L Amino acid

Certain other amino-acids occur in a few proteins, and others, not necessarily a- or l-amino-acids, are found naturally in the free state or as constituents of peptides. 

Effects of L- -amino acid ligands - Stepping on the tail of enantioselectivity The naturally occurring -amino acids form a class of readily available strongly coordinating ligands, which exhibit broad stmctural variation. Moreover, their availability in enantiomerically pure form offers opportunities for enantioselective catalysis. Some derivatives of these compounds have been... 

In a typical procedure, a solution of 0.175 mmol of L- -amino acid and 0.175 mmol of NaOH in 1 ml of water was added to a solution of 0.100 mmol of Cu(N03)2in 100 ml of water in a 100 ml flask. Tire pH was adjusted to 6.0-6.5. The catalyst solution was cooled to 0 C and a solution of 1.0 mmol of 3.8c in a minimal amount of ethanol was added, together with 2.4 mmol of 3.9. The flask was sealed carefully. After 48 hours of stirring at 0 C the reaction mixture was extracted with ether, affording 3.10c in quantitative yield After evaporation of the ether from the water layer (rotary evaporator) the catalyst solution can be reused without a significant decrease in enantioselectivity. 

L-phenylalanine L-amino acid oxidase and horseradish peroxidase (E) I-... 

Fig. 3. (a) Chemical stmcture of a synthetic cycHc peptide composed of an alternating sequence of D- and L-amino acids. The side chains of the amino acids have been chosen such that the peripheral functional groups of the dat rings are hydrophobic and allow insertion into Hpid bilayers, (b) Proposed stmcture of a self-assembled transmembrane pore comprised of hydrogen bonded cycHc peptides. The channel is stabilized by hydrogen bonds between the peptide backbones of the individual molecules. These synthetic pores have been demonstrated to form ion channels in Hpid bilayers (71). 

Enzymatic hydrolysis is also used for the preparation of L-amino acids. Racemic D- and L-amino acids and their acyl-derivatives obtained chemically can be resolved enzymatically to yield their natural L-forms. Aminoacylases such as that from Pispergillus OTj e specifically hydrolyze L-enantiomers of acyl-DL-amino acids. The resulting L-amino acid can be separated readily from the unchanged acyl-D form which is racemized and subjected to further hydrolysis. Several L-amino acids, eg, methionine [63-68-3], phenylalanine [63-91-2], tryptophan [73-22-3], and valine [72-18-4] have been manufactured by this process in Japan and production costs have been reduced by 40% through the appHcation of immobilized cell technology (75). Cyclohexane chloride, which is a by-product in nylon manufacture, is chemically converted to DL-amino-S-caprolactam [105-60-2] (23) which is resolved and/or racemized to (24)... 

Other L-amino acids are manufactured much more economically ia thousands of tons per year ia Japan by simplified fermentations direcdy from glucose, ethanol, acetic acid, glycerol, or / -paraffin, by means of selected auxotrophic, regulatory, and analogue-resistant bacterial mutants (94,95). 

L-tryptophan by hydroxylation to 5-hydroxy-L-tryptophan by the enzyme, ttyptophan-5-hydroxylase. 5-Hydroxy-L-tryptophan is then rapidly decarboxylated by aromatic-L-amino acid deacarboxylase to 5-HT. The actions of 5-HT as a neurottansmitter ate terminated by neuronal reuptake and metabobsm. 

Several strategies for the production of pure D- or L-amino acids rely on the use of enzymes. L-Serine (49) is synthesized by combining glycine (48) and formaldehyde in the presence of the enzyme serine hydroxymethyl transferase (66). 

Use of D-amino acids in the synthesis of a hairpin loop portion from the CD4 receptor provides a stable CD4 receptor mimic, which blocks experimental allergic encephalomyelitis (144). This synthetic constmct is not simply the mirror image or enantiomer of the CD4 hairpin loop, but rather an aH-D-constmct in the reverse sequence, thus providing stereochemicaHy similar side-chain projections of the now inverted backbone (Fig. 11). This peptide mimetic, unlike its aH-L amino acid counterpart, is resistant to en2yme degradation. As one would expect, the aH-D amino acid CD4 hairpin loop, synthesi2ed in the natural direction, the enantiomer of the natural constmct, is inactive. 

Enzymatic Method. L-Amino acids can be produced by the enzymatic hydrolysis of chemically synthesized DL-amino acids or derivatives such as esters, hydantoins, carbamates, amides, and acylates (24). The enzyme which hydrolyzes the L-isomer specifically has been found in microbial sources. The resulting L-amino acid is isolated through routine chemical or physical processes. The D-isomer which remains unchanged is racemized chemically or enzymatically and the process is recycled. Conversely, enzymes which act specifically on D-isomers have been found. Thus various D-amino acids have been... 

In another procedure, D-amino acid oxidase (52) is usehil to produce L-amino acids from DL-amino acids. a-Ketocarboxylic acids which are formed by the action of enzymes on D-amino acids, are aminated to form L-amino acids by coupling through the action of amino acid aminotransferases (53). 

L-Amino acid Threshold value, mg/dL Sweet Sour Bitter Salty Umami... 

In Parenteral and Enteral Nutrition. Amino acid transfusion has been widely used since early times to maintain basic nitrogen metaboHsm when proteinaceous food caimot be eaten. It was very difficult to prepare a pyrogen-free transfusion from protein hydrolysates. Since the advances in L-amino acid production, the crystalline L-amino acids have been used and the problem of pyrogen in transfusion has been solved. The formulation of amino acid transfusion has been extensively investigated, and a solution or mixture in which the ratio between essential and nonessential amino acid is 1 1, has been widespread clinically. Special amino acid mixtures (eg, branched chain amino acids-enriched solution) have been developed for the treatment of several diseases (93). 

L-amino acid oxidase (Uver) L-amino acids a-keto acids +NH3 2FAD O2 H202... 

An a helix can in theory be either right-handed or left-handed depending on the screw direction of the chain. A left-handed a helix is not, however, allowed for L-amino acids due to the close approach of the side chains and the CO group. Thus the a helix that is observed in proteins is almost always right-handed. Short regions of left-handed a helices (3-5 residues) occur only occasionally. 

L-Amino acid (Section 27.2) A description of the stereochemistry at the a-carbon atom of a chiral amino acid. The Fischer projection of an a-amino acid has the amino group on the left when the carbon chain is vertical with the carboxyl group at the top. 

The predominance of L-amino acids in biological systems is one of life s most intriguing features. Prebiotic syntheses of amino acids would be expected to produce equal amounts of L- and D-enantiomers. Some kind of enantiomeric selection process must have intervened to select L-amino acids over their D-connterparts as the constituents of proteins. Was it random chance that chose L- over D-isomers ... 

From this, it is clear that D-glyceraldehyde is (i )-glyceraldehyde, and L-alanine is (S)-alanine (see figure). Interestingly, the u-car-bon configuration of all the L-amino acids except for cysteine , (S). Cysteine, by virtue of its thiol group, is in fact (i )-cysteine. 

D- or L-amino acids, but a given helix must be composed entirely of amino acids of one configuration. a-Helices cannot be formed from a mixed copolymer of D- and L-amino acids. An a-helix composed of D-amino acids is left-handed. 

---