Functional expression of horsegram (Dolichos biflorus) Bowman-Birk inhibitor and its self-association

Protease Inhibitors from Plants as Therapeutic Agents- A Review

Plant-based diets are a great source of protease inhibitors (PIs). Two of the most well-known families of PIs are Bowman-Birk inhibitors (BBI) and Kunitz-type inhibitors (KTI). The first group acts mainly on trypsin, chymotrypsin, and elastase; the second is on serine, cysteine, and aspartic proteases. PIs can retard or inhibit the catalytic action of enzymes; therefore, they are considered non-nutritional compounds; nevertheless, animal studies and cell line experiments showed promising results of PIs in treating human illnesses such as obesity, cardiovascular diseases, autoimmune diseases, inflammatory processes, and different types of cancer (gastric, colorectal, breast, and lung cancer). Anticarcinogenic activity's proposed mechanisms of action comprise several inhibitory effects at different molecular levels, i.e., transcription, post-transcription, translation, post-translation, and secretion of cancer cells. This work reviews the potential therapeutic applications of PIs as anticarcinogenic and anti-inflammatory agents in human diseases and the mechanisms by which they exert these effects.

Multimeric Association of Purified Novel Bowman-Birk Inhibitor From the Medicinal Forage Legume Mucuna pruriens (L.) DC

A Bowman-Birk protease, i.e., Mucuna pruriens trypsin inhibitor (MPTI), was purified from the seeds by 55.702-fold and revealed a single trypsin inhibitor on a zymogram with a specific activity of 202.31 TIU/mg of protein. On sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) under non-reducing conditions, the protease trypsin inhibitor fraction [i.e., trypsin inhibitor non-reducing (TINR)] exhibited molecular weights of 74 and 37 kDa, and under reducing conditions [i.e., trypsin inhibitor reducing (TIR)], 37 and 18 kDa. TINR-37 revealed protease inhibitor activity on native PAGE and 37 and 18 kDa protein bands on SDS-PAGE. TINR-74 showed peaks corresponding to 18.695, 37.39, 56.085, and 74.78 kDa on ultra-performance liquid chromatography (UPLC) coupled with electrospray ionization/quadrupole time-of-flight-mass spectrometry (ESI/QTOF-MS). Similarly, TINR-37 displayed 18.695 and 37.39 kDa peaks. Furthermore, TIR-37 and TIR-18 exhibited peaks corresponding to 37.39 and 18.695 kDa. Multiple peaks observed by the UPLC-ESI/QTOF analysis revealed the multimeric association, confirming the characteristic and functional features of Bowman-Birk inhibitors (BBIs). The multimeric association helps to achieve more stability, thus enhancing their functional efficiency. MPTI was found to be a competitive inhibitor which again suggested that it belongs to the BBI family of inhibitors, displayed an inhibitor constant of 1.3 × 10^-6 M, and further demonstrates potent anti-inflammatory activity. The study provided a comprehensive basis for the identification of multimeric associates and their therapeutic potential, which could elaborate the stability and functional efficiency of the MPTI in the native state from M. pruriens.

Response of Midgut Trypsin- and Chymotrypsin-Like Proteases of Helicoverpa armigera Larvae Upon Feeding With Peanut BBI: Biochemical and Biophysical Characterization of PnBBI

Proteinase/Protease inhibitors (PIs) from higher plants play an important role in defense and confer resistance against various insect pests and pathogens. In the present study, Bowman-Birk Inhibitor (BBI) was purified from mature seeds of an interspecific advanced hybrid peanut variety (4368-1) using chromatographic techniques. The biochemical and biophysical characteristics such as low molecular mass, presence of several isoinhibitors and higher-ordered dimer/tetramer, predominance of antiparallel β-sheets and random coils in secondary structure, reactive sites against trypsin and chymotrypsin, broad spectrum of stability toward extreme pH and temperature along with MALDI TOF-TOF analysis (ProteomeXchange identifier PXD016933) ascertained the purified biomolecule from peanut as BBI (PnBBI). Surface plasmon resonance competitive binding analysis revealed the bifunctional PnBBI is a trypsin specific inhibitor with 1:2 stoichiometry as compared to chymotrypsin. A concentration-dependent self-association tendency of PnBBI was further confirmed by 'red shift' in the far-UV CD spectra. Furthermore, the insecticidal potential of PnBBI against Helicoverpa armigera was assessed by in vitro assays and in vivo feeding experiments. A significant reduction in larval body weight was observed with concomitant attenuation in the activity of midgut trypsin-like proteases of H. armigera (HaTPs) fed on PnBBI supplemented diet. The one and two-dimensional zymography studies revealed the disappearance of several isoforms of HaTP upon feeding with PnBBI. qRT-PCR analysis further suggests the role of PnBBI in not only inhibiting the activity of midgut trypsin and chymotrypsin-like proteases but also in modulating their expression. Taken together, the results provide a biochemical and molecular basis for introgressed resistance in peanut interspecific advanced hybrid variety against H. armigera.

