**4. Proteinogenic sulfur-containing amino acids**

The two proteinogenic sulfur-containing amino acids, methionine and cysteine, are critical to evaluating soybean meal as food and feed. While present in soybean, methionine and cysteine levels are both inadequate for consumer needs [8, 20, 24]. Similar to research determinations, the nutritional requirements for methionine and cysteine intake are often grouped together as overall sulfur consumption from protein. Adult humans are recommended to intake 910– 1120 mg methionine and cysteine (based on body weight) per day [13]. For livestock, sulfur amino acids recommendations can vary based on species, age, end-use, and diet formulation. The importance of dietary sulfur amino acids for livestock is greatly emphasized in literature, especially with soybean meal as base feed [8, 20, 34, 35].

deficient amino acids. While an increase in protein content would theoretically entail an increase in amino acids including methionine and cysteine [57], the opposite effect has been more notable [58, 59]. Molecular breeding techniques have recently improved the understanding of amino acid genomic regulation. Multiple quantitative trait loci (QTL) studies have been performed to identify genomic regions that control amino acids levels in soybean seed [60–64]. A myriad of QTL's were found to create amino acid phenotypic variation. Individual amino acids had reoccurring or proximal QTL's discovered such as Satt 518 [60], ss107913002 [61], and BARC-048619 [62] for glycine and threonine. QTL's for methionine and cysteine were also discovered which could lead to valuable improvements for soybean livestock feed [60–64]. Other genomic studies such as genome-wide association studies (GWAS) and genetic

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Genetic engineering experiments such as genetically modified organisms (GMOs) and gene editing are also promising avenues for improving amino acid profiles of soybeans. Compared to conventionally bred varieties, transgenic soybeans face additional adversity from registration requirements and public opinion. Transgenic efforts generally have one of three targets: magnifying biosynthesis genes, adjusting biosynthesis regulation, and modifying storage proteins. The earliest example would be a Brazil nut gene transfer in 1992. This successfully increased protein content and methionine biosynthesis, however a major food allergen was also transferred making commercialization impossible [65]. Expressing zein proteins from corn has also been shown to increase sulfur-containing amino acids levels in soybean [66, 67]. Altering biosynthesis feedback regulation amplified both non-proteinogenic and proteogenic lysine by circumventing normal enzymatic pathways [68]. Tryptophan in soybean also exhibited increased non-proteinogenic levels when a feedback-insensitive enzyme was transferred [25]. While soybean is not deficient in lysine or tryptophan, corn is deficient in both. By increasing lysine and tryptophan concentrations, soybean becomes an even more useful feed additive to corn rations. Even with limited research on modifying overall amino acids profiles in soybeans, modifying 11S and 7S storage proteins ratios [69] or silencing their expression entirely [70] has displayed increased amino acids levels. Similarly, a study using irradiated mutant soybeans lacking storage proteins as breeding parents demonstrated increased non-proteinogenic amino acids contents [54]. In addition, further research should be conducted to determine the bioavailability and digestibility of increased non-proteinogenic amino acids in soybean.

Amino acids concentrations in soybean are not only affected by their genetic potential. Agronomic properties greatly impact the final levels of amino acids. Agronomy encompasses all aspects of crop production including environmental effects, climatic variables, and abiotic factors. Perhaps the most considered agronomic factor is soil nutrient availability. Insufficient

diversity analyses would further improve genetic understanding.

**6. Genetic engineering**

**7. Agronomic relations**

Methionine and cysteine are vital to biological functions because of the sulfur contained in their R-groups and versatility in macromolecule synthesis. Methionine is well-known for being the typical initiating amino acid for protein synthesis and has hydrophobic properties when incorporated into proteins [36]. These hydrophobic properties usually result in methionine incorporation within the core of proteins. However, certain proteins have surface-exposed methionine susceptible to oxidation that is associated with age-related disease [30, 37, 38]. S-adenosylmethionine, a methionine metabolism intermediate, is widely-used with functions in methylation as well as amine, methylene, and sulfur atom donation [30, 39, 40]. Methionine is especially important for poultry production as birds have exceedingly high sulfur amino acid requirements, and low methionine levels can negatively affect growth rate, carcass yield, fat content, and disease immunity [41–43]. Cysteine's ability to form disulfide bonds makes it incredibly important to tertiary protein structure and can occur with and without enzyme interactions [30, 44]. Cysteine is involved with keratin and feather production in poultry and deficiencies have been correlated to poor breast muscle development [45, 46]. Swine also need higher amounts of cysteine as they age to compensate for body maintenance [29, 47].
