4.1 Comparison of degree of hydrolysis among edible fungi proteins
Three gastrointestinal enzymes were used to simulate the hydrolysis of 27 proteins in edible fungi. The highest degree of hydrolysis was squalene synthase (40.95%) in Ganoderma lucidum, followed by phosphoglycerate mutase-like protein (40.62%) in Pleurotus eryngii, and the other proteins see (Table 1, Fig. 1). The TDH of these proteins were similar to those of 28 plant proteins simulated by Luo et al.(Luo et al., 2020), indicating that these proteins were within the range of theoretical values, and the TDH of squalene synthase and phosphoglycerol ester mutant-like proteins were more than 40%, which were high-quality proteins derived from peptides in the gastrointestinal tract. Otherwise, squalene synthase and Phosphoglycerate mutase-like proteins are widely present in organisms, and the F values of MAO-A docking are 0.049 and 0.045, respectively. Through docking, it is found that they have good inhibitory effect on MAO-A, and high hydrolysis degree indicates that they are more conducive to the absorption and utilization of human body.
4.2 Frequency of high affinity oligopeptides
After simulated docking, F value was used to evaluate the ability of these edible fungi proteins to inhibit MAO-A. The F value was used to evaluate the ability of proteins from edible fungi to inhibit MAO-A. Among the high affinity oligopeptides, the highest F value was the mitochondrial carrier (0.069) in Pleurotus eryngii, followed by laccase (0.064) in Auricularia auricula and glutamate dehydrogenase (0.059) in Flammulina velutipes. The F value of mitochondrial carrier, laccase and glutamate dehydrogenase was higher than that of other proteins (enzymes), indicating that the inhibitory effect on MAO-A activity was relatively better than that of other proteins studied in this paper, and it was easier to obtain high affinity oligopeptides after hydrolysis. Since these three proteins are widely present in edible fungi and have high relative content, they are a major advantage for screening MAO inhibitory peptides from edible fungi.
4.3 Super high affinity oligopeptides
MAO-A inhibitors are currently used to treat refractory depression and some specific types of depression(Shulman et al., 2013). Computer simulation is helpful to reasonably explore the selection of more valuable compounds from a series of compounds with inhibitory potential, so as to screen inhibitors more reasonably, reduce the experimental cost and reduce the experimental burden.
After HPEPDOCK docking, the oligopeptides with docking scores≤-160 were called ultra-high affinity oligopeptides. There were 27 proteins involved in hydrolysis. After hydrolysis, 32 oligopeptides with ultra-high affinity were obtained by molecular docking. Among them, there were 20 oligopeptides with − 170 ≤ docking scores≤-160, which were PSTSY, PVVY, SISW, IIDAY, GPQW, PIPF, PGW, PPQH, PTATH, SGITY, TTSW, SSTAW, SAIW, GTICF, TITIL, PSW, VIW, SIPQF, PGVW, PEW. There were 9 oligopeptides with − 180 ≤ docking scores < − 170, which were ISDVW, QTGGW, PAAW, TIQVF, IIAEW, PTW, QETIY, SIVAW and IQIIH. There were 2 oligopeptides with − 190 ≤ docking scores < − 180, which were PTSGW and QCQW. And 1 oligopeptide with − 200 ≤ docking scores< -190, which was PQSTW (see table 2 for details). The results showed that the docking scores of ultra-high affinity oligopeptides was mainly between-170 and-160.These oligopeptides not only had low molecular weight and good inhibitory effect on MAO-A, but also had good bioavailability and were more likely to play a role through the intestine.
Bioactive peptides from food proteins are increasingly considered as a useful tool for improving health and preventing chronic diseases, and have attracted great attention from food manufacturers and consumers. Peptides are one of many food compounds with the ability to display biological activities in vivo. The application of bioactive peptides in functional foods, nutrients and therapeutic formulations has attracted more and more attention from the academic community(Acquah et al., 2019; Lammi et al., 2019). So far, researchers have found some antidepressant-active peptides. As Veyssiere et al.(Veyssiere et al., 2015) have shown, a peptide named spadin, consisting of 17 amino acids, works faster than classical antidepressants and does not induce side effects. Kimura et al.(Kimura et al., 2018) identified that the three peptides obtained from the hydrolysis of spinach green leaf protein showed strong anti-anxiety effects after oral administration. As a new therapeutic tool, food-derived peptides have shown outstanding advantages in finding new safe and effective antidepressants. In summary, these ultra-high affinity oligopeptides are the main target peptides for further exploring the inhibitory effect of MAO-A in vivo and in vitro.
4.4 Repeated high affinity oligopeptides
All 27 proteins hydrolysates contain high affinity oligopeptides with MAO-A. There were 430 high-affinity oligopeptides after simulated hydrolysis of 27 proteins, of which 27 oligopeptides sequences were repeated (see Fig. 2 for details). In the repeat oligopeptides, PY has 9 and repeats the most. Followed by AW, there are 7. The peptides with 4 repeats were IAF, PW, EW, VW, and SW. There were 3 repeats in ISR, and 2 repeats in the remaining 19 oligopeptides. Among the repeatable high affinity oligopeptides, there are 8 oligopeptides containing 2 amino acid sequences, namely PY, AW, CW, VW, PW, QW, SW, and EW. There are 16 kinds of oligopeptides with 3 amino acid sequences, namely VQF, IAF, VVR, ISR, PEY, ATW, TVY, QPR, VEW, SVF, PSW, PGF, VAY, CIH, ITY, and PGR. There are 3 kinds of oligopeptides containing 4 amino acids, namely QPSL, VSPR and TQSL. Thus, the same high affinity oligopeptide sequence can be obtained after different protein hydrolysis. These oligopeptides may have special effects on inhibiting MAO-A activity, which can be further tested in vitro and in vivo.