Eliminating anti-nutritional plant food proteins: the case of seed protease inhibitors in pea

Several classes of seed proteins limit the utilisation of plant proteins in human and farm animal diets, while plant foods have much to offer to the sustainable intensification of food/feed production and to human health. Reduction or removal of these proteins could greatly enhance seed protein quality and various strategies have been used to try to achieve this with limited success. We investigated whether seed protease inhibitor mutations could be exploited to enhance seed quality, availing of induced mutant and natural Pisum germplasm collections to identify mutants, whilst acquiring an understanding of the impact of mutations on activity. A mutant (TILLING) resource developed in Pisum sativum L. (pea) and a large germplasm collection representing Pisum diversity were investigated as sources of mutations that reduce or abolish the activity of the major protease inhibitor (Bowman-Birk) class of seed protein. Of three missense mutations, predicted to affect activity of the mature trypsin / chymotrypsin inhibitor TI1 protein, a C77Y substitution in the mature mutant inhibitor abolished inhibitor activity, consistent with an absolute requirement for the disulphide bond C77-C92 for function in the native inhibitor. Two further classes of mutation (S85F, E109K) resulted in less dramatic changes to isoform or overall inhibitory activity. The alternative strategy to reduce anti-nutrients, by targeted screening of Pisum germplasm, successfully identified a single accession (Pisum elatius) as a double null mutant for the two closely linked genes encoding the TI1 and TI2 seed protease inhibitors. The P. elatius mutant has extremely low seed protease inhibitory activity and introgression of the mutation into cultivated germplasm has been achieved. The study provides new insights into structure-function relationships for protease inhibitors which impact on pea seed quality. The induced and natural germplasm variants identified provide immediate potential for either halving or abolishing the corresponding inhibitory activity, along with associated molecular markers for breeding programmes. The potential for making large changes to plant protein profiles for improved and sustainable food production through diversity is illustrated. The strategy employed here to reduce anti-nutritional proteins in seeds may be extended to allergens and other seed proteins with negative nutritional effects. Additionally, the novel variants described for pea will assist future studies of the biological role and health-related properties of so-called anti-nutrients.

A salt-bridge stabilized C-terminal hook is critical for the dimerization of a Bowman Birk inhibitor

Legume Bowman-Birk inhibitors (BBIs) that inhibit mammalian proteases exist as dimers in solution. The structural basis governing dimerization of HGI-III (horsegram seed BBI) was investigated. An intra-monomer salt bridge (D76-K71) stabilizes an atypical hook-like conformation at the C-terminus. We postulate that this hook, positions D75 to enable an inter-monomer salt-bridge D75(a)-K24(b), which results in dimerization. We verify this by K71A and D76A mutations of HGI-III. The mutants were both monomers, likely due to destabilization of the C-terminal hook. Dimerization was sustained in a double mutant K71D/D76K that was anticipated to form a similar hook critical for dimerization. Conversely, K24(b) that interacts with D75(a) of the loop is the specificity determining residue that interacts with trypsin to inhibit its activity. The inter-monomer salt bridge D75(a)-K24(b) must be disrupted for the inhibition of trypsin, requiring HGI-III to transition into a monomer. Size exclusion studies and a model of HGI-III-trypsin complex support this notion. Interestingly, isoforms of the inhibitor present in germinated seeds (HGGIs) are monomers; and most strikingly, the C-termini of these inhibitors are truncated with the loss the C-terminal hook critical for dimerization. The tendency of HGI-III to self-associate seems to relate to its physiological function of a storage protein.

