17: Turning Trees into Plexiglass - Synthetic Biology For Production of Green Foods and Products
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(Learning goals written by Claude, Anthropic)
By the end of this chapter, students should be able to:
Food Production, Climate, and Alternative Proteins
- Quantify the climate impact of food production (~25% of global GHG emissions, ~35% from agriculture), explain why cattle require the most land per 1,000 kcal of any food source, and connect dietary choices to climate outcomes.
- Explain why soy leghemoglobin gives plant-based burgers a meat-like taste, describe its high O₂ affinity (kon ≈ 2×10⁸ s⁻¹, koff ≈ 20 s⁻¹) and structural similarity to myoglobin, and distinguish the C4 (succinyl-CoA → ALA, in fungi and animals) from the C5 (α-ketoglutarate → ALA, in bacteria and plants) heme biosynthesis pathways, identifying ALA as the critical shared precursor and describing the synthetic biology strategies (gene upregulation, downregulation, knockout) used to maximize heme production in Pichia pastoris under the AOX1 promoter.
- Describe at least four alternative protein sources under development through synthetic biology or sustainable agriculture (β-lactoglobulin from Trichoderma reesei, casein/egg-white proteins from Pichia pastoris, fungal fermentation using Fusarium venenatum, duckweed), and explain why duckweed is particularly promising — containing up to 50% protein (primarily RuBisCO), all essential amino acids, aquaculture compatibility, and no farmland requirement.
Biosourced Industrial Materials
- Explain the environmental significance of replacing fossil-fuel-based acrylic acid production (~4,000 kg CO₂/metric ton) with biological feedstocks, describe the free radical polymerization mechanism by which MMA and MBL form PMMA and PMBL, respectively, and outline the six key steps Arzeda used to engineer a fermentation pathway from lignocellulosic hydrolysate to MBL using Scylax™ (computational pathway design) and Archytas™ (enzyme engineering).
- Compare the physical properties of biosourced PMBL to fossil-fuel-derived PMBL and PMMA — including superior thermal stability (Tg = 195°C vs. 105°C for PMMA), tensile strength, elasticity, and solvent resistance — and explain why achieving ~5 g/L (approaching the ~20 g/L commercial threshold) represents a significant milestone for sustainable materials.
Introduction
Manufacturing is energy-intensive and environmentally damaging, contributing to climate change by releasing CO2 and other pollutants. A manufactured item has a lifetime after which it must be disposed of, often in a manner that involves little recycling. A circular economy in which used products are consistently recycled for further use would be highly beneficial for the environment if implemented well.
Synthetic biology seeks to genetically alter and redesign organisms to produce traditional or novel products more sustainably, with lower energy inputs and less polluting output. Although it is a nascent field, well-known products are being produced using it. We will explore several products made through synthetic biology, as well as those in which novel cells are the products.
Products from cells
Burgers by Impossible Foods
Agriculture has transformed the planet. About 50% of all land (excluding deserts and ice sheets, and comprising an area equal to the Americas) is used for agriculture, most of which is for animal production. In 2015, the US accounted for about 8% of the world's total greenhouse gas emissions from food, while China accounted for around 14%. Contributions to food production include agriculture, land-use change, and supply-chain emissions (transport, packaging, food processing, retail, cooking, and waste).
Estimates indicate that wild mammals account for only about 6% of the total mass of all mammals (including people, livestock, and pets) on Earth. In a parallel finding, the mass of "stuff" (plastics, metals, asphalt, concrete, etc) created by humans now exceeds the entire biomass of the planet!
Most of the agricultural land is used to produce meat and milk for human consumption. Collectively, cattle by far require the most land use, as shown in the interactive graph of Our World In Data in Figure \(\PageIndex{1}\) below, which shows how many square meters are required to produce 1000 kcals (1000 cal in the dietary sense) of food from each food type listed. The graph is similar when the measure is land use per 100 grams of protein produced.
Figure \(\PageIndex{1}\): Land use of foods/1000 kcals. https://ourworldindata.org/land-use-diets
The number of animals slaughtered daily worldwide is unbelievably high, as shown in Table \(\PageIndex{1}\).
| cows | goats | sheep | pigs | ducks | chickens | fish |
| 900,000 | 1.4 million | 1.7 million | 3.8 million | 11.8 million | 202 million | 100's of millions |
Table \(\PageIndex{1}\): Animals slaughtered daily for food. Max Roser (2023) - “How many animals get slaughtered every day?” Published online at OurWorldInData.org. Retrieved from: 'https://ourworldindata.org/how-many-...ered-every-day' [Online Resource]
The global percentage of all GHG emissions from the agricultural sector is about 35%, as illustrated in Figure \(\PageIndex{2}\) below. Averages for some specific countries are also shown.
