N- -amino acids

Draw resonance forms for the purple anion obtained by reaction of ninhydrin with an n-amino acid (Problem 26.53). 

Identify the chiral carbon atoms in a carbohydrate or n-amino acid. 

In all, there are 2 —16 different ways to combine two amino acids in chains of four units. The 20 common amino acids combine to give 20 tetrapeptides, which is 1.6x10 different molecules. Each time the chain adds an amino acid, the number of possibilities is multiplied by 20, so a set of n amino acids can form 20 different pol q)eptides. Because proteins can contain hundreds of amino acids, each of which can be any of the 20... 

Unidentified hydrolyzable N (UH-N) =% total hydrolyzable N - (% amino acid-N + NH3-N + amino sugar-N)... 

Protein N > % amino acid N + 10 % amino acid N (to include amide N of asparagine and glutamine lost during 6 M HC1 hydrolysis). 

A more complex pathway of activation is seen in N-amino acid derivative of phosphoramidic acid diesters of antiviral nucleosides, as exemplified by prodrugs of stavudine (9.79, Fig. 9.14) [153 -155], The activation begins with a carboxylesterase-mediated hydrolysis of the terminal carboxylate. This is followed by a spontaneous nucleophilic cyclization-elimination, which forms a mixed-anhydride pentacycle (9.80, Fig. 9.14). The latter hydrolyzes spontaneously and rapidly to the corresponding phosphoramidic acid monoester (9.81, Fig. 9.14), which can then be processed by phosphodiesterase to the nucleoside 5 -monophosphate, and by possible further hydrolysis to the nucleoside. 

In another process, which does not require any knowledge of the 3D structure of the enzyme, every single position is randomized separately in a systematic manner 45,46). Thus, in the case of an enzyme composed of N amino acids, libraries each containing about 300 clones are produced and screened. In still another strategy, hot spots are first identified by the epPCR screening process, and these are subsequently randomized separately 47,48). 

If the DNA sequence is altered by deletion or insertion of 3 nucleotides, then the mutant protein will lack or gain n amino acid(s), but its sequence will otherwise be normal. 

Rodriguez, M. S., Wildermann, M. N. Amino acid composition of the cementum matrix from human molar teeth. J. Periodont. 43, 438 (1972)... 

FIGURE 3-3 Stereoisomerism in n-amino acids, (a) The two stereoisomers of alanine, l- and o-alanine, are nonsuperimposable mirror images of each other (enantiomers), (b, c) Two different conventions for showing the configurations in space of stereoisomers. In perspective formulas (b) the solid wedge-shaped bonds project out of the plane of the paper, the dashed bonds behind it. In projection formulas (c) the horizontal bonds are assumed to project out of the plane of the paper, the vertical bonds behind. However, projection formulas are often used casually and are not always intended to portray a specific stereochemical configuration. 

Stevenson, F. J., Cheng, C-N., "Amino Acids in Sediments Recovery by... 

As there are 19 possibilities for the introduction of proteinogenic amino acids besides the one contained in the wild-type protein, the number of possible variants of a protein obtained by introducing M substitutions over the length of the protein sequence of N amino acids is given by Eq. (11.1). 

Decarbamoylation to D-amino acid was performed by treating the N-carbamoyl-D-amino acid with equimolar nitrite under acidic conditions [6]. But now, this step can also be carried out enzymatically. Recently, Shimizu and co-workers found a novel enzyme, D-decarbamoylase (IV-carbamoyl- n-amino acid amidohydrolase), which stereospecifically hydrolyzes JV-carbamoyl-D-amino acids, in several bacteria [7, 8], For example, Blastobacter sp. A17p-4 was found to produce D-decarbamoylase together with D-hydantoinase [8]. Therefore, a sequence of two enzyme-catalyzed reactions, the D-stereospecific hydrolysis of DL-5-(p-hydroxyphenyl) hydantoin and subsequent hydrolysis of the D-carbamoyl derivative to D-p-hydroxyphenylglycine, is possible (Fig. 1). Based on these results, a new commercial process for the production of D-p-hy-droxyphenylglycine has been developed [9]. 

In a peptide or protein that contains n amino acids, the number of possible amino acid sequences is A", where A is the number of different amino acids. 

Tamarappoo, B.K. Kilberg, M.S. (1991). Functional reconstitution of the hepatic system N amino acid transport activity. Biochem. J. 274,97-101. 

Wong and co-workers [22] also applied the methodology in the preparation of a library of neomycin B mimetics (Fig. 6). Two libraries were prepared by a multiple-component condensation of neamine derived aldehyde, f-butyl isocyanide or isocyanoacetic acid methyl ester, CBZ-N amino acids (xl3) and a glycine conjugated polyethylene glycol methyl ether. The products of the reaction were isolated by precipitation. 

N Amino acid side chains, nucldc adds I... 

Is the following structure a n amino acid or an i amino acid Identify it. 

The reaction can be followed by O2 uptake (manometrically or oxygen electrode), by determination of ammonia, or by determination of hydrogen peroxide. A fluorimetric assay procedure for determining the hydrogen peroxide produced permits determinations of n-amino acids in the 1-100 /xg/ml range (53). 

Glucose oxidase is obtained from molds such as Aspergillus niger and species of Penicillium, In its gross molecular properties at least, the enzyme from A. niger resembles l- rather than n-amino acid oxidase (19) since it is a dimer of 186,000 MW with two tightly, but noncovalently, bound FAD molecules per dimer. These FAD moieties are kinetically independent and probably, therefore, are present at an active site on each subunit. The enzyme is remarkably stable over a wide range of pH and other variables, which makes it ideal for analytical and bulk conversion applications. 

The expected result was obtained since n-amino acid oxidase converted )8-chloroalanine to pyruvate under anaerobic conditions, to chloropyruvate at high O2 concentrations, and to mixtures of these at intermediate O2 concentrations. Under steady state conditions, the reaction behaved as if cleavage of the a C-H bond were the rate-limiting process in turnover, although stopped-flow spectrophotometric measurements showed that this interpretation can not be entirely correct in this case (53) or in the case of )8-choloro-a-aminobutyrate (54), a-yS-Elimination has now been observed in three flavoprotein oxidase reactions (54) and can be considered strong circumstantial evidence for a-proton removal from compounds which closely resemble the physiological substrates. 

Studies of nitroalkane oxidation by n-amino acid oxidase (55) and glucose oxidase 49, 56) have provided strong evidence both for intermediate substrate carbanions and for subsequent covalent adduct formation between these and the N position of the flavin nucleus. The rationale for using nitroalkanes can be seen in the following reaction stoichiometries for D-amino acid oxidase (55) ... 

The intermediate EiP, which is the major species of reduced enzyme with which O2 reacts in the amino acid oxidase reaction, is more reactive with O2 than Er in one case (49) (D-amino acid oxidase) but less reactive in the other (18) (n-amino acid oxidase). The reasons for such seemingly inconsistent behavior, as well as the virtual lack of reactivity of reduced flavins with O2 in systems such as succinic dehydrogenase, will only become clear when the molecular details of the oxidation mechanism of reduced flavin are elucidated. 

---