Bowman-Birk inhibitors from legumes as colorectal chemopreventive agents

Aberrant functioning of serine proteases in inflammatory and carcinogenic processes within the gastrointestinal tract (GIT) has prompted scientists to investigate the potential of serine protease inhibitors, both natural and synthetic, as modulators of their proteolytic activities. Protease inhibitors of the Bowman-Birk type, a major protease inhibitor family in legume seeds, which inhibit potently and specifically trypsin- and chymotrypsin-like proteases, are currently being investigated as colorectal chemopreventive agents. Physiologically relevant amounts of Bowman-Birk inhibitors (BBI) can reach the large intestine in active form due to their extraordinary resistance to extreme conditions within the GIT. Studies in animal models have proven that dietary BBI from several legume sources, including soybean, pea, lentil and chickpea, can prevent or suppress carcinogenic and inflammatory processes within the GIT. Although the therapeutic targets and the action mechanism of BBI have not yet been elucidated, the emerging evidence suggests that BBI exert their preventive properties via protease inhibition; in this sense, serine proteases should be considered as primary targets in early stages of carcinogenesis. The validation of candidate serine proteases as therapeutic targets together with the identification, within the wide array of natural BBI variants, of the most potent and specific protease inhibitors, are necessary to better understand the potential of this protein family as colorectal chemopreventive agents.

Horse gram- an underutilized nutraceutical pulse crop: a review

Horse gram is an underutilized pulse crop grown in wide range of adverse climatic conditions. It occupies an important place in human nutrition and has rich source of protein, minerals, and vitamins. Besides nutritional importance, it has been linked to reduced risk of various diseases due to presence of non-nutritive bioactive substances. These bioactive substances such as phytic acid, phenolic acid, fiber, enzymatic/proteinase inhibitors have significant metabolic and/or physiological effects. The importance of horse gram was well recognized by the folk/alternative/traditional medicine as a potential therapeutic agent to treat kidney stones, urinary diseases, piles, common cold, throat infection, fever etc. The inception of nutraceutical concept and increasing health consciousness the demand of nutraceutical and functional food is increased. In recent years, isolation and utilization of potential antioxidants from legumes including horse gram are increased as it decreases the risk of intestinal diseases, diabetes, coronary heart disease, prevention of dental caries etc. Keeping in view the increasing demand of food having nutraceutical values, the present review ascribed with recent scientific knowledge towards the possibilities of exploring the horse gram, as a source of food and nutraceuticals compounds.

The contribution of two disulfide bonds in the trypsin binding domain of horsegram (Dolichos biflorus) Bowman-Birk inhibitor to thermal stability and functionality

The major Bowman-Birk inhibitor (BBIs) of horsegram (Dolichos biflorus) HGI-III, contains seven interweaving disulfides and is extremely stable to high temperatures. The contributions of two disulfide bonds in the trypsin domain to thermal stability and functionality were evaluated using disulfide deletion variants of wild type protein. Thermal denaturation kinetics, differential scanning calorimetry and urea denaturation studies indicate that the absence of either of the two disulfides destabilizes the protein significantly. C20-C66 contributes substantially to both thermal stability and controls trypsin and chymotrypsin inhibitor activity. These two disulfides act in synergy as deletion of both disulfides leads to a complete loss of thermal stability. The data indicate that the two subdomains are not entirely independent of each other. Long range interactions, between the domains are facilitated by C20-C66. The deletion of the disulfide bonds also increased proteolytic susceptibility in a manner similar to the decreased thermal stability. From this study of rHGI a prototype of legume BBIs in can be concluded that among the array of seven evolutionarily conserved disulfide bonds, the disulfide C20-C66 that connects a residue in the trypsin domain with a residue at the border of the same domain plays a dominant role in maintaining functional and structural stability.

Mobilization of seed protein reserves

The mobilization of seed storage proteins upon seed imbibition and germination is a crucial process in the establishment of the seedling. Storage proteins fold compactly, presenting only a few vulnerable regions for initial proteolytic digestion. Evolutionarily related storage proteins have similar three-dimensional structure, and thus tend to be initially cleaved at similar sites. The initial cleavage makes possible subsequent rapid and extensive breakdown catalyzed by endo- and exopeptidases. The proteolytic enzymes that degrade the storage proteins during mobilization identified so far are mostly cysteine proteases, but also include serine, aspartic and metalloproteases. Plants often ensure early initiation of storage protein mobilization by depositing active proteases during seed maturation, in the very compartments where storage proteins are sequestered. Various means are used in such cases to prevent proteolytic attack until after imbibition of the seed with water. This constraint, however, is not always enforced as the dry seeds of some plant species contain proteolytic intermediates as a result of limited proteolysis of some storage proteins. Besides addressing fundamental questions in plant protein metabolism, studies of the mobilization of storage proteins will point out proteolytic events to avoid in large-scale production of cloned products in seeds. Conversely, proteolytic enzymes may be applied toward reduction of food allergens, many of which are seed storage proteins.