3.1. Animal Origin
The proteins in dairy products have high nutritional value. They also contain a significant amount of animal-derived peptides and are considered to be a high-quality source of bioactive peptides. Milk-derived proteins are a precursor of bioactive peptides with various physiological functions. The main protein in milk is casein, which is around 80% of the total amount. Casein is structurally diverse with many sites exposed so that many proteases break down these proteins to form functional bioactive peptides. Numerous studies have shown that milk hydrolysates such as camel milk, cow milk, and goat milk have significant inhibitory effects on α-glucosidase. Casein hydrolysates extracted from bovine and camel milk were found to have a strong inhibitory effect on α-glucosidase, with IC
50 values of 1.04 mg/mL and 0.59 mg/mL, respectively; LPTGWLM, MFE, and GPAHCLL were the most effective α-glucosidase inhibitory peptides
[37][29].
Notably, some peptides composed of amino acids such as Gly, Ser, Glu, Tyr, Arg, Phe, and Pro are considered to be α-glucosidase inhibitors. Lee et al.
[44][30] identified the tripeptides GEY and GYG with α-glucosidase inhibitory activities (IC
50 of 2.70 and 1.50 mg/mL). Zhang et al.
[45][31] used the quantitative structure–activity relationship screening method and silkworm protein database to identify α-glucosidase inhibitory peptides. Four peptides that have α-glucosidase inhibition effects were obtained and they were QPGR (IC
50 of 65.80 mol/L), SQSPA (IC
50 of 20.00 mol/L), QPPT (IC
50 of 560.0 mol/L), and NSPR (IC
50 of 205.00 mol/L). QPGR could form hydrogen bonds with Lys776 in the active site ofα-glucosidase and SQSPA had potential interactions with Arg520, Lys519, and Asp777 of α-glucosidase. Compared with QPGR, QPPT and NSPR only form a single bond with Lys776 of α-glucosidase. In the three-dimensional structure, Lys776 is located at the edge of the α-glucosidase activity pocket and may be a key target for these peptides to inhibit α-glucosidase activity
[45][31]. SEDSSEVDIDLGN was a new peptide derived from sericin, and it had been found to non-competitively bind α-glucosidase by hydrogen bonding forces or van der Waals forces. The binding might be dominated by the Asn and Ser side chains which were contained in SEDSSEVDIDLGN
[46,76][32][33]. In addition, CSSV (IC
50 = 206.00 μg/mL), SAAP (IC
50 = 66.90 μg/mL), PGGP (IC
50 = 63.50 μg/mL), LGGGN (IC
50 = 42.93 μg/mL), YSFR (IC
50 = 162.00 μg/mL) from the protein of Giant Salamander, and GPPGPA from skin collagen hydrolysates of Giant Salamander contained higher contents of Gly and Pro
[47,48][34][35]. This might be why these peptides have a strong inhibitory effect on α-glucosidase. Peptide sequences were identified from edible insects, KVEGDLK, YETGNGIK, AIGVGAIR, IIAPPER, and FDPFPK exhibited the highest enzyme inhibitory activity
[49][36]. The only peptide containing phenylalanine FDPFPK was identified as the most powerful α-glucosidase inhibitor (IC
50 = 5.95 μg/mL). These results suggest that some peptides composed of specific amino acids can be used as α-glucosidase inhibitors.
3.2. Plant Origin
Plants have always been an important source of drug development due to their wide distribution, rich resources, effectiveness, and high safety. Moreover, plant secondary metabolites also can be widely used in pharmaceuticals and other fields. Many researchers have screened α-glucosidase inhibitors from plants and they are a rich source of α-glucosidase inhibitory peptides
[77,78,79][37][38][39].
