Peptide natural occurrence

Relatively few heteroaromatic N-oxides occur in nature. The chemistry of compounds that contain the oxidized peptide bond (the so-called hydroxamic acids) and their role in iron metabolism have been reviewed (67SC1443). Another review deals with the natural occurrence of N-oxides (68MI1). 

To date, the O-sulfation of hydroxyamino acids as residues in proteins or as free amino acids has not been detected. Similarly, desulfation by sulfatases has been observed only for sulfated serine, but it could not be attributed unambiguously to a sulfatase.11301 This lack of natural occurrence explains why only marginal attention has been paid to the synthesis of sulfated hydroxyamino acid peptides. 

Natural Occurrence of Pyroglutamic Acid and Pyroglutamyl Peptides... 

Natural occurrence, syntheses, and the use of 1-aminocyclopropanecar-boxylic acid (ACA) and other 2,3-methanoamino acids for preparation of cyclic peptides and heterocyclic derivatives of ACA 07CRV4493. 

The widespread occurrence of cyclic peptides and depsipeptides in natura makes the research on the development of rapid and efficient approaches for their generation mandatory. The rational design of synthetic cycfic peptide analogs, focused on biological activities that imitate natural structural motifs (turns, hefices, etc.), can help to increase their native properties and to adapted them for human applications, for example, in medicine [12-14]. 

In this Section, the nature of the linkages between the protein chain and the carbohydrate moieties in glycoproteins will be considered, and some indication of their occurrence will be given. It will become clear that, in a number of examples, a given polypeptide chain is glycosidically substituted at more than one point. Sometimes, the amino acid residue at two such points is of a different type, so that more than one type of carbohydrate-peptide bond occurs in the same protein, It is likely that more examples of this kind of structure will be found as more features of the constitution of glycoproteins involved in membranes and connective tissue are determined. 

Although the participation of amino acids in the biosynthesis of some shikimate metabolites and particularly in the pathways leading to alkaloids has already been explored in Chapters 4 and 6, amino acids are also the building blocks for other important classes of natural products. The elaboration of shikimate metabolites and alkaloids utilized only a limited range of amino acid precursors. Peptides, proteins, and the other compounds considered in this chapter are synthesized from a very much wider range of amino acids. Peptides and proteins represent another grey area between primary metabolism and secondary metabolism, in that some materials are widely distributed in nature and found, with subtle variations, in many different organisms, whilst others are of very restricted occurrence. 

Aspartyl peptides are of far less frequent occurrence in nature than those containing 7-glutamyl linkages. The first peptide shown to contain... 

BMP may become a savoury peptide and a flavour modifier of the future [26]. It occurs naturally in beef. One method of production of BMP already initiated by several concerns is biotechnology [28[. The taste threshold of BMP has been reported to be 1.600 ppm [28[. There are also reports in the literature claiming that BMP cannot be considered as a flavour modifier and that its occurrence in beef is highly unlikely [30, 31 [. 

The foregoing section indicates some reasons why peptide-based drugs are needed. If a peptide hormone is not produced in sufficient quantities or is defective in structure, then a replacement is required. Peptides, especially very small molecules, have a very short half life in the body. The reason for this is the ubiquitous occurrence of proteolytic enzymes that effect hydrolysis of peptides to the constituent amino acids. Although longer peptides, especially those with structural features such as disulphide bonds, survive longer in vivo, they are more likely to stimulate the body s immune system to produce antibodies and effect removal of the peptides. This is particularly likely to occur with molecules that differ structurally from the naturally occurring hormones. Thus, treatment of juvenile-onset diabetes mellitus with insulins from animal sources can occasionally stimulate the patient s immune system... 

The double CP experiment9,10 involves the sequential transfer of polarization among three spin-3 systems. This technique can be used to observe selectively one type of spin (e.g. 15N) which is directly bonded to another nucleus (e.g. 13C). Since both 15N and l3C nuclei are rare spins, 13C-15N bonds are extremely rare occurrences in natural isotopic abundance peptides. The 13C-15N bonds observed in this experiment originate from doubly-labelled compounds. The different kinds of labelled 13C-15N bonds and their concentrations can be detected. 

---