三萜皂苷是一类苷元为三萜类的皂苷，在豆科植物中广泛存在。乙酰化的A类三萜皂苷具有苦味，影响大豆的品质。先前的研究表明大豆三萜皂苷的含量与Sg-5（Glyma.15g243300）基因相关，该基因编码细胞色素P450家族成员CYP72A69，负责大豆甾醇A的生物合成。
最近，研究人员获得了一个新的大豆突变体CWS5095，该突变体几乎不含有A类三萜皂苷。基因定位和分析结果表明，突变体中含有4个SNP位点，其中一个SNP（G1127A）导致了sg-5编码的蛋白在EXXR结构域发生一个氨基酸的改变（Arg376Lys）。该突变位于细胞色素P450家族的保守区域，蛋白结构分析表明赖氨酸对精氨酸的取代阻碍了血红素辅酶因子对CYP72A69的结合，导致大豆甾醇A的生物合成受阻，从而不产生A类三萜皂苷。
该研究为大豆品质改良提供了新的思路。
PLoS One. 2018 Jan 30.
Molecular elucidation of a new allelic variation at the Sg-5 gene associated with the absence of group A saponins in wild soybean.
Author
Sundaramoorthy J, Park GT, Mukaiyama K……Kim JH, Seo HS, Song JT*.
*: School of Applied Biosciences, Kyungpook National University, Republic of Korea.
Abstract
In soybean, triterpenoid saponin is one of the major secondary metabolites and is further classified into group A and DDMP saponins. Although they have known health benefits for humans and animals, acetylation of group A saponins causes bitterness and gives an astringent taste to soy products. Therefore, several studies are being conducted to eliminate acetylated group A saponins. Previous studies have isolated and characterized the Sg-5 (Glyma.15g243300) gene, which encodes the cytochrome P450 72A69 enzyme and is responsible for soyasapogenol A biosynthesis. In this study, we elucidated the molecular identity of a novel mutant of Glycine soja, 'CWS5095'. Phenotypic analysis using TLC and LC-PDA/MS/MS showed that the mutant 'CWS5095' did not produce any group A saponins. Segregation analysis showed that the absence of group A saponins is controlled by a single recessive allele. The locus was mapped on chromosome 15 (4.3 Mb) between Affx-89193969 and Affx-89134397 where the previously identified Glyma.15g243300 gene is positioned. Sequence analysis of the coding region for the Glyma.15g243300 gene revealed the presence of four SNPs in 'CWS5095' compared to the control lines. One of these four SNPs (G1127A) leads to the amino acid change Arg376Lys in the EXXR motif, which is invariably conserved among the CYP450 superfamily proteins. Co-segregation analysis showed that the missense mutation (Arg376Lys) was tightly linked with the absence of group A saponins in 'CWS5095'. Even though Arg and Lys have similar chemical features, the 3D modelled protein structure indicates that the replacement of Arg with Lys may cause a loss-of-function of the Sg-5 protein by inhibiting the stable binding of a heme cofactor to the CYP72A69 apoenzyme.