α-Glucosidase inhibitory peptides from various protein-rich plant hydrolysates, such as grains, legumes, and seeds, have been extensively studied. Enzymatic hydrolysis increased the α-glucosidase inhibitory activity of protein hydrolysates
[80][40]. Different proteases have different restriction cutting sites, peptides with different molecular weights, amino acid sequences, and biological activities can be released by cutting peptide bonds at different parts
[81][41]. Commonly used enzymes include alkaline protease, acid protease, pepsin, trypsin, neutral protease, bromelain, flavor protease, etc. Alkaline protease is an endonuclease with a wide range of specificity. It preferentially cleaves the C-terminal peptide bonds of hydrophobic amino acid residues such as Try, Phe, Leu, Ile, Val, Met, etc. This gave it more catalytic sites to cleave peptide bonds in proteins, thus allowing deeper enzymatic cleavage
[82][42]. A longer enzymatic digestion time would result in a higher degree of hydrolysis value of the hydrolyzed product
[83][43]. Trypsin only cleats the C-terminus into Arg and Lys. Flavourzyme are exopeptidases that break the N-terminus of the peptide chain; papain prefers to cleaved peptide bonds between carboxylic acid groups of Lys or Arg and adjacent amino acid residues
[84][44]. Compared with other hydrolysates, the plant protein peptides hydrolyzed by alkaline protease showed the highest α-glucosidase inhibition. Therefore, alkaline proteases has been used to hydrolyze many plant proteins, such as soy protein,
Luffa cylindrical (L.) M. Roem seed, and
Ginkgo biloba seed
[52,53,85][45][46][47]. For better comparison, previous studies used different commercial enzymes for the enzymatic preparation of α-glucosidase inhibitory peptides under the same conditions. Soybean protein peptides prepared using alkaline protease had the highest α-glucosidase inhibitory activity where compared to those prepared using papain and trypsin. Three new α-glucosidase inhibitory peptides LLPLPVLK, SWLRL, and WLRL were found with IC
50 values of 237.43 ± 0.52 μmol/Lol/L, 182.05 ± 0.74 μmol/Lol/L, and 165.29 ± 0.74 μmol/Lol/L
[52][45]. Using different proteases to hydrolyze
Luffa cylindrical (L.) M. Roem seed, the alkaline protease hydrolysate showed the strongest inhibition of α-glucoside, followed by tryptic hydrolysate, with concentration-dependent inhibition of α-glucosidase (IC
50 of 0.48–0.80 mg/mL)
[85][47].
Ginkgo biloba seed was hydrolyzed using alkaline protease and three new peptides with α-glucosidase inhibitory activity were screened: LSMSFPPF, VPKIPPP, and MPGPPSD. LSMSFPPF showed the strongest inhibitory activity (IC
50 of 454.33 ± 32.45 μmol/Lol/L), followed by MPGPPSD (IC
50 of 943.82 ± 73.10 μmol/L) and VPKIPPP (IC
50 of 1446.81 ± 66.98 μmol/Lol/L)
[53][46].
The candidate peptides were further screened based on molecular weight, amino acid composition, and binding energy to α-glucosidase. Low molecular weight peptides have better stability and higher bioavailability, resulting in better biological function in vivo
[87,88,89][48][49][50]. Digestion of soy protein with trypsin to obtain hydrolysates with α-glucosidase inhibitory activity (IC
50 of 0.27 mg/mL) showed the highest inhibitory activity corresponding to MW < 5 kDa grade fractions and a yield of 76.08%
[54][51]. Two tripeptides GSR (IC
50 of 20.4 μmol/L) and EAK (IC
50 of 520.2 μmol/L) were obtained, the inhibitory effect of peptides on α-glucosidase is mainly due to the formation of five strong hydrogen bonds between GSR and His674, Asp518, Arg600, Asp616, and Asp282 in α-glucosidase; four hydrogen bonds were formed between EAK and residues Asp282, Asp518, and Asp616. Asp residues are import targets to inhibit the activity of α-glucosidase
[54][51]. In the easy-to-cook and difficult-to-cook bean hydrolysates, the ultrafiltration fractions with MW < 3 kDa showed the highest inhibition of α-glucosidase (34.4–89.2%)
[90][52]. The subunits of gluten hydrolysates were digested using kiwifruit actinidin. The WGLYH (≤1 kDa) group have the highest inhibitory activities on α-amylase and α-glucosidase
[55][53].
