A-Imino acids

The nonpolar amino acids (Figure 4.3a) include all those with alkyl chain R groups (alanine, valine, leucine, and isoleucine), as well as proline (with its unusual cyclic structure), methionine (one of the two sulfur-containing amino acids), and two aromatic amino acids, phenylalanine and tryptophan. Tryptophan is sometimes considered a borderline member of this group because it can interact favorably with water via the N-H moiety of the indole ring. Proline, strictly speaking, is not an amino acid but rather an a-imino acid. 

While their physiologic role is uncertain, L-amino acid oxidases of liver and kidney convert amino acids to an a-imino acid that decomposes to an a-keto acid with release of ammonium ion (Figure 29-6). The reduced flavin is reoxidized by molecular oxygen, forming hy-... 

Figure 29-6. Oxidative deamination catalyzed by L-amino acid oxidase (i-a-amino acidiOj oxidoreduc-tase). The a-imino acid, shown in brackets, is not a stable intermediate.

Proteins are composed of a-amino acids and a-imino acids. 

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. 

In an original application, Yasuda et al have used both l-AAO and d-AAO, and L-lysine oxidase to oxidize o ,Ci -diamino acids. The reactions produce the expected a-keto w-amino acid products, but these then spontaneously cyclize to form cyclic a-imino acids. These compounds are then substrates for the authors recently discovered A methyl amino acid dehydrogenase (NMAADH) from Pseudomonas putida, producing the pure L-cyclic amino acid (Scheme 5). 

Addition of (S)-a-amino acids to P,p-dibromo-a-imino acids proceeded to produce the corresponding optically active a,ot-diamino acid derivatives with d.s. = 33 to 85 % 243). 

In the above reactions under modest conditions, traces of pyruvate have been found (13), which indicates that a-keto or a-imino acids are intermediates in the pathways to a-hydroxy or a-amino acids. The reductive amination of a-keto acids has been demonstrated experimentally under alkaline conditions with FeS/H2S or Fe(OH)2 as catalyst and reducing agent (14). The formation of significant amounts of pyruvate from CO and FeS/nonylmercaptan at 250° C and 2000 bar has been reported (15). Remarkably, pyruvate is stable under these conditions and apparently not reduced to lactate. 

The only preparative-scale reactions of synthetic value in this category are those catalyzed by the amino acid dehydrogenases. These enzymes catalyze the reductions of a-imino acids to a-amino acids. This can be done on a very large scale, as demonstrated by the LeuDH-catalyzed reduction of (120) to f-butyl leucine (121) shown in Scheme 59. Another enzyme of this group with preparative promise is... 

Decarboxylation, a-Imino acids undergo catalytic decarboxylation in the presence of BU3P (catalytic quantities) at room temperature. Acetic acid or pyridine can accelerate the process. 

The twenty L-amino acids (actually, nineteen a-amino acids and one a-imino acid (Table 1.1)) which, in preparation for their role in protein synthesis, are joined in vivo through their carboxy group to tRNA to form a-aminoacyl-tRNAs, are organised by ribosomal action into specific sequences in accordance with the genetic code (Chapter 8). 

Table 1.1. The twenty coded amino acids (nineteen coded L-a-amino acids, and one coded L-a-imino acid) structures and definitionsa... 

Planar cis and trans isomers (Figure 2.1(a)) are the most stable configurations, because the planar structure involves maximum orbital overlap. For the majority of peptides built up from a-amino acids, the amide bond adopts the trans geometry, a-Imino acids (notably proline but also /V-methylamino acids), as well as a-methyl-a-... 

Imino-acid residues modify these torsion angles considerably replacement of the amide proton, for example by methylation, induces conformational changes, since cis-trans isomerisation is more easily brought about (because there is a lower energy barrier). However, hydrogen-bonding ability and, of course, other types of closepacking interactions, are substantially modified at a-imino-acid residues. 

Proteins are formed from a-amino acids which, with the exception of proline, have a primary amino group and a carboxylate group on the same carbon (proline is an a-imino acid, with a secondary amino group). With the exception of glycine, they all have the L (or R) conhguration, and are joined by peptide (amide) bonds (Figure 3.4) to form polypeptide chains. Since, at pH 7, they are present as dipolar ions, it follows that proteins... 

