Peptides absolution

Nauser T, Schoneich C. (2003) Thiyl radicals abstract hydrogen atoms from the aC-H bonds in model peptides Absolute rate constants and effect of amino acid structure. / Am Chem Soc 125 2042-2043. 

Nauser T, Casi G, Koppenol W, Schoneich C. (2008) Reversible intramolecular hydrogen transfer between cysteine thiyl radicals and glycine and alanine in model peptides Absolute rate constants derived from pulse radiolysis and laser flash photolysis. JPhys Chem B 112 15034-15044. 

One glycosylation site exists on the P suburhts of human LH [53664-53-2 and TSH [64365-92-OJ ie, Asn-30 (Fig. 4). In some species, Asn-13 of LH-P is glycosylated (48). FSH-P suburnt [58857-12-8] is glycosylated at two sites, Asn-13 and 30. Based on interactions of synthetic peptides with the LH receptor, loops formed by P93—100 and P38—57 may be essential for hormone bioactivity (48). Highly conserved sequences between residues 31—37 have been implicated in the formation of the a—P suburnt dimer (48), which is absolutely necessary for the expression of bioactivity. 

Polypeptides. These are a string of a-amino acids usually with the natural 5(L) [L-cysteine is an exception and has the R absolute configuration] or sometimes "unnatural" 7f(D) configuration at the a-carbon atom. They generally have less than -100 amino acid residues. They can be naturally occurring or, because of their small size, can be synthesised chemically from the desired amino acids. Their properties can be very similar to those of small proteins. Many are commercially available, can be custom made commercially or locally with a peptide synthesiser. They are purified by HPLC and can be used without further purification. Their purity can be checked as described under proteins. 

Even though these enzymes have no absolute specificity, many of them show a preference for a particular side chain before the scissile bond as seen from the amino end of the polypeptide chain. The preference of chymotrypsin to cleave after large aromatic side chains and of trypsin to cleave after Lys or Arg side chains is exploited when these enzymes are used to produce peptides suitable for amino acid sequence determination and fingerprinting. In each case, the preferred side chain is oriented so as to fit into a pocket of the enzyme called the specificity pocket. 

Since there are a number of excellent and extensive reviews of peptide chemistry,6-9 no attempt will be made here to describe the known methods of peptide synthesis. Absolute comparisons of the procedure presented herein with other methods of peptide synthesis are impossible... 

Many enzymes have absolute specificity for a substrate and will not attack the molecules with common structural features. The enzyme aspartase, found in many plants and bacteria, is such an enzyme [57], It catalyzes the formation of L-aspartate by reversible addition of ammonia to the double bond of fumaric acid. Aspartase, however, does not take part in the addition of ammonia to any other unsaturated acid requiring specific optical and geometrical characteristics. At the other end of the spectrum are enzymes which do not have specificity for a given substrate and act on many molecules with similar structural characteristics. A good example is the enzyme chymotrypsin, which catalyzes hydrolysis of many different peptides or polypeptides as well as amides and esters. 

Both absolute quantitation and relative quantitation of species in mixtures is of interest in some circumstances. Quantitation in a 5-minute analysis can be achieved by addition of an internal standard, ideally the target microorganism grown in special media to incorporate heavy isotopes92-95 and determination of the relative peak heights of pairs of proteins from the analyte and the standard. Isotope-labeled proteins or peptides, selected to match proteins or peptides characteristic of target microorganisms, can also serve as internal standards for isotope ratio measurement. The addition of unmatched proteins or peptides is less reliable for either ESI or MALDI measurements because of unpredictable suppression in the variable mixture. 

Brandsch, M., et al. Evidence for the absolute conformational specificity of the intestinal H+/peptide symporter, PEPT1. /. Biol. Chem. 1998, 273, 3861-3864. 

Quantitation and qualification of large molecules follow similar principles as procedures for small molecules but involve different foci and approaches. The quantitation of proteins and peptides includes relative and absolute amount evaluations. Most proteomic applications in drug discovery are concerned with relative abundances of proteins. 

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