According to previous investigations, the peptides with strong α-glucosidase inhibitory activity are short peptides with relative molecular weights less than 1 kDa; this is because lower-molecular-weight peptides can enter the active site of α-glucosidase and bind to it.
[28][54]. However, VVDLVFFAAAK (MW = 1179.4 Da) from black tea protein also exhibited the best α-glucosidase inhibitory ability compared to peptides with MW < 1 kDa
[56][55]. Therefore, MW < 1 kDa was only used as the first screening condition. On the other hand, amino acid composition also contributes significantly to α-glucosidase inhibitory activity. α-Glucosidase inhibitory peptides are usually accompanied by a high degree of hydrophobicity
[37,95][29][56]. It suggests that hydrophobic amino acid contents in bioactive peptides are closely related with the inhibition activities of α-glucosidases. Quinoa proteins were subjected to simulated gastrointestinal digestion in vitro. Three peptides were isolated and the peptide IQEGGLT (IC
50 of 109.48 μmol/L) containing three hydrophobic residues showed strong inhibitory activity against α-glucosidase. In contrast, at 250 μmol/L, peptides DKDPYPK (22.16 ± 0.61%) and GEHGSDGNV (30.84 ± 0.69%) showed lower inhibition than IQEGGLT (55.85 ± 0.26%), probably due to their higher hydrophilicity
[57]. Quinoa proteins contain high amounts of Gln, Glu, Asp, Asn, Arg, Ser, Leu, and Pro
[96][58]. Ujiroghene et al.
[58][59] identified four antidiabetic peptides from germinated quinoa yogurt drink including VAHPVF, LAHMIVAGA, KLTPQMA, and KSFGSSNI. Among these peptides, LAHMIVAGA and VAHPVF showed significant α-glucosidase inhibitory activity with IC
50 values of 10.90 mg/mL and 9.00 mg/m.
For the different amino acid composition of peptides, leucine and valine can participate in muscle repair and blood glucose control; proline can improve the hypoglycemic activity of peptide
[98][60]. The peptide GLLGY from rice bran fermentation broth was a non-competitive inhibitor that forms five hydrogen bonds with Asp282, Ser523, Asp616, and His674 of α-glucosidase
[25][61]. GLLGY also maintained excellent α-glucosidase inhibition in the gastrointestinal digestive system
[25][61]. From the structure of α-glucosidase, Asp616 and His674 were the key amino acid residues in the catalytic structural domain, which might be the target of molecular docking of α-glucosidase inhibitors.
The binding energy of plant-derived peptides to α-glucosidase can be predicted by docking. The lower the binding energy required for peptides to bind to α-glucosidase, the easier it is to inhibit the activity of α-glucosidase. Four peptides FYNPAAGR, FFVPPSQQ, PGVLPVAS, and FSYNPQAG were screened from the hydrolysates of hot-pressed peanut meal. Molecular docking indicates that peptides can occupy the active pocket of α-glucosidase through hydrogen bonding, hydrophobic interaction, salt bridges, and π stacking, thus preventing α-glucosidase from forming a complex with substrates. Among these four peptides, PGVLPVAS had the lowest binding energy to α-glucoside, followed by FYNPAAGR, FFVPPSQQ, and FSYNPQAG
[64][62]. Therefore, lower binding energy of peptides than that of acarbose could be used as another screening criterion. The peptide TTGGKGGK (−8.97 kcal/mol) obtained from black bean (
Phaseolus vulgaris L.) proteins had a higher α-glucosidase inhibitory potential than acarbose. Black soybean peptides inhibit α-glucosidase through hydrogen bonding, polarity, and hydrophobicity. The main binding sites are Asp34, Thr83, and Asn32
[65][63]. Among the peptides purified from chickpea (
Cicerarietinum L.) protein hydrolyzates, FGKG showed optimal inhibition of α-glucosidase
[66][64]. Since FGKG exhibited the highest inhibition rate and lowest binding energy (−10.047 kcal/mol), the hydrophobic interactions of FGKG with Leu162-Phe1 and Ala198-Phe1 appeared to contribute significantly to the stabilization of the inhibitor–enzyme complex
[66][64].
