Problem 7 - Bioseparations Science and Engineering

Question

Therapeutic Proteins from Transgenic Tobacco Transgenic plants (e.g., corn, tobacco, etc.) have the potential to produce complex bioactive proteins at significantly lower cost than production via transgenic animals or mammalian cell cultures. The advantages of transgenic plant production are the following: easy and efficient introduction of stable foreign genes, cost- effective biomass production \((\$ 0.04-\$ 0.1 / \mathrm{kg})\), no possible contamination with human disease agents, and the ability to perform the complex protein processing needed for many bioactive human therapeutics. Cost- effective biomass production makes this mode of production suitable for high- volume recombinant proteins. The ability to perform complex protein processing is advantageous for production of therapeutic glycoproteins and bioactive peptides. Downstream processing costs are a major portion of the total unit production cost associated with transgenic plant production of high-volume therapeutic proteins. Therefore, primary recovery requires significant volume reduction. Once volume reduction and biomass removal have been achieved, chromatographic purification is required to remove plant protein impurities. Design a purification process for use in a facility manufacturing 100 metric tons per year of recombinant human serum albumin protein from transgenic tobacco. Assume that the expression level is \(0.5 \mathrm{~g}\) of product protein per kilogram of tobacco. Assume that biomass production and primary recovery are performed at a separate site. The feed to the purification section is 4000 liters/day (containing \(200 \mathrm{~g} /\) liter of product protein), and this material is purchased for \(\$ 0.7 / \mathrm{g}\) of product protein. The process should include the appropriate filtration and chromatographic steps. Assume that a combination of affinity and ion exchange chromatography will provide a product that is more than \(90 \%\) pure.

Step-by-Step Solution

Answer

The first step is to calculate the annual requirement of protein which is 100 million grams. Then, recognizing that the daily procurement of protein is 800,000 grams, it is apparent that less than 50% of the procured protein is used efficiently. The purification process involves first removing any remaining solid or particulate impurities from the feed material by filtration. Following this, chromatographic purification through affinity chromatography and ion-exchange chromatography yields a final product that is over 90% pure.

Step 1: Calculate the total protein requirement

Firstly, make clear to yourself the required amount of protein to meet the production goal of 100 metric tons per year. Since 1 metric ton equals 1000 kg, 100 metric tons would be equal to \(100,000 \, \mathrm{kg}\) or \(100,000,000 \, \mathrm{g}\) of protein per year.

Step 2: Calculate the daily protein requirement

Knowing that each day's feed contains \(200 \, \mathrm{g/L}\), and a total of 4000 L is processed daily, we can calculate the total amount of protein procured each day by multiplying the volume by the concentration. This would give \(200 \, \mathrm{g/L} \times 4000 \, \mathrm{L} = 800,000 \, \mathrm{g}\) daily. To clarify the annual requirement, multiply this by the number of days in a year (365), which should equal \(800,000 \, \mathrm{g/day} \times 365 \, \mathrm{days/year} = 292,000,000 \, \mathrm{g/year}\). However, this is significantly more than the requirement of 100 million grams per year, indicating a process efficiency of less than 50%.

Step 3: Design the Purification Process

Having determined the requisite quantities, the purification process can be planned accordingly. The biomass is already reduced before this stage, so the focus here is on filtration to remove remaining impurities and then chromatographic purification. First, the feed is passed through a filtration system to remove solid impurities or other particles. The filtrate is then subjected to chromatographic purification. This involves two major steps: affinity chromatography, which will separate the desired protein based on its specific affinity for certain molecules, followed by ion-exchange chromatography, which separates molecules based on their charge. These combined steps should yield a product with at least 90% purity.

Key Concepts

Therapeutic Protein Purification

The process of isolating a specific protein from complex mixtures while maintaining its biological activity is known as therapeutic protein purification. This is extremely important in the production of substances like recombinant human serum albumin, which are used for medical purposes. To purify proteins from transgenic tobacco, as in our example, a series of steps are required.

These include homogenization to break open cells, centrifugation to remove cell debris, and then a series of filtration and chromatographic methods. Since the end product must be of the highest purity for therapeutic use, techniques like affinity chromatography, which binds specifically to the protein of interest, and ion-exchange chromatography, based on charges of molecules, are employed. An optimized purification process ensures that the therapeutic proteins are not only pure but also active, achieving the efficacy needed for medicinal use.

Biomass Production Cost

Biomass production cost is a significant factor when considering the use of transgenic plants for protein expression. In the given problem, we have a cost-effective way of producing biomass using transgenic tobacco, with costs ranging from \(\$ 0.04-\$ 0.1 / \mathrm{kg}\). The low cost of biomass production in plants as compared to mammalian systems is largely due to the scalability of plant farming and the minimal requirements for maintenance and care.

However, this is just one part of the overall cost; downstream processing, especially the purification of the desired protein, can add substantially to the total expenses. Reducing these costs without compromising the purity and quality of the desired product is one of the challenges faced in using transgenic plants for protein production.

Chromatographic Purification

Chromatographic purification is a critical step in the production of therapeutic proteins. It involves separating the desired protein from various impurities based on differences in specific properties, such as size, charge, and binding affinity. In the context of transgenic tobacco protein expression, after initial volume reduction, chromatography plays a pivotal role in achieving high levels of purity.

A combination of different chromatographic techniques can be utilized. Affinity chromatography is particularly useful for capturing the target protein due to its high specificity. It uses a ligand that binds to the protein of interest, facilitating its separation from other components. Subsequently, ion-exchange chromatography separates proteins based on their charge, further refining the purity. The design of these chromatography steps needs to be precise and efficient to ensure that purification is cost-effective and meets the required purity standards of more than 90%.

Transgenic Tobacco for Protein Expression

Transgenic tobacco plants are engineered to carry and express foreign genes, allowing them to produce proteins not typically found in the plant. This technology is advantageous for mass-producing therapeutic proteins because it is cost-effective, scalable, and poses no risk of contamination with human pathogens.

In terms of therapeutic protein production, tobacco plants are especially appealing due to their biomass yield and growth rate. Transgenic tobacco has been successfully used to express proteins including enzymes, vaccines, and antibodies. In our specific scenario, we're interested in the expression of recombinant human serum albumin. Through genetic engineering, tobacco plants can be modified to produce this protein in their leaves, which can then be harvested and processed to extract and purify the protein of interest. Understanding the protein expression system in transgenic tobacco is crucial for optimizing yield and efficiency in protein production.