Stereoisomers of amino acids

An update on the occurrence of these unusual stereoisomers of amino acids. 

The enantiomeric separation of the D- from the L-stereoisomers of amino acids is an area of growing interest. It is generally recognized that heat- and alkali-treatment of proteins can result in the racemization of L-isomers of amino acid residues to the D-analogs. Almost without exception, humans cannot utilize the D-isomers of amino acids, and some are thought to be toxic (although... 

Gibson, Q. H. and Wiseman, G. (1951). Selective absorption of stereoisomers of amino acids from loops of the small intestine of the rat. Biochem. J. 48 426-429. 

Two appendices are included at the end of this chapter. The first is intended to serve as a reminder, for those of you who might need it, of the nomendature and representation of stereoisomers. The second appendix contains descriptions of various chemo-enzymatic methods of amino acid production. This appendix has been constructed largely from the recent primary literature and includes many new advances in the field. It is not necessary for you to consult the appendix to satisfy the learning objectives of the chapter, rather the information is provided to illustrate the extensive range of methodology assodated with chemo-enzymatic approaches to amino add production. It is therefore available for those of you who may wish to extend your knowledge in this area. Where available, data derived from die literature are used to illustrate methods and to discuss economic aspects of large-scale production. 

The two forms of mirror images are called enantiomers, or stereoisomers. All amino acids in proteins are left-handed, and all sugars in DNA and RNA are right-handed. Drug molecules with chiral centers when synthesized without special separation steps in the reaction process result in 50/50 mixtures of both the left- and right-handed forms. The mixture is often referred to as a racemic mixture. 

For all other amino acids, with the exception of glycine (Gly), the a-carbon is bonded to four different groups, and the two stereoisomers are mirror images that cannot be superimposed. Eukaryotic proteins are always composed of L-amino acids although D-amino acids are found in certain peptide antibiotics and some peptides of bacterial cell walls. ° ° The physical properties of amino acids are influenced by the degree of ionization at different pH values. 

Three approaches can be employed to separate peptide stereoisomers and amino acid enantiomers separations on chiral columns, separations on achiral stationary phases with mobile phases containing chiral selectors, and precolumn derivatization with chiral agents [111]. Cyclodextrins are most often used for the preparation of chiral columns and as chiral selectors in mobile phases. Macrocyclic antibiotics have also been used as chiral selectors [126]. Very recently, Ilsz et al. [127] reviewed HPLC separation of small peptides and amino acids on macrocyclic antibiotic-based chiral stationary phases. 

Nearly all biological compounds with a chiral center occur naturally in only one stereoisomeric form, either d or L. The amino acid residues in protein molecules are exclusively L stereoisomers. D-Amino acid residues have been found only in a few, generally small peptides, including some peptides of bacterial cell walls and certain peptide antibiotics. 

It is remarkable that virtually all amino acid residues in proteins are L stereoisomers. When chiral compounds are formed by ordinary chemical reactions, the result is a racemic mixture of d and l isomers, which are difficult for a chemist to distinguish and separate. But to a living system, D and L isomers are as different as the right hand and the left. The formation of stable, repeating substructures in proteins (Chapter 4) generally requires that their constituent amino acids be of one stereochemical series. Cells are able to specifically synthesize the l isomers of amino acids because the active sites of enzymes are asymmetric, causing the reactions they catalyze to be stereospecific. 

In 1899 Fischer turned his attention to the study of proteins, wishing to understand their chemical structures. It was known at that time that proteins were composed of amino acids, and thirteen naturally occurring ones were identified. Fischer was able to isolate via the hydrolysis of proteins three additional naturally occurring amino acids valine, proline, and hydroxyproline. Amino acids exhibit stereoisomerism, and Fischer was able to separate individual forms from mixtures of stereoisomers for several of these compounds. 

At the present time, the reported applications of the CR CSP have been limited to the separation of amino acids and some dipeptides as bulk substances. One example of the use of the CR CSP in a complex matrix was the direct stereochemical resolution of aspartame stereoisomers and their degradation products in coffee and diet soft drinks (76). Aspartame (N-DL-a-aspartyl-DL-phenylalanine methyl ester) is a dipeptide whose L,L-isomer is a low-calorie sweetener sold under the name NutraSweet. The structure of aspartame and its major degradation products are presented in Fig. 9 aiwl the stereochemical separation of these compounds on the CR CSP in Fig. lOA. The resolutions were accomplished using a mobile phase... 

Numerous biotranformation processes for fhe synthesis of amino acids have been described and for fhe purpose of this chapter, we have restricted the discussion to fhe unnatural amino acids fhat are not accessible by fermentation. For this class of amino acids, commercialized biotranformations are either based on asymmetric synthesis starting from a prochiral compound or on (dynamic) kinetic resolutions of a racemate. As an illustration the published processes for (R)- and (S)-tert-leucine are outlined in Scheme 4.4. Both stereoisomers of tert-leucine have been used for fhe synfhesis of peptides that serve as protease inhibitors acting against viral infections (e.g. Hepatitis C, HIV), bacterial infections, autoimmune diseases and cancer [27]. This particular amino acid is versatile in fhese applications since fhe tert-butyl moiety provides resistance against endogenous proteases and can enhance the binding affinity of fhe peptide to fhe target protease. 

Amino Acid Classes Biologically Active Amino Acids Modified Amino Acids in Proteins Amino Acid Stereoisomers Titration of Amino Acids Amino Acid Reactions PEPTIDES PROTEINS... 

Bondy S.C. and Harrington M.E. (1979) The preparation high-affinity binding of the biological forms of amino acids and hexose stereoisomers to bentonite. Stud. Phys. Theor. Chem. 7, (Origin of Optical Activity in Nature), p. 141-149. 

Let us look more closely at the chirality of enzymes. An illustration is chymotrypsin, an enzyme in the intestines of animals, which catalyzes the hydrolysis of proteins during digestion. Chymotrypsin, like all proteins, is composed of a long molecular chain of amino acids that folds up into the active enzyme. Human chymotrypsin has 268 chiral centers that result firom the amino acids so the maximum number of stereoisomers possible is 2 , a staggeringly large number, almost beyond comprehension. 

Proteins are long chains of amino acids covalently bonded together by amide bonds formed between the carboxyl group of one amino acid to the amine group of another amino acid. Because they are made from pure amino acid stereoisomers, protein themselves are single stereoisomers despite having several hundred or more chiral centers. 

These two objects are not identical no matter which way you turn them, they cannot be superimposed on each other any more than a left hand can on a right. They are in fact mirror images. They are enantiomers (from the Greek for opposite), also called stereoisomers. The amino acids from which all proteins are constructed have this property, and their two forms are designated L and D, deriving from the Latin laevus (left) and dexter (right), based on the direction in which they rotate the plane of polarized light. 

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