3.3. Microbial Origin
As mankind’s knowledge of the world deepens, the vast resources contained in microorganisms are being understood. For peptides, microbiota is a high-quality source that is gradually being exploited by mankind. Due to the short life cycle, fast growth, reproduction, and low culture cost of microorganisms, more and more scholars are focusing on obtaining α-glucosidase inhibitory peptides from microorganism.
4. Mechanism of α-Glucosidase Inhibitory Peptide
Many studies have shown that naturally active peptides could treat type 2 diabetes by lowering blood sugar, but screening for them is a time-consuming process
[106,107][65][66]. To discover safer and more effective α-glucosidase inhibitory peptides, it is particularly important to explore the interaction mechanism between α-glucosidase inhibitory peptides and α-glucosidase. Based on recent studies, it is concluded that the main mechanism of action is to inhibit α-glucosidase activity in small intestine mucosa (
Figure 43). The bioactive peptide mainly binds to oligosaccharides and enzymes, producing competitive inhibition. The peptides act on the enzyme through the hydrophobic effect and hydrogen bonding van der Waals forces, modulating the amino acids at the catalytic sites of the enzyme to affect its original activity and reduce glucose intake
[46,108,109][32][67][68]. In recent years, researchers have summarized the structural features and mechanism of α-glucosidase inhibitory peptides: (1) Low-molecular-weight peptides have better α-glucosidase inhibitory activity
[110][69]; (2) Amino acids contain at least a hydroxyl or basic side chain at the N-terminus, with Pro present in the penultimate position at the
C-terminus
[8]; (3) The presence of Arg at the
C-terminal end improves the stability of the peptide–enzyme binding, ensures strong binding of peptides to key active amino acid residues of the enzyme, and promotes increased inhibitory activity
[61][70]; (4) The higher the content of hydrophobic amino acids, the better the inhibitory activity of α-glucosidase.
Figure 43. Mechanism of α-glucosidase inhibitory peptides.
Hydrogen bond and hydrophobic interaction play an important role in α-glucosidase inhibition. Qiu et al.
[70][71] used papain to hydrolyze soft-shelled turtle (
Pelodiscus sinensis) egg to obtain potential anti-diabetic peptides. Furthermore, three oligopeptides, HNKPEVEVR, ARDASVLK, and SGTLLHK, were synthesized, which had strong inhibitory effects on α-glucosidase activity (IC
50 = 56,195 and 289 μmol/Lol/L, respectively). Li et al.
[71][72] investigated the action mechanism of ARDASVLK and HNKPEVEVR on α-glucosidase using synchronous fluorescence, circular dichroism spectroscopy, and molecular modeling. The amino acid residues that ARDASVLK binds to α-glucosidase mainly include Pro1159, Arg1250, Tyr1251, Gln1254, Glu1258, Trp1355, Trp1369, Gln1372, Arg1377, Asp1526, Phe1559, Ile1587, Gly1588, and Arg1591. HNKPEVEVR interacted with amino acid residues of α-glucosidase as follows: Arg1156, Tyr1251, Trp1355, Leu1367, Asp1368, Trp1369, Asp1370, Phe1427, Asn1429, Val1432, Ser1452, Arg1455, Ser1459, Lys1460, and Ile1587
[71][72]. Molecular docking and thermodynamic results demonstrated that α-glucosidase interactions with ARDASVLK and HNKPEVEVR were driven by hydrogen bonding and hydrophobic action
[71][72]. The detection by circular dichroism spectroscopy showed that the content of some secondary structures (α-helix and β-sheet) of HNKPEVEVR were altered. Although the cause of these secondary structure changes was unknown, it was clear for minor conformational changes that blocking active center or preventing substrate binding would inactivate the enzyme. Therefore, ARDASVLLK and HNKPEVEVR, which were obtained from turtle eggs, could burst α-glucosidase by forming α-glucosidase-peptide complexes. In this case, the active vacancy of the enzyme was occupied and the conformation was changed, eventually the enzyme activity was diminished.