Figure \(\PageIndex{2}\): https://ourworldindata.org/grapher/f...SA~IND~RUS~POL
The percentage of greenhouse gas emissions from all aspects of food products through the entire food chain is about 25%. Figure \(\PageIndex{3}\) below shows the best estimate of global greenhouse gas per sector in 2016. The number today would probably differ little from these.
Figure \(\PageIndex{3}\): Breakdown of global greenhouse gas emissions in 2016. Hannah Ritchie (2020) - “Sector by sector: where do global greenhouse gas emissions come from?” Published online at OurWorldInData.org. Retrieved from: 'https://ourworldindata.org/ghg-emissions-by-sector' [Online Resource].
An expanded analysis of the approximate 25% of global emissions that arise from food is shown in Figure \(\PageIndex{4}\) below.
Figure \(\PageIndex{4}\): Global greenhouse gas emissions from food production. Hannah Ritchie (2019) - “Food production is responsible for one-quarter of the world’s greenhouse gas emissions,” Published online at OurWorldInData.org. Retrieved from: 'https://ourworldindata.org/food-ghg-emissions' [Online Resource]
Making plant-based foods that taste more like meat, if people would eat them, could have a large effect on greenhouse gas emissions and climate change. One example is Impossible Burgers and similar products from Impossible Foods. They use soy leghemoglobin, a monomeric heme-binding protein found in legume root nodules, to give meat a blood-like appearance and taste. As a single-chain heme-binding protein, it has a high affinity for O2, similar to animal myoglobin. The high affinity derives from very high on-rates for binding O2 (almost diffusion-controlled at around 2x108 s-1, and an off rate of around 20 s-1. This high affinity keeps O2 bound, preventing it from inhibiting nitrogenase and nitrogen fixation by root-associated microbes. Heme is important for the positive taste of red meat. Plant-based burgers containing leghemoglobin require much less land and lead to far lower greenhouse gas emissions.
Figure \(\PageIndex{5}\) shows an interactive iCn3D model of the alignment of sperm whale myoglobin (1MBO) and soy leghemoglobin (1BIN).
Figure \(\PageIndex{5}\): Alignment of sperm whale myoglobin (1MBO, cyan) and soy leghemoglobin (1BIN, magenta). (Copyright; author via source). Click the image for a popup or use this external link: https://structure.ncbi.nlm.nih.gov/i...4pUi1S6qFtGn4A
The leghemoglobin in the Impossible burgers is produced in yeast. Large amounts of heme are required to produce leghemoglobin in yeast; this can also be achieved in engineered cells by introducing the appropriate genes. The C4 pathway (humans, animals, fungi, and purple non-sulfur phototrophic bacteria top) and C5 pathway (archaea, plants, and other bacteria) for heme synthesis are shown in Figure \(\PageIndex{6}\) below (heme synthesis described in more detail in Chapter 22.3). The succinyl-CoA is derived from the citric acid cycle.
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Figure \(\PageIndex{6}\): Top - C4 heme synthesis pathway (humans, animals, fungi, and purple non-sulfur phototrophic bacteria top). Bottom-heme synthesis C5 pathway (archaea, plants, and other bacteria). Heme biosynthetic pathway. Wikimedia Commons: Heme-Synthesis-Chemical-Details-Mirror (top) and Heme pathway in E. coli. Zhang, J., Kang, Z., Chen, J. et al. Optimization of the heme biosynthesis pathway for producing 5-aminolevulinic acid in Escherichia coli. Sci Rep 5, 8584 (2015). https://doi.org/10.1038/srep08584. Creative Commons Attribution 4.0 International License. http://creativecommons.org/licenses/by/4.0/ (bottom).
Succinyl-CoA (C4 pathway) and α-ketoglutarate (C5 pathway) are derived from the citric acid cycle. The key precursor, 5-aminolevulinate (ALA), needs to be elevated by engineering the C4 or C5 pathways, with the C5 pathway generally producing more ALA in engineered E. coli.
Leghemoglobin from soy (species name Glycine max) can also be synthesized in the methylotrophic (uses methanol as a sole carbon source) yeast Pichia pastoris, which is often used for recombinant protein expression. Three groups of enzymes are needed.
- group 1: porphobilinogen synthase (PBGS)
- group 2: uroporphyrinogen III synthase (UROS), uroporphyrinogen III decarboxylase (UROD), coproporphyrinogen III oxidase (CPO)
- group 3: Ala synthase (ALAS), protoporphyrinogen oxidase (PPO), and ferrochelatase (FECH)
Transcription of these genes in P. pastoris can be controlled by using the methanol-induced alcohol oxidase (AOX1) promoter, which is often used to achieve high expression of recombinant proteins.
