Team:Edinburgh/Cellulases (C model)

How does this whole synergy thing even work? It can seem almost magical that having enzymes closer together can increase their usefulness, but consider the following statements about cellulose degradation, all of which are true ([http://www.annualreviews.org/doi/abs/10.1146/annurev-biochem-091208-085603 Fontes and Gilbert, 2010])

1. Exoglucanase chews away at the end of a cellulase chain, producing cellobiose sugars.
2. Endoglucanase cuts cellulose chains in the middle, essentially turning one chain into two.
3. Cellobiose inhibits the action of the above enzymes.
4. β-glucosidase cuts cellobiose in half, producing two glucose molecules.

Facts 1 and 2 mean that exoglucanase works best with endoglucanase nearby, since every time endoglucanase acts, it produces new chain-ends for exoglucanase to attack. Meanwhile, facts 1, 3, and 4 mean that β-glucosidase helps nearby copies of the other enzymes by preventing the cellobiose from inhibiting them.

Syn: a simple demonstration of synergy

We can demonstrate that there is (potentially) a huge difference between the synergistic and non-synergistic systems with Edinburgh's Syn C program. This is a 2D simulation that works in the following way:

• The world is a 2D grid.
  • Each spot in the grid can contain a sugar, a bond between sugars, or nothing.
• Cellulose is modelled as alternating sugars and bonds: s-b-s-b-s-b-s-b-s-b-s etc etc.
• There are three enzyme types: endoglucanase, exoglucanase, and β-glucosidase.
  • The enzymes move about randomly, in a "brownian motion" manner.
  • If an enzyme is in the same place as a bond, it can cut it:
    • Endoglucanase can only cut bonds away from the ends of a chain.
    • Exoglucanase can only cut bonds if this results in a cellobiose molecule (s-b-s) forming.
    • β-glucosidase can only cut cellobiose bonds.
• There is inhibition of exoglucanase by cellobiose; it does not cut bonds if there is a nearby cellobiose molecule.

Sugars are dark green squares, bonds between sugars are light green. From left to right are shown the actions of endoglucanase, exoglucanase, and β-glucosidase.

Simulating synergy / non-synergy

We can run two different simulations with the same settings. However, in one simulation the enzymes float about freely, whereas in the other they travel in triplets, each triplet containing one of each type of enzyme, side by side:

The enzymes' current positions are displayed as white, yellow, or blue squares. Note that in the synergistic system (right) they travel together.

So, what happens if we run a large simulation? This:

Iteration 4000 of a run with 20 copies of each enzyme per simulation. The left side has 732 free glucose molecules. The right (synergistic) side has 2492.

C code

There are two releases. The first is for people (probably Unix users of some type) who have the development files for the [http://www.libsdl.org/ SDL graphics library] (called libsdl1.2-dev in Ubuntu) installed. This version gives a nice, visual display of what's happening, but is much slower. The second version simply spits out .bmp image files at set intervals, but is fast and should compile on any machine with a C compiler.

The actual code is the same; the only difference is that the first version contains a Makefile that causes the SDL functions to be compiled in, whereas if the program is simply compiled with "gcc syn.c" or similar, the SDL functions are left out.

• Syn-sdl.zip
• Syn-nosdl.c

Conclusion

Synergy - it actually works!

References

• Fontes CMGA, Gilbert HJ (2010) [http://www.annualreviews.org/doi/abs/10.1146/annurev-biochem-091208-085603 Cellulosomes: highly efficient nanomachines designed to deconstruct plant cell wall complex carbohydrates]. Annual Review of Biochemistry 79: 655-81 (doi: 10.1146/annurev-biochem-091208-085603).