The presence of van der Waals forces also affects the activity of α-glucosidase. Three new peptides including LSMSFPPF, VPKIPPPHE, and MPGPPS which had α-glucosidase inhibitory activity were selected from the protein isolates of
Ginkgo biloba seed cake. There were 12 effector sites of van der Waals force between LSMSFPPF and α-glucosidase
[53][46]. For these sites, Ala454, Phe455, Ala514, Asp440, Thr517, Thr519, Leu525, Asn447, and Arg520 were on α-glucosidase A chain and Leu93, His91, and Lys89 were on the chain B. In addition, VPKIPPP and α-glucosidase also had 12 effector sites of van der Waals force, including Glu377, Thr339, Leu227, Leu300, Pru602, Gly228, Arg400, Tyr389, Glu396, Asn233, Ala229, and Glu231 on chain A.
For the α-glucosidase inhibitory peptide, it can form hydrogen bonds with the active site of the α-glucosidase which makes the binding more stable. At the same time, the inhibitory peptide interacts with the hydrophobic part of the enzyme to enhance the affinity between the inhibitory peptide and enzyme; the stability of the binding site has also been enhanced. This ultimately prevents the enzyme from binding to oligosaccharides, thereby effectively inhibiting changes in blood glucose. Van der Waals force interactions between α-glucosidase inhibitory peptides and enzymes can affect the binding and dissociation kinetics of inhibitory peptides, thus regulating enzyme activity, and some inhibitory peptides can improve the affinity and selectivity of inhibitory peptides by enhancing van der Waals force interactions with enzymes.
5. Bioavailability of α-Glucosidase Inhibitory Peptides
Systemic delivery of functional peptides via the oral route has been a daunting task. Low bioavailability of peptide was due to denaturation in the digestive tract, susceptibility in enzymatic reaction, and poor permeability in the cell membrane
[111][73].
In tThe
current study, the bioavailability of peptide components was usually assessed via simulating gastrointestinal digestion. The stability of peptide bioactivity depends not only on structural maintenance but also on other factors
[112][74]. Because the behavior of peptides that mimic the digestive species in the gastrointestinal tract can provide important information about their possible behavior in the digestive system. α-Glucosidase inhibiting peptides could exert hypoglycemic effects in vivo after entering the bloodstream in their active form. Therefore, peptides must withstand hydrolysis by gastrointestinal enzymes and continue to exert biological activity through the intestinal wall after oral administration
[57]. Whether the peptide is consumed as a functional food ingredient or as a nutritional product, the ability to resist gastrointestinal digestion or interpretation is very important.
6. Conclusions
Foods not only provide nutrition, but also provide many functional ingredients that are beneficial to health. Food-derived α-glucosidase inhibitory peptides are non-toxic and can inhibit the increase in blood glucose concentration, which shows great potential in the management and treatment of diabetes. It will be helpful to screen α-glucosidase inhibitory peptides more efficiently. Meanwhile, the discoveries of α-glucosidase inhibitory peptides can be accelerated by bioinformatics analysis. As dietary active peptides, it is also necessary to consider whether they will be degraded by digestive enzymes; thus, increasing the stability and bioavailability of α-glucosidase inhibitory peptides are important issues. Especially, will these bioactive peptides affect the gut microbiota and provide additional mechanisms for lowering blood glucose? These issues require further study.