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Prof. Eric Arnoys

catalase

The bombardier beetle relies on simple but effective chemistry to defend itself from predators. When threatened, it shoots a stream of boiling hot spray at the attacker:

Figure 1A from T. Eisner and D.J. Aneshansley, "Spray aiming in the bombardier beetle: Photographic evidence," PNAS USA 96, pp. 9705–9709.

You can also see a bombardier beetle in action from this (and many others) YouTube video.

How is the mix of boiling irritants propelled at the attacker? It turns out that a sample decomposition reaction provides both the gaseous expansion for propulsion and the heat needed to boil the solution. Hydrogen peroxide is held in one pouch, awaiting the proper catalyst for its decomposition:

2H2O2(aq) → 2H2O(l) + O2(g)

In one simple reaction the beetle generates the heat and the explosive force for its self-defense. Heating of the solution results from the standard enthalpy of decomposition of -98.2 kJ/mol, and oxygen evolution expands to a volume ~500 fold for the molecules that react.

catalase

This brings us to our protein: the appropriately named catalase (this is the structure of the human isoform, PDB code 1QQW, structure generated with PyMOL). Most species of catalase are found as tetramers and employ the iron in a heme to oxidize the peroxide, resulting in a reaction rate only limited by diffusion. If RuBisCO is a tortoise, catalase is an overly-caffeinated hare--it is unlikely that you'll find a faster enzyme..

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