Unlocking the Flavor Secrets of Soybeans: World Vegan Day Celebration

A diptych artwork displaying a protein structure of lipoxygenase with the image of a soybean pod, displayed in two contrasting colour schemes

Wed, 11/01/2023 - 12:00

On this World Vegan Day, we’re celebrating soybeans. Soybean is a fantastic superfood emerging as an excellent protein alternative. Let's explore what makes soybean so extraordinary and uncover the secret behind the distinctive "beany" flavour.

Superfood Soybeans

All plants contain all nine essential amino acids - crucial building blocks that our bodies cannot produce on their own and must be obtained from external sources. What makes soybean remarkable is that it contains all nine amino acids and has around 40 % pure protein - the highest amongst any plant-based diet. Packed with fibre, healthy polyunsaturated fats (PUFAs), and an array of vitamins and minerals, including calcium, iron, potassium, folate, and magnesium, soybeans are a complete package of nutrition. Soya foods have been shown to lower cholesterol, preventing conditions that can lead to cardiovascular diseases, stroke, and coronary heart disease. Soybean and soya are also loaded with antioxidants, which give the body a shield against several diseases, including deadly cancers.

The secret behind the beany flavour

Ever wondered what gives soy-based dishes that "beany" flavour? The answer lies in the unsaturated fatty acids abundant in soybeans, such as linoleic and linolenic acids. Under the influence of an enzyme called lipoxygenase (LOX), these fatty acids undergo oxidation, resulting in the formation of hydroperoxides. These compounds are the primary contributors to the unique sensory experience of consuming soy-based products.

LOX: A versatile enzyme

Lipoxygenases have a wide substrate specificity, meaning they can act on different types of fatty acids. This versatility allows them to participate in a variety of metabolic pathways. In plants, LOXs are involved in many physiological functions including seed germination, plant growth and development, fruit ripening and senescence. LOXs are also involved in the response mechanisms to diverse environmental stresses. In humans and animals, they play a significant role in inflammation and immune response.

LOX structural features and substrate specificity

In plants, these enzymes primarily target linoleic and linolenic acids (LA), while in animals, the main focus is on arachidonic acid (AA). Now, let's look into the structural characteristics that facilitate such distinct substrate specificity.

Lipoxygenase is a non-heme iron-containing protein consisting of a small N-terminal PLAT domain and a major C-terminal catalytic domain, which contains the active site, the region of enzyme where the substrate binds and undergoes a chemical reaction. The N-terminal domain contains an eight-stranded antiparallel β-barrel, while the C-terminal catalytic domain has an arched helix, which covers the substrate-binding channel, acting as a protective shield. The helix 2 runs parallel to the arched helix, serving as a lining for the substrate cavity. Within helix 2, there is a stretch of six amino acids (residues 261-267) arranged in a π-helix. π-helix is a rare structural element in proteins, formed by the insertion of an additional amino acid into a standard α-helix.

Image displaying protein structure with various regions highlighted. — *Figure 1: Structure of Soybean LOX-1 (UniProt accession P08170, PDB 3PZW) showing the two domains along with the arched helix, π-helix and helix 2 in the catalytic domain.*

The catalytic activity of LOX is related to iron (Fe) ion. The active state of LOX is oxidized Fe3+, and the inactive state is reduced Fe2+. In soybean LOX-1, the iron ion coordinates with five amino acids- His499, His504, His690, Ile839, Asn694 and a water molecule.

Image showing two panels from a website with the left panel containing a network of interacting nodes and the right hand panel displaying protein structure visualisation — Figure 2: The active site of Soybean LOX-1 (UniProt accession P08170, PDB 3PZW) in 2D (left) and 3D view (right): The bound iron ion interacts with HIS499, HIS504, HIS690, ILE839, ASN694 and HOH870. (Source: https://www.ebi.ac.uk/pdbe/entry/pdb/3pzw/bound/FE2)

Both plant and mammalian enzymes have the N-terminal domain, but in the soybean lipoxygenases, this domain is significantly larger and has additional surface loops. The π-helix, embedded in helix 2, is only found in plant LOXs. Moreover, the hydrophobic substrate binding channel exhibits variations between plant and animal LOXs. In plant enzymes, it's less curved, possibly to accommodate the more saturated linoleic acid (LA), while in animal LOXs, it takes on a U-shaped configuration to suit arachidonic acid (AA).

Image showing two superposed protein structures with additional chemicals also displayed at the bottom of the image. — Figure 3: 3D Superposed view for comparison of Soybean LOX-1 (UniProt accession P08170, PDB 3PZW) and Caribbean Sea Whip LOX (UniProt accession O16025, PDB 4QWT): The grey structure represents Caribbean Sea Whip LOX with a distinct U-shaped channel, specialized for accommodating arachidonic acid. The Soybean LOX has a less curved channel but a significantly larger N-terminal domain shown in orange. Additionally, both plant and animal substrates, including linoleic acid (LA) and arachidonic acid (AA), are indicated for reference.

Unmasking the beany flavour

In industrial processes, heat treatment has been employed to deactivate these lipoxygenase enzymes. This intervention aims to enhance the nutritional and sensory attributes of soybeans, ultimately leading to more appealing end-products. By inhibiting the activity of lipoxygenases, the formation of hydroperoxides, which are associated with bitter and beany flavours, is minimized.

Schematic image depicting the steps required in the process of soybean flavour adaptation. — Figure 4: Use of microorganisms to decrease the “beany” off-flavour for Soyabean (Source: Potential of Microorganisms to Decrease the “Beany” Off-Flavor: A Review, Estelle Fischer, Nathalie Cayot, and Rémy Cachon. Journal of Agricultural and Food Chemistry 2022 70 (15), 4493-4508. DOI: 10.1021/acs.jafc.1c07505e)

Furthermore, genetic modification techniques have been leveraged to eliminate specific lipoxygenase variants. This strategic intervention leads to an improvement in the overall taste profile of soybean products. With fewer hydroperoxides in production, the undesirable flavours are reduced, giving rise to a more pleasant, 'light', and even slightly sweet taste in soybeans. This transformation makes soybeans not only nutritionally rich but also inherently more palatable.

Preeti Choudhary

About the artwork

Thanatpohn Siriwatdeachakul (Year 12) from the Stephen Perse Foundation School drew his inspiration from the intricate spiral seed pod of the soybean plant for creating this artwork. He was fascinated by the natural perfection of its spirals and curls, which prompted him to choose Lipoxygenase-1 protein as the focal point of his artwork.

Using a blend of traditional and digital techniques, Thanatpohn created a monoprint with a digital overlay using the software Procreate. The artwork masterfully incorporates a realistic depiction of a spiral soybean seed pod, seamlessly integrated with the 3D structure of Lipoxygenase-1. The colour palette is harmoniously shared between the seed pod and the protein structure, creating a cohesive and visually striking composition. Additionally, Thanatpohn skillfully produced a contrasting version of the painting, adding depth and dimension to his artistic expression.

View the artwork in the virtual 2022 PDB Art exhibition.

You can also check out the new 2023 PDB art exhibition here bit.ly/PDBart2023