Team:TU Delft-Leiden/Modeling/EET/Deterministic

Deterministic Model of the EET Module

One of the parts of our project is to enable cells to transport electrons to the extracellular environment, thus generating a current, as a response to a signal. To do this, the cell needs EET-complexes (Extracellular Electron Transport complexes). The EET complex consists of three proteins: MtrA, a cytochrome on the inside of the outer membrane, MtrB, a β-barrel protein located in the outer membrane, and MtrC, another cytochrome, located on the cell surface. This complex enables the cell to transport electrons from the cytoplasm of the cell to the extracellular environment [1].

The assembly of the trans-membrane EET complex depends on many factors other than transcriptional and translational control, as it requires a large amount of post-translational modifications. In this section, we will set up a simplified model of this assembly process, largely based on section 1.3 of the thesis of Jensen [1]. Our goal is to predict how many EET complexes are formed under different initial conditions.

In our modelling of the assembly of the EET complex, we will, in addition to the assembly mechanism, also focus on the apparent reduced cell viability. Jensen proposes two possible explanations for this: the formation of cytosolic aggregates, and reduced membrane integrity due to the high amount of trans-membrane protein complexes. We speculate that the specific carbon source (L-lactate) needed to enable extracellular electron transport might also reduce cell viability. For more information on this, please refer to Flux Balance Analysis.

Extensive Model

Since localization is an important part of the assembly process, we will consider compounds in different areas of the cell as different species in our model. We will distinguish between the cytosol (abbreviated as cyt), the periplasm (peri), the inner membrane (mem1) and the outer membrane (mem2).

