carcinogenesis. Tumor-promoting bile acids activate PKC, and PKC expression studies in rat colonic epithelial cells and human breast cancer cells indicate a positive role for PKC in the proliferation of the cells. Altered expression of PKC in human colon and breast tumors indicates that PKC isozymes may be useful markers for these diseases.