Uncommon Amino Acids

In many cases only the racemic mixtures of a-amino acids can be obtained through chemical synthesis. Therefore, optical resolution (42) is indispensable to get the optically active L- or D-forms in the production of expensive or uncommon amino acids. The optical resolution of amino acids can be done in two general ways physical or chemical methods which apply the stereospecific properties of amino acids, and biological or enzymatic methods which are based on the characteristic behavior of amino acids in living cells in the presence of enzymes. 

Natural products derived from amino acids form a broad and divergent group, including simple amino acid derivatives, alkaloids, and small, often cyclic, polypeptides. Simple amino acid derivatives, which are not uncommon in algae, are often oxidation or rearrangement products of one of the 20 common amino acids. Alkaloids and polypeptides are more complex in their structural modifications. 

Chapt. 5 is entirely devoted to the hydrolytic ring opening of lactams. The reverse reaction, namely lactam formation by cyclization-dehydration, has been seldom reported in the literature, suggesting that the reaction is, indeed, uncommon. However, it is also conceivable that lactam formation has sometimes been overlooked, and that much remains to be discovered regarding structural conditions for lactam formation, the factors that influence the lactam/amino acid equilibrium, and the role of enzymes. 

Sulser, H., and Sager, F. (1976). Identification of uncommon amino acids in the lentil seed (Lens culinaris Med.). Experientia 32, 422-423. 

Reduced forms of folic acid are required for essential biochemical reactions that provide precursors for the synthesis of amino acids, purines, and DNA. Folate deficiency is not uncommon, even though the deficiency is easily corrected by administration of folic acid. The consequences of folate deficiency go beyond the problem of anemia because folate deficiency is implicated as a cause of congenital malformations in newborns and may play a role in vascular disease (see Folic Acid Supplementation A Public Health Dilemma). 

Uncommon Amino Acids Also Have Important Functions... 

In addition to the 20 common amino acids, proteins may contain residues created by modification of common residues already incorporated into a polypeptide (Fig. 3-8a). Among these uncommon amino acids are 4-hydroxyproline, a derivative of proline, and 5-hydroxylysine, derived from lysine. The former is found in plant cell wall proteins, and both are found in collagen, a fibrous protein of connective tissues. 6-N-Methyllysine is a constituent of myosin, a contractile protein of muscle. Another important uncommon amino acid is y-carboxyglutamate, found in the bloodclotting protein prothrombin and in certain other proteins that bind Ca2+ as part of their biological function. More complex is desmosine, a derivative of four Lys residues, which is found in the fibrous protein elastin. 

FIGURE 3-8 Uncommon amino acids, (a) Some uncommon amino acids found in proteins. All are derived from common amino acids. Extra functional groups added by modification reactions are shown in red. Desmosine is formed from four Lys residues (the four carbon backbones are shaded in yellow). Note the use of either numbers or Creek letters to identify the carbon atoms in these structures, (b) Ornithine and citrulline, which are not found in proteins, are intermediates in the biosynthesis of arginine and in the urea cycle. 

There are many types of vertebrate collagen. Typically they contain about 35% Gly, 11% Ala, and 21% Pro and 4-Hyp (4-hydroxyproline, an uncommon amino acid see Fig. 3-8a). The food product gelatin is derived... 

Transfer RNAs base-pair with mRNA codons at a three-base sequence on the tRNA called the anticodon. The first base of the codon in mRNA (read in the 5 —>3 direction) pairs with the third base of the anticodon (Fig. 27-8a). If the anticodon triplet of a tRNA recognized only one codon triplet through Watson-Criclc base pairing at all three positions, cells would have a different tRNA for each amino acid codon. This is not the case, however, because the anticodons in some tRNAs include the nucleotide inosinate (designated I), which contains the uncommon base hypoxanthine (see Fig. 8-5b). Inosinate can form hydrogen bonds with three different nucleotides (U, C, and A Fig. 27-8b), although... 

Amino acids have common features that permit them to be linked together into polypeptide chains and uncommon features that give each polypeptide chain its unique character. 

Most transfer RNAs have common parts and uncommon parts. The common parts facilitate binding of the aminoacyl-tRNAs to common sites on the ribosome. The uncommon sites permit specific reactions with charging enzymes that covalently attach the correct amino acids to the correct tRNA. Another uncommon site on the tRNAs is the anticodon, which leads to specific complex formation with the complementary codon site on the messenger. 

A is correct You may read this question and be uncertain of what it means by "monomer". Such an experience is not uncommon on the MCAT. The minimum that you need to understand for this question is that a monomer is some type of "equivalent unit". Since it is an MCAT question, you should assume that monomer refers to something that you already know. You make this assumption because MCAT doesn t test science that is not covered in these books. Next you need to recognize the chemical shown as a polypeptide. You need to know that polypeptides are divided into amino acid residues, and you must be able to recognize where these residues begin and end. 

A Caribbean cyanobacterium Hormothamnion enteromorphoides was found to produce a complex mixture of cytotoxic and ichthyotoxic peptides, hormothamnins, which may explain the apparent absence of predation on these potentially palatable life forms. Initial chemical characterization suggested that the major peptide, hormothamnin A, is a cyclic undecapeptide containing six common and five uncommon amino acid residues (320). 

Interpretation of complex metabolic profiles and communication of lab results to clinicians has been one of the greatest challenges in dried blood spot analysis by MS/MS for acylcamitines and amino acids. Following its introduction, MS/MS defined numerous diseases of fatty acid and organic acid metabolism that were uncommonly encountered in a typical practice and not well understood by most pediatricians. Furthermore, even in disorders such as PKU that were well known, the improvements made by MS/MS screening for PKU compared with older technology such as BIA or fluorometry were not well... 

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