Molecular Biology: Protein Structure and Function - Stepanov V.M. 2005

Amino Acids
Methods for the Synthesis of α-Amino Acids

1.3.1. Chemical Synthesis

Methods of Chemical synthesis of Amino acids are quite diverse. Many of them have found application in the industrial production of amino acids used as animal feed additives and in medicine. Let us consider only the principal ones, without dwelling on methods that yield only a single amino acid.

The simplest Amino acids can be obtained through the aminolysis of $\alpha$-halocarboxylic acids; for example, Alanine is produced by treating chloropropionic acid with a large excess of ammonia to suppress the side reaction of iminodicarboxylic acid formation:

Class="center">

The synthesis proposed in the last century by A. Strecker is based on the reaction of an aldehyde R—CHO with potassium cyanide and ammonium salts, the latter of which can be replaced by urea. The resulting substituted hydantoin (a cyclic derivative of the corresponding amino acid) is subsequently hydrolyzed with alkali to yield the D,L-amino acid. In this way, D,L-Methionine (an amino acid used in medicine and animal husbandry) is synthesized industrially starting from $\beta$-methylthiopropionaldehyde, which in turn is formed via the reaction of acrolein and methyl mercaptan:

Aldehydes can be reacted with pre-synthesized hydantoin. Subsequent hydrogenation of the double bond and Hydrolysis lead to the racemate of the corresponding amino acid:

1.3.2. Resolution of Racemic Amino Acids

Regardless of how chemical synthesis is carried out, it leads to The formation of a mixture containing strictly equal amounts of L- and D-isomers of The amino acid—its racemate. Naturally occurring L-isomers are generally of primary practical interest; therefore, methods for resolving racemates to isolate the L-isomers are of great importance. Chemical resolution methods are based on the interaction of racemic amino acids or their derivatives with asymmetric compounds.

Thus, researchers resort to preparing salts of amino acid esters with a pre-isolated stereoisomer of an amino acid, such as L-glutamic acid, an acylated L-amino acid, or another asymmetric acid, notably D-tartaric acid. The resulting diastereomeric salts generally exhibit different solubilities. The salt formed by the ester of the L-isomer of the target amino acid with D-tartaric acid differs in solubility from the salt composed of the D-amino acid ester and D-tartaric acid. This makes it possible to isolate the isomers via fractional crystallization. The complexity of this route is obvious, so it is used only when other approaches fail.

The most widespread method for resolving amino acid racemates is the enzymatic approach. Most commonly, this utilizes the ability of the enzyme amino acid acylase, found in animal Kidneys or produced by certain microscopic Fungi, to selectively hydrolyze N-acyl-L-amino acids without affecting the D-isomers. Following exhaustive hydrolysis of the acylated (most often acetylated) amino acid in the presence of acylase, a mixture of the free D-Amino Acid and acetyl-amino acid is formed, which are easily separated by crystallization or Ion Exchange:

Typically, after Separation, the acetyl-amino acid undergoes racemization, and the resulting racemate is again hydrolyzed by acylase, ultimately achieving virtually complete Conversion of the acetyl-D,L-amino acid into the D-amino acid.

Of particular interest is The conversion of the racemic lactam of D,L-Lysine (obtained from an accessible starting material, caprolactam) into D-lysine through the concerted action of two Enzymes. One of these, L-lysine lactamase, hydrolyzes only the D-isomer of the lactam to form D-lysine, while the other, a racemase, converts the unreacted L-isomer lactam into the racemic D,L-lysine lactam. The D-isomer is then hydrolyzed again by the lactamase. The net result is the complete conversion of D,L-lysine lactam into D-lysine, which is used as a feed additive:

1.3.3. Enzymatic Synthesis of Amino Acids

METHODS FOR PRODUCING amino acids through precursor conversions catalyzed by specific enzymes are highly practical. In this approach, the enzyme ensures the stereoselectivity of the reaction, yielding exclusively a single isomer, typically the L-isomer. A widely used example is the synthesis of L-aspartic acid via The addition of ammonia to fumaric acid, catalyzed by aspartase or microbial Cells containing this enzyme:

To produce L-phenylalanine, the reaction between cinnamic acid and ammonia, catalyzed by phenylalanine ammonia-lyase, is employed:

1.3.4. Microbiological Synthesis of Amino Acids

This method is based on The production of free amino acids by certain microorganisms using simple raw Materials, such as sugar industry wastes, starch, acetate, and the like. Microbial mutants are employed that have lost certain enzymes in the branched Amino acid Biosynthesis pathway, leading to an enhanced production of specific amino acids. For instance, blocking homoserine dehydrogenase—which is responsible for converting aspartate semialdehyde into homoserine and subsequently into Threonine and methionine—directs virtually all the aspartate semialdehyde toward lysine synthesis. Additionally, Mutations are introduced to remove the feedback inhibition of pathway-end enzymes (such as lysine); sometimes, researchers resort to multiplying the genes encoding Key Enzymes for The biosynthesis of the given amino acid.

The combination of these techniques has made it possible to obtain mutants capable of synthesizing, for example, up to 100 g of L-lysine per liter of culture broth with a high conversion rate of starting materials into the amino acid. Microbiological, chemical, and partly enzymatic methods of Amino acid synthesis form The basis of their industrial production, which reaches tens and hundreds of thousands of tons per year (lysine, glutamic acid, phenylalanine, aspartic acid, methionine). These amino acids are used as feed additives, flavor enhancers, Pharmaceuticals, and starting materials for obtaining physiologically active peptides.