An important part of the cytochromes that form part of the Mtr complex are heme molecules. Heme molecules enable the MtrA and MtrC proteins to accept and donate electrons. Heme is formed in the cytosol in a series of steps from the compound δ-aminolevunlinic acid (δ-ALA). After the heme is formed, it is transported to the periplasm. This process is catalyzed by the ccm cluster (ccmAH), which is located in the inner membrane [1]. These processes are described by reactions (1) and (2): $$ \delta - \ ALA(cyt) \ \xrightarrow{k_{1}} \ heme(cyt) \tag{1}$$ $$ heme(cyt) + \ ccmAH(mem1) \ \xrightarrow{k_{2}} \ heme(peri) + \ ccmAH(mem1) \tag{2}$$ As already mentioned, the ccm cluster, bound to the inner membrane transports the heme molecules from the cytosol to the periplasm. Also, the cluster catalyzes the binding of MtrA and MtrC to the heme, thereby forming cytochromes. The ccmAH proteins are produced in the cytosol. From there, they are transported to the periplasm. When in the periplasm, the ccm proteins will bind to the inner membrane [1]. To take into account the fact that the membrane is not capable of taking up an unlimited amount of ccm clusters, or that a high amount of such membrane proteins might reduce cell viability, we introduce a limited amount of “membrane binding sites” (for the inner membrane those are called mem1bindingsite, for the outer membrane mem2bindingsite). The processes involving the assembly of the ccm cluster are described by the reactions (3), (4) and (5). $$ \emptyset \ \xrightarrow{k_{3}} \ ccmAH(cyt) \tag{3}$$ $$ ccmAH(cyt) \ \xrightarrow{k_{4}} \ ccmAH(peri) \tag{4}$$ $$ ccmAH(peri) + \ mem1bindingsite \ \xrightarrow{k_{5}} \ ccmAH(mem1) \tag{5}$$ The Mtr complex consists of three proteins, MtrA, MtrB, and MtrC, which are expressed from the same operon. To model the production and initial transport of these proteins, we will consider them as being one protein, MtrCAB. This is of course not a completely accurate description of reality, but, since the proteins are produced and transported in the same amount, this simplification does not change the outcome of deterministic modeling and reduces the number of reaction equations by three. The production of the MtrCAB proteins in the cytosol is described by equation (6). From the cytosol, the proteins are transported to the periplasm by a type II secretion system (equation (7)). $$ \emptyset \ \xrightarrow{k_{6}} \ MtrCAB(cyt) \tag{6}$$ $$ MtrCAB(cyt) \ \xrightarrow{k_{7}} \ MtrCAB(peri) \tag{7}$$ From the moment the mtr proteins enter the periplasm, the different proteins start to undergo different post-translational modifications and therefore need to be considered separately. To facilitate this, we enter reaction (8), which describes the MtrCAB separating in the three individual proteins MtrA, MtrB and MtrC. Since the fact that MtrCAB behaved as one protein is a model simplification and not occurring in real life, this reaction also has no physical meaning. Its only purpose is to reduce the amount of reactions. Therefore, the reaction rate (\(k_{8}\)) will be extremely high compared to the rates of the other reactions. In this way, this reaction will not influence the overall speed of the assembly process. $$ MtrCAB(peri) \ \xrightarrow{k_{8}} \ MtrA(peri) + \ MtrB(peri) + \ MtrC(peri) \tag{8}$$ In the periplasm, the MtrA protein will react with a heme molecule to form a MtrA-heme complex. This allows the protein to accept and donate electrons. This heme ligation reaction is catalyzed by the ccm cluster. This process is described by equation (9): $$ MtrA(peri) + \ heme(peri) + \ ccmAH(mem1) \ \xrightarrow{k_{9}} \ MtrA-heme(peri) - \ heme(peri) + \ ccmAH(mem1) \tag{9}$$ The MtrC protein undergoes a similar heme ligation process, which is also catalyzed by ccmAH. This is described in equation (10). After heme ligation, the MtrC-heme complex has to undergo yet another post-translational modification, namely lipidization. In this reaction, a lipid is added to the MtrC-heme complex. This is described by equation (11). Since we have made the assumption that there will be an abundance of lipids, we do not consider the lipidization as a reaction between the protein and the lipid, but rather as a process that happens at a steady rate, independent of the lipid concentration. $$ MtrC(peri) + \ heme(peri) + \ ccmAH(mem1) \ \xrightarrow{k_{10}} \ MtrC-heme(peri) + \ ccmAH(mem1) \tag{10}$$ $$ MtrC-heme(peri) \ \xrightarrow{k_{11}} \ MtrC-heme-lipid(peri) \tag{11}$$ When the lipidized mtrC-heme complex is formed, it is translocated to the outer membrane, thereby filling one membrane binding site. The mtrB protein undergoes a similar process, also filling one membrane binding site. These processes are described by equations (12) and (13). $$ MtrC-heme-lipid(peri) + \ mem2bindingsite \ \xrightarrow{k_{12}} \ MtrC-heme-lipid(mem2) \tag{12}$$ $$ MtrB(peri) + \ mem2bindingsite \xrightarrow{k_{13}} \ MtrB(mem2) \tag{13}$$ Once the MtrB and lipidized MtrC-heme proteins are in place in the outer membrane, they can react with the MtrA-heme complex to form a transmembrane protein complex, which occupies one outer membrane binding site. This complex is capable of transporting electrons to the extracellular environment and will therefore be called EET (Extracellular Electron Transport complex). Since the MtrB and MtrC-heme complex both occupied one membrane binding site, this reaction frees one of those binding sites. This reaction is described by equation (14). $$ MtrA-heme(peri) + \ MtrB(mem2) + \ MtrC-heme-lipid(mem2) \ \xrightarrow{k_{14}} \ EET(mem2) + \ mem2bindingsite \tag{14}$$ The fourteen aforementioned reactions describe the assembly of the EET complex in quite some detail. However, it does not include the presumed formation of protein aggregates in the cytosol, which might be harmful for cell viability. Therefore, we included reaction (15), which describes the aggregation of the MtrCAB protein aggregates. $$ n \cdot MtrCAB(cyt) n\mbox{-} \ \xrightarrow{k_{15}} \ Aggregates(cyt) \tag{15}$$

Simplified Model