Legumins

Lupins, a good source of storage proteins - legumins
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One of the main causes of food allergy is due to the presence of proteins called legumins. These are the major seed storage proteins of oilseeds. Their presence is enough to cause severe allergic reaction in sensitive individuals. They are the subject of considerable research because food allergies are severe enough in some individuals to produce anaphylactic shock.

Legumins  can also be sourced from treenuts and non-treenuts. They all appear to elicit different levels of allergic reactions but it is not clear why. Considerable effort is being expended on determining the structural similarities and differences between these storage proteins.

One feature that is common to all legumins is that they are  equivalent in weight to the 11S  protein which is found in soybean. This means they sediment at a rate based on sucrose density gradient centrifugation equivalent to an 11S protein from soya. In reality the mass varies between 11S and 12S.

The storage proteins have been isolated from broad beans, ground nut, lupin, pea and soya to try and understand where legumin fit in the overall pattern of allergic reactions. The related proteins such as vicilin are 7S proteins which are also found in soybeans.

Lupin legumins make up a third of the total storage proteins in the seed. In pea, the 20 to 30 per cent protein is made up of legumin and vicilin. All thαe legumins are hexamers. They are usually arranged as six subunits aligned as two trimers. each subunit consists of a pair of disulphide-bonded polypeptides, α and β. The alpha polypeptides are the larger of the two being about 40 kDa with an acidic isolectric point. the beta polypeptides are smaller being 20 kDa and have an alkaline pI (isoelectric point). They are synthesized from a large precursor protein of 60 kDa to produce the 40 kDa and 20 kDa peptides 

At the SHIFT 2020  on-line symposium, Gupta et al., (2021) examined the legumins from different sources. They found that all of them had different polypeptide profiles, shapes, sizes and tertiary structures. This makes interpreting their different properties when it comes to understanding allergenic response as well as product development opportunities rather a challenge. All of them however had a preponderance of Glx amino acids. In terms of secondary and tertiary protein structures, all legumins have alpha-helices, beta-sheets, beta -turns and random coils in surprisingly similar quantities.

The alpha-helix in most legumins amounts to between 11.0 and 12.4 per cent of the total secondary structure. Only cashew strangely is 6 per cent. the majority of structure is devoted to beta-sheets accounting for between 34  and 44 per cent.

Genes

There are four major legumin genes. these all code for similar polypeptides. There are five to six minor genes that make rather different proteins. These represent the A and B subfamilies which are found in bean and soybeans.

Product Development With Legumins

One of the wide-spread uses for legumins assuming allergy is not an issue is in the formation of hydrogels. Given that thermal gelation of soybean globulins is extensive, it is not surprising to find similar research interest into other leguminous seed proteins. All these proteins have notable advantages when it comes to forming thermally sensitive gels.

References

Gupta, S., Liu, C., Sathe, S.K. (2021) Compositional, Hydrodynamic, and Spectral properties of Select Oilseed legumin Proteins. IFT Poster session 2021

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