Figure \(\PageIndex{7}\) below shows a more detailed representation of the heme synthesis pathway.
Figure \(\PageIndex{7}\): The biosynthetic pathway of heme. Su, H.; Chen, X.; Chen, S.; Guo, M.; Liu, H. Applications of the Whole-Cell System in the Efficient Biosynthesis of Heme. Int. J. Mol. Sci. 2023, 24, 8384. https://doi.org/10.3390/ijms24098384. Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/)
The two biosynthetic pathways of 5-aminolevulinic acid (in bold to indicate their importance) are shown in green (C4 pathway) and red (C5 pathway). The three downstream synthetic pathways of heme are marked with blue (CPD), indigo (SHD), and purple (PPD). Solid lines indicate single reactions, and dashed lines indicate more than two. The names of genes encoding the individual enzymes are in italics, and some reactions have alternative genes. The abbreviations of the corresponding enzymes are shown in the grey rectangle.
Figure \(\PageIndex{8}\) below complements this figure and shows the synthetic biology strategies to enhance heme production.
Figure \(\PageIndex{8}\): Synthetic biology strategies to enhance heme production. Green, orange, and red color blocks indicate genes that need to be up-regulated, down-regulated, and knocked out, respectively. See Table 1 for a list of names and abbreviations for heme synthesis enzymes. Su, H. et al., ibid.
Computational tools such as AI can help design new pathways and novel enzymes to enhance production. It is becoming easier to transfer large pathways into yeast.
Sales of alternative meat products have recently decreased. This decrease probably stems from more vocal opposition from the meat industry and people's reticence to eat what many consider ultra-processed foods. High-protein, nutritious burgers can be made very simply using beans and grains.
Other food products from microbes and sustainable plants
Significant effort is being devoted to growing meat in the lab using cell culture. This nascent field faces many challenges, including consumer resistance to eating lab-grown meat. At present, meat grown in tissue culture is very expensive. Three key steps in growing meat are finding the best cells, determining the nutrient conditions that maximize their growth, and adjusting conditions to make the lab-grown meat taste like meat.
Muscle stem cells have been used because they can proliferate extensively, but they have growth limits. Alternatively, immortal cells, such as those derived from chicken fibroblasts, could be used. They can also be converted into fat cells. Yet they could accrue mutations with possible but unlikely health consequences. Animal cells grown in culture often use fetal cow serum for its rich composition of growth factors and nutrients. However, it is expensive and has ethical concerns since it's derived from animals. Synthetic growth medium can be used, but it is also expensive. Whether lab-grown meat can overcome high costs and consumer resistance will determine its potential as a substitute.
More simply, people can increase their intake of peas, soy, grains, and nuts in their diet (i.e., being a vegan or vegetarian is the best approach to reducing their carbon footprint). Soy products have an extensive history of use as a source of protein but contain potential allergens (significant in babies who use soy formulas) and isoflavones, which mimic human estrogen derivatives. Pea-based infant formula is increasingly used as a substitute.
Expressed recombinant proteins made in genetically modified bacteria and yeast are also becoming more popular. Examples (other than leghemoglobin) include the production of β-lactoglobulin in the fungus Trichoderma reesei, a cow's whey protein, for dairy and animal-free milk products. The genetically modified yeast Pichia pastoris has also been engineered to make milk casein proteins, egg-white proteins, muscle myoglobin, and human breast milk proteins. Enzymes used to manufacture cheese (derived from calves' stomachs) can be replaced by chymosin made in yeast. Production of many of these proteins is linked to fermentation. Filamentous proteins with a texture similar to chicken fiber can be made through fermentation in the filamentous fungi Fusarium venenatum. Macroalgae, such as seaweed, can provide a high-protein food and have long been used in many cultures. Kelp farming can help not only provide protein but also capture carbon. Finally, long eaten in many cultures, insects could become more climate-friendly protein sources.
If humans are in search of non-animal sources of protein to fight climate change, why not produce and eat the most abundant protein in the biosphere, Rubisco? New products derived from the duckweed plant (genus Lemna) are coming to market. Figure \(\PageIndex{9}\):
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Figure \(\PageIndex{9}\): Duckweed (and a frog). https://commons.wikimedia.org/wiki/F...7678481%29.jpg
Duckweed is nutrient-rich, fast-growing, and a great source of Rubisco. It can be grown in aquaculture and does not require farmland. It contains up to 50% protein. After harvesting, the plants are filtered, milled, and dried using very simple technologies. Proteins, the most abundant of which is Rubisco, are then extracted. Duckweed can be used in baked goods and as a meat and dairy substitute. It is equal to eggs and meat because it supplies all the essential amino acids humans require.
