Adjustable Multi-Enzyme to Cell Surface Anchoring Protein

Abstract:

There are a plethora of enzymes that occur in the natural world which perform reactions that could be immensely useful to humans. Unfortunately, the efficiency of some of these reactions may render their applications impractical. The cellulosome scaffolding protein produced by Clostridium thermocellum has been shown to significantly increase the efficiency of cellulose degradation. It is possible that the scaffolding protein can be reduced in size and adapted for the cell surface of Escherichia coli. Different cohesion sites on the new cell surface display protein can also be introduced to allow attachment of desired enzymes. Future applications would include producing a collection of distinct versions of the scaffolding protein for unique arrangements and concentrations of enzymes, enabling construction of an extra-cellular assembly line for a variety of multi-enzymatic reactions. This would lay the foundation for making previously infeasible applications of enzymes possible through increased efficiency.

Inspiration: ^top^

Tuberculosis is caused by bacterial infections by Mycobacterium tuberculosis. The use of antibiotics in the treatment of tuberculosis is a double-edged sword: while curing the disease in some cases, it can also cause resistant mutants to emerge. Proper treatment of tuberculosis is typically a multi-drug regimen using first line anti-TB drugs: isoniazid, rifampin, pyrazinamide, ethambutol, and streptomycin (Long 425-428). However, if the regimen is not prescribed correctly or is not followed diligently by the patient due to misinformation or financial problems, the patient's population of tubercle bacilli may contain naturally drug-resistant mutants and those mutants can become a larger percentage of the population (Long 425-428). Tuberculosis is highly contagious and the spread of resistant mutants is causing more and more drug-resistant tuberculosis cases every year (“World Health Organization”).

A tuberculosis lesion within the body can contain 10^7–10^9 bacilli and generally 10-1000 of those are resistant to only one of the first line anti-TB drugs (Long 425-428). During mistreatment is more than 1% of the population is resistant to only one of the anti-TB drugs then it is termed drug-resistant (Long 425-428). In cases of multi- and extensive-drug resistant strains of Mycobacterium tuberculosis the bacteria is resistant to only two first line anti-TB drugs and more than three but not all anti-TB drugs respectively. Drug resistance theory is the most widely accepted explanation for why multi- and extensive-resistant tuberculosis strains are emerging. Drug-resistance is due to the selection of pre-existing resistant mutants in the original bacterial population by drug pressure (Long 425-428). The drug pressure in the case for tuberculosis is because Mycobacterium tuberculosis produces a mycolic acid, complex fatty acid, biofilm that protects it from the host’s immune system and makes drug delivery extremely difficult (Ojha, and et al 164-174). So with the mistreatment of the disease resistant mutants can become the majority infection and the drug selection pressure of another of the first line anti-TB drug can cause multi- and extensive-resistant tuberculosis cases (Long 425-428).

Background: ^top^

The original idea for our project was to start the first steps towards an anti-mycobacteria microbe capable of breaking down the mycolic acid biofilm around the bacilli and allow drugs and the host’s immune system to get rid of the infection. This proposal meant that standardized fatty acid degradation enzymes and an easy implementation of the degrading enzymes needed to be created. E. coli's own fatty oxidation pathway breaks down a couple different fatty acids including long chain and complex fatty acids, and done by a multi-subunit enzyme that has a alpha^2beta^2 conformation (Binstock, Pramanik, and Schulz 492-495). This multi-enzyme pathway can be isolated and over expressed in E. coli along with a cell surface display system engineered from Clostridium thermocellum's cellulosome. Clostridium thermocellum among other cellulose degrading organisms naturally produce and utilize a scaffolding protein known as the cellulosome. The structure has been shown to significantly increase the efficiency of the organisms’ cellulose degrading enzymes. The structure itself is composed of a number of smaller parts.

