Amino acids are carboxylic acids containing amino groups and are the building blocks of various proteins in living organisms. The amino acid molecule contains two functional groups: amino group and carboxyl group. Although amino acids are divided into α-, β-, γ-, w-amino acids according to the different positions of the amino groups on the carbon chain, the amino acids obtained after protein hydrolysis are all α-Amino acids, And there are only 22 kinds, including glycine, alanine, valine, leucine, isoleucine, methionine (methionine), proline, tryptophan, serine, tyrosine, cysteine, phenylalanine, asparagine, glutamine, threonine, aspartic acid, glutamic acid, lysine, arginine, histidine, selenocysteine and pyrrole Lysine.
Figure 1. General formula of amino acid structure
The first amino acid discovered in history was aspartic acid, which was isolated from asparagus in 1806 by a French chemist. It was not until 1940 that more than 20 kinds of amino acids in nature were discovered. In recent years, major progress has been made in the development and application of amino acids. The types and numbers of newly discovered amino acids have reached more than 1,000, including more than 100 types of medicinal amino acids and amino acid derivatives. At present, amino acids have been widely used in industrialized production, covering multiple industries such as food and health care, medicine, agriculture, and animal husbandry. They have developed into a major global industry and have become an inseparable part of human life (Table 1).
Table 1. Partial applications of amino acids
|Food industry||Fortified food, flavor enhancer, sweetener|
|Feed industry||Animal food|
|Pharmaceutical industry||Treatment of nutritional or metabolic disorders and treatment of bone marrow tumors|
|Chemical industry||Detergent, fiber|
In industry, amino acid synthesis methods are mainly divided into the following types: enzymatic method, extraction method, chemical synthesis method, fermentation method, etc.
Since the successful industrial production of L-aspartic acid by the enzymatic method in 1972, a variety of amino acids have been produced by the enzymatic method. The so-called enzymatic method is to use whole bacteria or extract enzymes from microbial cells to produce amino acids. The advantages of enzymatic method are simple production process, high product concentration, high conversion efficiency, few by-products, simple extraction process, high extraction yield, and immobilized enzyme or immobilized cell technology can be applied. Representative amino acids produced by enzymatic method include alanine, aspartic acid, tyrosine, dopa, tryptophan, 5-hydroxy-tryptophan, and lysine. For example, the commonly used method for producing L-alanine in industry uses L-aspartic acid as a raw material to obtain L-alanine through L-aspartate-β-decarboxylase catalysis.
Extraction method mainly refers to the extraction and production of amino acids from protein hydrolysates. It is one of the earliest applied methods and is also an important amino acid production technology. Our country has developed a process for extracting cystine from hair in the 1970s. After half a century of development, the technology has become more mature, with the increasing variety of products, and the continuous improvement of extraction quality and efficiency. It can extract from a single amino acid to multiple amino acids at the same time, and the production cost is continuously reduced, which is more environmentally friendly. Hair is a kind of high-quality keratin. The main practical applications in industry are human hair, pig hair and feathers. The content of amino acids such as cystine in hair is high. The target amino acid extracted from hair is cystine. Amino acids produced by typical extraction method include cysteine, cystine and tyrosine. At present, almost all amino acids extracted from hair are based on cystine, and the process route used to extract cystine is no different from the earlier ones. It is still a three-step method, namely: first hydrolyze the hair into amino acids with acid, and then obtain cystine through three times of neutralization and purification.
The biggest advantage of chemical synthesis method is that the types of amino acids produced are not limited. In addition to natural amino acids, it can also be used to prepare unnatural amino acids with various special structures. Because the amino acid produced by chemical synthesis method contains two optical isomers (chiral isomers) of D and L, it is necessary to consider how to solve the problem of isomers and the racemic utilization of D-isomers in the synthesis process. The amino acids that have been industrialized and mass-produced are glycine, methionine, tryptophan, etc., using chemical synthesis method. The main chemical synthesis process of glycine includes 4 kinds of chloroacetic acid ammonolysis method, Streck method, hydantoin method and biosynthesis method. Almost all domestic enterprises use chloroacetic acid ammonolysis technology to prepare glycine (aqueous chloroacetic acid process). The process uses chloroacetic acid and liquid ammonia as raw materials and is prepared under the action of a catalyst.
Fermentation methods include direct fermentation and fermentation of intermediate products. In fact, most amino acids are produced by fermentation. This method has the advantages of cheap and easy-to-obtain raw materials, mild production conditions, etc. With the aid of the amino acid synthesis ability of the microorganism itself, and the mutagenesis treatment of the strains, various required defective or resistant mutant strains can be selected to achieve the purpose of excessive synthesis of the target amino acid.