New methods are needed to make environmentally sustainable foods that minimize damage to our climate and biosphere. Over 150 Nobel and World Food Prize Laureates have signed a letter (January 2025) urging political and financial institutions to fund the development of new food technologies to prevent "an estimated 700 million people going hungry today and an additional 1.5 billion people to feed by 2050".
Genetic Manufacturing of Industrial Feedstocks
Let's look at one example of how synthetic biology and computational techniques are used to create products such as plexiglass from biological acrylate sources. Acrylates are esters of acrylic acid (typically made from propylene) synthesized by reacting it with alcohols like methanol. Life cycle analyses show that nearly 4,000 kg of CO2 are emitted per metric ton of acrylic acid produced. Biological feedstocks such as glycerol and 3-hydroxypropanoic acid can be used to reduce the climate impact, but large-scale supplies are needed. Figure \(\PageIndex{10}\) overviews acrylate production from fossil and biological feedstocks.
Figure \(\PageIndex{10}\): Production pathway of acrylates using fossil fuel and renewable resources. Souza, L.R.d.; Whitfield, B.; Al-Tabbaa, A. Biobased Acrylate Shells for Microcapsules Used in Self-Healing of Cementitious Materials. Sustainability 2022, 14, 13556. https://doi.org/10.3390/su142013556. Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/)
When the alcohol is methanol, the final product is methylacrylate (MA). The structure of the cyclic acrylate monomer feedstock used to polymerize plexiglass (Lucite) is methyl methacrylate (MBL), whose structure is shown in Figure \(\PageIndex{11}\) below.
Figure \(\PageIndex{11}\): Structure of methyl methacrylate and its lactone (a cyclic ester)
Methyl methacrylate can undergo a free radical polymerization in the presence of an initiator (In.), as shown below in Figure \(\PageIndex{12}\).
Figure \(\PageIndex{12}\): Mechanism of free radical polymerization of MMA
This reaction can form large polymers, such as plexiglass. The market for acrylic acid, the feedstock for plexiglass, is estimated to reach 12 million metric tons by 2030.
MBL, the lactone of MMA, is made in tulips from pathways that are not completely elucidated. It can also be used as a feedstock for plexiglass polymerization. Figure \(\PageIndex{13}\) shows the polymerization products from MMA and MBL.
Figure \(\PageIndex{13}\): Structure of poly-MMA and poly-MBL
Using synthetic biology and advanced computational methods, plexiglass can be produced from biological sources rather than fossil fuels. To accomplish this, Azerda has designed synthetic pathways from millions of potential metabolic pathways (using a software package called Scylax™) and intelligently redesigned key enzymes to maximize their catalytic potential for MBL synthesis (using the software Archytas™). They used high-throughput DNA and protein analyses to maximize expression. Finally, they engineered expression strains and downstream purification processes to maximize the final output of MBL. In summary, the key steps in the process were:
- identifying a pathway from millions of reactions in databases of pathways that could produce MBL from simple sugars through a fermentation process;
- engineering pathway enzymes to significantly increase catalytic efficiency and decrease inhibition;
- producing test quantities of the products in cell strains;
- scaling up production to levels needed for purification and reactions of the MBL
- purifying sufficient amounts of MBL from large fermentation broths
- making the desired product (plexiglass, for example) from the feedstock.
Strains of bacteria, yeast, and filamentous fungi were modified to meet the criteria outlined above. The ultimate substrate for the process was a lignocellulosic hydrolysate, so in the end, the process converts trees into plexiglass (incredible to think about)! Of course, it is also amazing that CO2 from the air, water, and minerals/ions from the soil can become a tree!
Starting with a detectable product level, the process was continually improved and scaled to yield 5 g/L in broth, which is close to the 20 g/L required for commercial viability. Figure \(\PageIndex{14}\) below shows plexiglass created from the lignocellulosic stock!
Figure \(\PageIndex{14}\): Plexiglass made from biosourced MBL.