• The enzymatic subunits of the cellulosome include a variety of cellulose degrading enzymes which include binding regions know as type 1 dockerin regions.
• The type 1 dockerin regions of these enzymes bind to the type 1 cohesin regions located on the cellulosome scaffoldin protein.
• The scaffoldin also includes a single type 2 dockerin region which binds to a corresponding type 2 cohesin region.
• The type 2 cohesin region is part of a S-layer binding protein and effectively anchors the cellulosome to the surface of the cell.
• The scaffoldin also includes a cellulose binding domain which attaches to the substrate and further increases the efficiency of C. thermocellum’s cellulose degradation process.

There are a couple of characteristics that make the C. thermocellum cellulosome a good candidate for this project.

• The entirety of the cellulosome is located outside the cell. This means that substrates do not have to be taken into the cell before reactions can occur.
• The cellulosome keeps enzymes within close proximity to the cell. This would be ideal for applications in sensitive environments (like another organism).
• The cellulosome significantly increases the efficiency of C. thermocellum’s cellulose degrading enzymes. The system may do the same for other multiple enzyme processes.
• The cohesin dockerin interactions of a given species are specific to that species. The cohesin of one will not bind to the dockerin of another.

Project Goals: ^top^

There are a number of issues that will have to be addressed before the cellulosome can be used in more practical applications.

• The S-layer binding module is not compatible with gram negative bacteria like E. coli. It is unable to bind to the S-layer of E. coli due to the organism’s outer membrane.
• The cellulosome scaffoldin is coded with a larger gene (roughly 7kb long) that is more difficult to work with during PCR and in plasmids.
• The addition of a greater variety of cohesin and dockerin regions (from other organisms) would be necessary to give the user more control over how the enzymatic subunits bind to the scaffoldin.
• The attachment of enzymes of the user’s choice to the scaffoldin will require that said enzymes are made to incorporate the correct cohesin regions.

Our team will address the S-layer binding module issue, by hybridizing the type 2 cohesin of C. thermocellum with the LPP-OmpA part previously submitted to the parts registry by the washington iGEM team. The LPP-OmpA part is a transmembrane protein with an extracellular amino acid string that will allow us to attach the type 2 cohesin of Clostridium's sdbA anchoring module without affecting the protein's structure or functionality. The LPP-OmpA part will then act as the cellulosome's new anchoring module. To address the difficult size of the cipA cellulosome gene, the team will develop an abbreviated version of the cellulosome scaffoldin by removing 6 of its nine cohesion regions as well as its cellulose binding domain from its coding domain by PCR. This will be accomplished by using two sets of primers that will amplify 2 fragments of the scaffoldin gene during PCR. The first fragment termed miniA1 will code for the type 2 dockerin and the first of the nine type 1 cohesin modules. The second fragment termed miniA2 will code for the last two cohesin modules and a signaling peptide for exocytosis. The miniA1 fragment will begin with the iGEM standard prefix and end with a blunt restriction site, sac1, while the miniA2 fragment will begin with sac1 as miniA1 and end with the standard iGEM suffix. During assembly the two fragments will be combined at the shared blunt restriction site producing the miniA part.

Next Steps:

In the future, a library of cohesin and dockerin modules could be built for simplified assembly of customized scaffodins. To do this, the team would isolate cohesin and dockerin modules of a variety of types from a variety of cellulosome producing species.

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References: ^top^

Binstock, J. F., A. Pramanik, and H. Schulz. "Isolation of a Multi-enzyme Complex of Fatty Acid Oxidation from

"Escherichia coli"." Biochemistry. 74.2 (1977): 492-495. Web. 29 Sep. 2012.

Chao, Tiffany. "Tuberculosis Becoming More Drug-Resistant Worldwide." ABC News. ABC News Medical Unit, 30 August

2012. Web. 28 Sep 2012. .

Long, Robert. "Drug-resistant tuberculosis." Canadian Medical Association Journal. 163.4 (2000): 425-428. Web. 28

Sep. 2012. .

Ojha, et al. "Molecular Microbiology." Growth of "Mycobacterium tuberculosis" Biofilms Containing Free Mycolic Acids

and Harbouring Drug-tolerant Bacteria. 69.1 (2008): 164-174. Web. 29 Sep. 2012. .

"Tuberculosis." World Health Organization. N.p., March 2012. Web. 28 Sep 2012. .