Rudnick and Abeles purified proline racemase to 95% homogeneity from Clostridium sticklandii, and characterized it 92. The enzyme is composed of two identical subunits with a molecular weight of about 38000, and is independent of any cofactors or metals. Most amino acid racemases require pyridoxal 5 -phosphate, which labilizes the bond between the a-hydrogen and the chiral center by aldimine formation with the a-amino group of the substrate. However, PLP is not involved in the reaction of proline racemase acting on an a-imino acid. The enzyme also acts on 2-hydroxy-L-proline and 2-allo-hydroxy-D-proline although slowly they are epimer-ized at a rate of 2 and 5% of the rate of L-proline racemization, respectively. L-Proline and D-proline showed Km values of 2.9 and 2.5 mti, respectively1119. ... 

The above hypothetical mechanism is, of course, based on the view that a-iminoglutarate is indeed an intermediate in the reaction. It has been shown that a-imino acids are intermediates in the reactions catalyzed by the flavin-containing l- and n-amino acid oxidases (328,3 7). In addition, such a mechanism would be consistent with the observed difference in pH optima for the forward and reverse reactions (Table... 

As an initial working hypothesis it will be assumed that the peptide theory is valid, in other words, that a protein molecule is built up only of chains of a-amino (and a-imino) acids bound together by peptide bonds between their a-amino and a-carboxyl groups. While this peptide theory is almost certainly valid (see Vickery and Osborne, 1928 Pauling and Niemann, 1939 Synge, 1943), it should be remembered that it is still a hypothesis and has not been definitely proved. Probably the best evidence in support of it is that since its enunciation in 1902 no facts have been found to contradict it. It is to be expected that investigations of the types described in the present article will throw further light on the accuracy of the peptide theory and on the possible existence of nonpeptide bonds in proteins. 

We are interested here only in the role of amino acids in proteins— that is, in the simple proteins (consisting only of ot-amino and a-imino acids) and the corresponding parts of conjugated proteins the structure and links es of prosthetic groups will be ignored. 

Initially, the reactants are searched for these substructures. The acid chloride substructure is found four times in the skeleton of M, these are the positions open for substitution . The a-imino acid substructure is found in each amino acid exactly once. Taking into account the numbering (labeling) of the positions open for substitution in the skeleton of M, there are 7 possibilities to substitute y different amino acids to these four positions. Some of these possibilities may be equivalent due to the point... 

Recently, asymmetric PTC using chiral quaternary ammonium salt 110 has proven to be an effective method for the enantioselective a-arylation of a-imino acid derivatives 108 via asymmetric nucleophilic aromatic substitution, to give a,a-disubstituted a-amino acids 111 in good to high enantiomeric purity (Scheme 8.22) [86]. 

Transformation of the carboxylate to amide functions gives rise to a species which can participate in hydrogen bonding important to biological function. The two a-imino acids are ... 

As indicated above, the reaction catalyzed by the general d- and l-amino acid oxidases has been represented by a dehydrogenation of an amino acid by a flavoenzyme to jneld reduced flavoenzyme and the corresponding imino acid [reactions (2) - - (4)]. Indirect support for the formation of the hypothetical imino acid has been provided by a number of studies which exclude a,/3-unsaturation in the course of the reaction. For example, it has been shown that (a) the four isomers of isoleucine are enzymically oxidized by the appropriate amino acid oxidase to the corresponding optically active a-keto-/3-methylvaleric acids (5, 6), (b) the l-isomers of /3-phenylserine are converted by L-amino acid oxidase to the respective isomers of mandelic acid (7), (c) the l- and D-isomers of a-aminophenylacetic acid, which have no /3-hydrogen atom, are attacked by the amino acid oxidases 8, 9), aod (d) on the oxidation of L-leucine in the presence of D,0 by L-amino acid oxidase, no deuterium is found in the isolated a-ketoisocaproic acid (10). More direct evidence for the formation of the a-imino acid as an intermediate has been provided by Pitt (10a) in studies on the oxidation of aromatic amino aci by ophto-n-amino acid oxidase in the presence of a tautomerase. 

AAOs catalyze the enantioselective oxidation of natural and noimatural a-AAs to the corresponding a-imino acids. In case a racemate is used as starting material, an oxidative kinetic resolution occurs thus, in the ideal case, only one enantiomer gets transformed [99]. Spontaneous hydrolysis affords the a-keto acid along with the nonconverted stereoisomer of the AA (Scheme 2.24) [100]. 

Thionyl chloride has been used as an oxidant for cobalt phthalo-cyanines " and [Co(en)2(gly)]. In the latter reaction, the product is the a-imino acid complex. When glycine is replaced by sarcosine, the product is the W-methylthiooxamato complex. Other coordinated amino acids have also been studied. ... 

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