Table \(\PageIndex{2}\) below compares the key physical properties of the polymers from Arzeda's PMBL compared to literature values for fossil-fuel-based PMBL and PMMA.
| Property | Measure | Lit PMBL | Arzeda PMBL | PMMA |
| Thermal | Glass transition pt Tg (oC) | 194-195 | 195 | 105 |
| Mechanical | Elasticity (mPa) | 1999/3439 | 5972 | 2855 |
| Tensile strength (mPa) | 36.7/62.7 | 72.7 | 70 | |
| Elongation at break | 1.3%.6.5% | 1.3% | 2.5 | |
| Optical | Light transmission | NA | >88% | 92% |
| Solvent resistance | toluene, 30 days, 20 oC | NA | Pass | Fail |
Table \(\PageIndex{2}\): https://www.energy.gov/sites/default...-korkegian.pdf
Summary
(Summary written by Claude, Anthropic)
Manufacturing and food production together account for a substantial share of global greenhouse gas emissions. This chapter examines how synthetic biology can produce foods and industrial materials from biological rather than fossil fuel sources — contributing to a more sustainable, circular economy.
The climate cost of food. Food production accounts for approximately 25% of global GHG emissions, with the agricultural sector alone contributing ~35%. Animal agriculture is the dominant driver: cattle require the most land of any food source per 1,000 kcal or per 100 g of protein, and over 200 million chickens and nearly 4 million pigs are slaughtered globally every day. Wild mammals now account for only ~6% of all mammalian biomass on Earth, and the mass of human-made materials exceeds the planet's entire biomass. Shifting toward plant-based diets is one of the most impactful individual responses to climate change.
Leghemoglobin and Impossible Burgers. Impossible Foods uses soy leghemoglobin — a monomeric heme-binding protein from legume root nodules, structurally homologous to myoglobin — to give plant-based burgers their meat-like taste, appearance, and aroma. Heme is a key contributor to the flavor of red meat. Leghemoglobin's extremely high O₂ affinity (kon ≈ 2×10⁸ s⁻¹, koff ≈ 20 s⁻¹) normally serves to buffer free O₂ away from nitrogenase in root nodules. For commercial production, the protein is expressed in the methylotrophic yeast Pichia pastoris under the methanol-inducible AOX1 promoter. Sufficient heme must also be produced in the engineered yeast; this requires co-expression of three groups of enzymes spanning both the C4 (succinyl-CoA → ALA, in animals and fungi) and C5 (α-ketoglutarate → ALA, in bacteria and plants) heme biosynthesis pathways, with synthetic biology strategies including targeted gene upregulation, downregulation, and knockout to maximize flux through the pathway. AI-assisted pathway design is increasingly used to optimize this.
Other alternative proteins. A wide range of sustainable protein sources is under development. Recombinant proteins produced by engineered microbes include β-lactoglobulin (cow whey protein) from Trichoderma reesei, casein and egg-white proteins from Pichia pastoris, and chymosin (replacing calf-stomach rennet for cheese making). Filamentous fungi (Fusarium venenatum) produce fibrous proteins with chicken-like texture through fermentation. Lab-grown meat from muscle stem cells or immortalized cell lines (e.g., chicken fibroblasts) remains technically challenging and expensive, with fetal bovine serum dependence, high costs, and consumer resistance as major barriers. More immediately, veganism and vegetarianism — increasing intake of peas, soy, grains, and nuts — remain the most effective individual dietary strategies for reducing carbon footprint. Duckweed (Lemna spp.) is an especially promising emerging source: it grows rapidly in aquaculture without farmland, contains up to 50% protein (primarily RuBisCO), requires minimal processing, and provides all essential amino acids. Macroalgae, insects, and other non-animal protein sources complete the portfolio of lower-impact alternatives.
Biosourced industrial materials: plexiglass from trees. Beyond food, synthetic biology can replace fossil fuels in manufacturing. Acrylic acid — the feedstock for plexiglass (PMMA) — currently generates nearly 4,000 kg CO₂ per metric ton produced from propylene. Arzeda used computational tools (Scylax™ for pathway design from millions of metabolic reactions; Archytas™ for enzyme engineering) to identify and optimize a fermentation pathway from lignocellulosic hydrolysate (wood) to methylene-butyrolactone (MBL), a cyclic lactone that polymerizes into a plexiglass-like material (PMBL). Key steps included computational pathway selection, enzyme engineering to increase catalytic efficiency and reduce product inhibition, strain construction in bacteria, yeast, and filamentous fungi, and scale-up from initial detection to 5 g/L in fermentation broth — approaching the ~20 g/L threshold for commercial viability. The resulting biosourced PMBL matches or exceeds fossil-fuel-derived PMBL and PMMA in thermal stability (Tg = 195°C vs. 105°C for PMMA), tensile strength, elasticity, and optical clarity, and outperforms PMMA in solvent resistance. In essence, the process converts atmospheric CO₂ (fixed by trees) into a durable plastic — demonstrating the potential for synthetic biology to close the carbon loop in industrial materials production.