Many of the processes occurring in multi-cellular organisms are regulated via a complex network of extra-cellular signals and cell surface receptors. Attachment of the signal compound to its specific cell surface receptor results in conveying a signal through the receptor to the interior of the cell to start a reaction(s) until the signal is switched off. There is a large variety of signals, such as hormones, oligosaccharides, lipids, proteins and so on, while all of the receptors are of the protein transmembrane type, such as G protein-coupled receptors and receptor-tyrosine kinases. An effective way to elucidate the stimulatory mechanisms is by using compounds that have only the region(s) critical for linkage to the receptor and usually covalently bound to a reporter group such a fluorescent tag or an enzyme forming a conjugate. Identification in signal compounds of the regions critical for binding to their receptors, has allowed the development of agonist and antagonist compounds of those receptors that may have potential therapeutic applications Following is a description of some of these compounds, their structures and mode of action.
* Hyaluronic acid (HA) peptide conjugates for formyl peptide receptor like 1 (FPRL1) receptor. FPRL1 is a G protein-receptor that in phagocytes binds chemotactic peptides, some of them formylated, and mediate important biological functions, e.g. regulation of immune responses against pathogens by engulfing and destroying bacteria. These peptidic ligands can be either agonistic or antagonistic and therefore have a significant therapeutic potential. A potential application of antagonistic peptides would be the down-regulation of immunity to reduce inflammatory disease, e.g. arthritis and asthma. Recently it has been shown that conjugation of the peptide agonists or antagonists to HA results in good serum stability suitable for clinical use. The peptides were linked to the HA via a Michael addition reaction between a methacrylate group added to the HA and the thiol group from a cysteine residue present in the peptides. The efficacy of the conjugates increased if the HA size was reduced by enzymatic degradation, indicating some steric hindrance by large HA chains.
* The study of glycopeptide ligands to cell surface receptors has been advanced by the use of neoglycopolymers of which neoglycopeptides are an example. For instance, while some monomeric oligosaccharide act as inhibitors of L-selectin shedding, attachment of multiple copies of that oligosaccharide to a polymeric carrier, usually a polypeptide, resulted in the selectin’s shedding but independent of activation. An elegant example of how conjugates can help to dissociate the functions of the different moieties of a ligand.
* Lipid-modified peptides. Signal transducing proteins frequently have covalently attached lipids required for their biological functions. Lipidation of peptides derived from some G protein-coupled receptors, e.g. ras proteins, has provided information about the effectiveness of different lipid residues. For instance, it has been shown that geranylgeranylated or palmitoylated peptides have a higher membrane affinity than their analogous myristoylated or farnesylated peptides. Also increasing the degree of lipidation of a peptide showed that addition of a second lipid residue resulted in its stable insertion in the membrane. Thus, like in the glycosylation case the addition of lipids to model peptides allow the elucidation of the roles for the different protein’s moieties.
* Oligonucleotide-peptide conjugates. The objective of signal transduction is to stimulate a cell response that is usually the expression of a protein by transcription of DNA. In some viruses replication the formation of nucleopeptides is an obligatory step, also some processing of DNA by certain enzymes require the formation of intermediary nucleopeptides. Usually oligonucleotides are linked via the hydroxyl group of tyrosine, serine or threonine, either directly or via a spacer, depending on the requirements of the system. Synthesis of these conjugates is usually carried out by cross-linking an oligonucleotide to a peptide where both components had been prepared separately.
Use of conjugates in elucidating the roles on the different components that participate in signal transduction is a powerful tool in the development of new therapeutic agents to up-regulate or down-regulate certain biological responses. The design and production of these compounds frequently requires a deep knowledge of the chemistry used in their synthesis. BioSynthesis, with 25 years of experience in the areas of peptide and nucleic acid synthesis is well positioned to assist any researcher in the design and production of these agents.