|MadSci Network: Biochemistry|
Hi there Juniper,
Thanks for the question. This kind of question is a biochemist's dream.
You are probably aware that catalase is one of the most intensively
studied of all enzymes; it was one of the first to be purified to
homogeneity and it's of special interest because of the great speed at
which it works. Catalase can approach a turnover number of 200 000
reactions per second, which is close to the theoretical rate at which the
reactants can diffuse together. So you quite rightly ask, how can it do
this so much better than iron alone? We might extend this to a more
general question: how do enzymes catalyse their reactions at rates so
much greater than the uncatalysed reaction? But let's stay with catalase
First, let's look at the reaction that is catalysed by catalase. It is the decomposition of hydrogen peroxide (an oxidised species) into water and molecular oxygen.
2H2O2 -----> 2H2O + O2
OK. What do we need for this reaction to occur? First, we need a place
for the hydrogen peroxide molecule to bind to. Second, because water is a
less oxidising species than hydrogen peroxide, we need a way to move
electrons around. Third, we need a source of protons,
because wherever reactions involve electrons moving around, we need to
have protons moving around too.
Now, imagine a single iron ion in solution. This ion may be capable of binding to hydrogen peroxide. And the iron ion is capable of simple redox transformations between the +2 and +3 oxidation states. But of course, these reactants have to diffuse together and stay together for the reaction to occur. Clearly, if we had all the required elements assembled in one place, things would be much faster. This is how catalase works-like all enzymes, it has evolved to have an active site, at which the reactants can be assembled.
So, how does catalase work? First, it contains a channel into which hydrogen peroxide can diffuse. Within this channel is a chemical group called a haem (or heme) group, consisting of Fe(III) bound at the centre of a ring-like structure called a porphyrin ring. Haem groups are common in biology and are frequently involved with electron transfer reactions (as in cytochrome c) or with ligand binding (as in haemoglobin). The haem group in catalase is vital to the reaction, because it can be oxidised from Fe(III) to the very oxidised and less common Fe(IV), or ferryl species.
H2O2 + Fe(III)-enzyme ----> H2O + O=Fe (IV)-enzyme
The first part of our reaction is complete; hydrogen peroxide has bound to
the haem group and oxidised it to Fe(IV). But things are more complicated
than they appear! As you rightly suggested, the protein structure is also
involved. In this case, one of the protons of the hydrogen peroxide
molecule has been removed from one end of the molecule and placed at the
other end. The proton is transferred via a histidine residue in the
active site. This action polarises and breaks the O-O bond in hydrogen
peroxide, releasing the water molecule.
Now for the second part of the reaction-we must return the enzyme to the Fe (III) state and reduce the second molecule of hydrogen peroxide to water.
H2O2 + O=Fe(IV)-enzyme ----> H2O + Fe (III)-enzyme
Because we have made a highly-oxidising Fe(IV) species, we can now react with the second peroxide molecule, releasing water and an oxygen molecule.
The key factor in catalase is the ability of the iron in the haem group to
form a ferryl species. This is enhanced by the presence of a nearby
tyrosine residue, which is a ligand to the iron in the haem group. The
tyrosine is in the ionised phenolate form (O-) - it has lost
its proton due to the electron-withdrawing power of the haem ring and of a
nearby arginine residue. In summary, the properties of the haem group and
its environment are everything!
I hope that this has aided your understanding of catalase to some degree - you specified a desire to understand the high-level biochemistry in your question! There are lots of sources of information about catalase on the Internet, but I especially recommend this one:
Here you will find a lot of the information in this answer and more. To view the rotating molecules you will need the CHIME plug-in for your web browser-all the instructions can be found at the above website.
Try the links in the MadSci Library for more information on Biochemistry.