Plant Based Proteins

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Introduction

Plant-based burgers have gained a considerable amount of attention in the past couple of years. As big fast-food chains start to incorporate plant-based meat in their menu, we thought it would be a good idea to explore the science behind the ImpossibleTM Burger and address the genetic engineering aspects of the process.

Lab-Grown vs Plant-Based

There is a number of alternatives to the traditional meat derived from livestock. Lab-Grown meat refers to the meat that is grown as muscle cells in the lab. While the cell cultures did originate from actual animals, a large portion of the tissue has developed and matured under controlled laboratory environment, instead of a farm. Plant-based meat is entirely made of proteins harvested from plants, which are combined in a specific way to mimic the taste, smells and texture of actual meat.

The Secret Ingredient

Obviously, plants don’t usually taste like meat, and food scientists need to figure out the combination of proteins that make up the taste of meat in order to replicate it. According to Impossible Foods, the compound that gives ground beef the red colour and slight metallic taste is Heme [1]. Heme is commonly known to be a component of hemoglobin, the protein that carries oxygen in red blood cells. The iron molecule in heme binds oxygen, resulting in its signature red-rusty colour. The problem is, plants do not normally have hemoglobin, so Impossible Foods had to find a more creative way around the problem.

Figure 1. Hemoglobin in Red Blood Cell [2].

Leghemoglobin


Deep in the root of some legumes exist a symbiotic relationship between the plant and soil bacteria Rhizobium. As the host plant provides organic food sources, it also forms protective root nodules to house these nitrogen-fixing bacteroids. Rhizobium is able to convert atmospheric nitrogen (N2) into ammonia for the host, but its enzymes require a low oxygen environment to remain active. Therefore, leghemoglobin, leguminous hemoglobin, acts as a scavenger and carrier of oxygen to control the concentration of oxygen within the nodules [3].


Figure 2. Root nodules containing leghemoglobin [3,6]. When a nodule is cut open, the red colour can be seen due to the presence of leghemoglobin.

Just like human blood, leghemoglobin can give off a red colour due to the iron in its heme group binding oxygen (Fig 2). Leghemoglobin is both structurally and functionally similar to mammalian hemoglobin, as well as myoglobin, a form of hemoglobin that functions in muscles. They all contain the oxygen-binding heme group and alpha helices 3-D structure as seen in the figure below. Myoglobins exist in mammalian meat that is commonly consumed, so one would assume that leghemoglobin, a protein that is similar in structure and function, could be a part of our diet as well. So Impossible Foods decided to add soy leghemoglobin into their formula of plant-based foods to create that meat-like colour and flavour.


Figure 3. Comparison between hemoglobin, myoglobin, and leghemoglobin [5].

GM-yeast


While all of these sounded like a genius idea, it is in fact not that easy to execute. Leghemoglobin is only present in certain legumes and is usually present in a very small amount. In other words, to support mass production of plant-based burgers, Impossible Foods would need a LOT of soy, thus a lot of land, resources, and time for them to grow. That is really not ideal, so they have to resort to engineering another organism to produce leghemoglobin.

While all of these sounded like a genius idea, it is in fact not that easy to execute. Leghemoglobin is only present in certain legumes and is usually present in a very small amount. In other words, to support mass production of plant-based burgers, Impossible Foods would need a LOT of soy, thus a lot of land, resources, and time for them to grow. That is really not ideal, so they have to resort to engineering another organism to produce leghemoglobin.

Another good candidate for genetic modification would be yeast. They are unicellular eukaryotes, thus possess the ability to produce more complex proteins. They are also very affordable and easy to culture just like bacteria. For this purpose, Impossible Foods employed Pichia pastoris [7], a fairly commonly used host system for recombinant protein expression. There are multiple ways to incorporate a target gene, which is the LegHb for soy leghemoglobin in this case, into yeast cells. The diagram below depicts various methods of inserting a gene into yeast genome (Fig 4). There is an enormous amount of details in each procedure, thus Figure 4 is shown merely shown to provide an idea of the differences between genetically engineering bacteria and yeast.


Figure 4. Different methods to incorporate a desired gene into yeast genome [8].

GM-yeast has enabled the production of plant-based meat, which has the potential to significantly decrease land occupation and water consumption and might be a potential solution to relieve the environmental impacts caused by agricultural activities.


References

[1] http://ulogs.org/@scienceangel/lab-diaries-7-creating-your-own-super-cool-strikekiller-mutant-strike-safe-to-use-virus-from-scratch-part-i. [Accessed: 05-Jun-2019].
[2] J. W. Sahl et al., “Insights into enterotoxigenic Escherichia coli diversity in Bangladesh utilizing genomic epidemiology,” Scientific Reports, vol. 7, no. 1, p. 3402, Jun. 2017.
[3] http://carlottachlaurenzi.blogspot.com/2018/08/charge-gateway-for-worldwide-customers.html. [Accessed: 05-Jun-2019].
[4] K. Morgan, “Plasmids 101: Origin of Replication.” [Online]. Available: https://blog.addgene.org/plasmid-101-origin-of-replication. [Accessed: 05-Jun-2019].
[5] https://www.biologicscorp.com/fab-fragment-antibody/. [Accessed: 05-Jun-2019].
[6] http://biomaster12.weebly.com/1/archives/02-2017. [Accessed: 05-Jun-2019].