Imino acid, proline

There are approximately 20 amino acids found in proteins, all of which are a-amino acids with the exception of the two a-imino acids proline and hydrox-yproline (Figure 10.2), which for the purpose of this discussion will be considered with the amino acids because of their similarity. The a-amino acids are so called because the amino group is attached to the a-carbon of the chain which is, by convention, the carbon atom adjacent to the carboxyl group. Succeeding carbon atoms are designated 8, y, S and e (Figure 10.3). Hence in... 

Ninhydrin (triketohydrindene hydrate) reacts with an amino acid when heated under acidic conditions (pH 3-4) to produce ammonia, carbon dioxide and a blue-purple complex. This reaction forms the basis of many widely used methods (Figure 10.11). One mole of carbon dioxide is liberated from each mole of amino acid, exceptions being the dicarboxylic amino acids, which produce two moles of carbon dioxide, and the a-imino acids, proline and hydroxyproline, which do not produce carbon dioxide. Although this formed the basis of a gasometric technique, colorimetric methods are now the most common. 

Figure 11.3 The effect of imino acids on protein structure. The presence of the imino acids proline and hydroxyproline introduces a constraint into the angles of the peptide bond which results in a bend in the previously regular chain structure.

Amino acids and some small peptides are absorbed into the enterocytes in the jejnnnm. The transport of amino acids from the lumen into the ceU is an active process, coupled to the transport of Na ions down a concentration gradient. There are at least six carrier systems with different amino acid specificities neutral amino acids (i.e. those with no net charge, e.g. branched-chain amino acids) neutral plus basic amino acids imino acids (proline, hydroxyproline) and glycine basic amino acids (e.g. arginine and lysine) P-amino acids and taurine acidic amino acids (glutamic and aspartic acids). 

Note also that the imino acid proline must distort the regular zigzag array and introduce a bend into the chain two configurations may be considered, and... 

Amino acids are by definition low-molecular-weight mono- or dicarboxylic acids with one or more amino groups. A few imino acids (proline, hydroxyproline, pipe-colic acid) are also considered to belong to this group of biologically important substances. 

J The imino acid proline. Because the side chain is fused to the oc-amino group, the entire structure, not just the side chain, is shown. 

As a protein, collagen is unusual in both chemistry and structure. Nearly one-third of its residues are glycine, and an additional 20-25% are imino acids (proline and hydroxyproline). In terms of sequence, glycine occurs regularly in essentially every third position, following as a steric requirement of the secondary-tertiary structure. 

Not only the smallest optically active amino acid, for example alanine, but also valine, leucine, several (substituted) aromatic amino acids, heterosubstituted amino acids (methionine, homomethionine and thienylglycine) and even an imino acid, proline, are obtainable in both the l- and the D-form. Furthermore, this biocatalyst has recently been reported to hydrolyze azido amino acid amides with high enantioselectivities as well (vide infra)[32). 

Protein synthesis in the body is constrained to 20 amino acids (including the imino acid proline), but modifications made after translation greatly extend the range of side chains found in mature proteins. Reversible modifications provide opportunities for regulation of protein function. 

Amino acids, the monomeric units of peptides and proteins. From analysis of the vast number of proteins, it follows that 20 proteinogenic or standard amino acids are the building blocks of aU proteins. These amino acids are specified by the genetic code. With selenocysteine and pyrrolysine two additional members have been identified. Besides the imino acid proline, all other building blocks are known as a-amino acids, H2N-CHR-COOH, but the zwitterion form, H3N+-CHR-C00, occurs at physiological pH values. The amino acids can therefore act as either acids or bases. Depending on the side-chain residue R, amino acids can be classified into those with (a) non polar side chains [Gly/G Ala/A Val/V Leu/L Ile/I Met/M Pro/P Phe/F Trp/W] ... 

Because of its ring stmcture, the imino acid proline cannot be accommodated in the substrate binding sites of trypsin, chymotrypsin, or carboxypeptidase A. Therefore these proteases fail to cleave peptide bonds involving proline. 

Reaction 1 is a transamination reaction to form mercaptopyruvate. A transaminase in higher plants which will utilize cysteine or cystine as the amino donor has never been reported. In fact Forest and Wightman (1972) showed that cystine was not an amino donor to any keto acid tested with extracts of bush bean cotyledons or seedlings. It was unique in this respect in that the only other protein amino acids which acted in this manner were the two imino acids proline and hydroxyproline. 

A number of other nitrogenous compounds accumulate in response to certain mineral deficiencies, including the imino acid pipecolic acid which has been reported to accumulate in Mg- (Frieberg and Steward, 1%0) and Cl-deficient plants (Freney et al., 1959). Another imino acid, proline is also reported to accumulate in Cl-deficient plants, in cabbage a 50-fold increase occurred while in cauliflower there was a sevenfold increase (Freney et al., 1959). It is curious that proline accumulates in Cl-deficient plants and those grown at high NaCl levels (see Section II,C). 

The second glass transition was associated with gelatin rigid blocks composed of sequences mainly made up of the imino acids proline and hydroxyproline including glycine at every third position which are predominant in mammalian gelatin. So the transition at -30 C can be attributed at the glass transition of plastified soft blocks in WG. 

The imino acids proline and hydroxyproline are practically excluded so far from our consideration either as acyl or as acceptor components since the enzymes commonly used in the studies discussed have no specific affinity for imino acids. 

Because of resonance, which gives the N-C bond a partially double bond character, the peptide bond is planar. Furthermore, the a-carbons are almost always trans. These two features of the peptide bond play a dominant role in determining protein structure. The other covalent linkage of importance is the disulfide bond that joins different parts of the protein chain in enzymes secreted outside the cells. Special note should also be made of an imino acid proline, which creates a very rigid peptide bond. 

The nitrogen of the basic amino acids lysine, histidine and arginine is not derived from wort amino acids but possibly from ammonium ions. In these cases intracellular oxoglutarate is aminated with ammonia to produce glutamate for biosynthesis. The imino acid proline is synthesized, without... 

Amino add raagants reagents for the colorimetric identification and quantitation of amino acids. One of the most important is ninhydrin (2,2-dihy-droxy-lH-indene-13(2H)-dione), which reacts with amino acids to form a blue-violet dye called Ruhe-mann s purple (absorbance maximum 570 nm). With the imino acid, proline, ninhydrin forms a yellow product, absorbance maximum 440 nm. 

The reaction is the basis of various methods for the determination of amino acids since it is possible to measure (1) the CO2 produced, (2) the NH3 produced and (3) the colour intensity obtained when the liberated ammonia reacts with a further molecule of ninhydrin to produce a purple compound which can be assayed photometrically. This is the method by which amino acids are estimated using an amino acid analyser. The imino acids (proline and hydroxyproline) give yellow products instead of a purple one. Amino acids give a strong reaction with ninhydrin, but proteins and polypeptides, which contain far fewer free amino groups, give a much weaker reaction. 

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