http://www.howstuffworks.com/question115.htm

The reason why it foams is because blood and cells contain an enzyme called catalase. Since a cut or scrape contains both blood and damaged cells, there is lots of catalase floating around. 

When the catalase comes in contact with hydrogen peroxide, it turns the hydrogen peroxide (H2O2) into water (H2O) and oxygen gas (O2). 


H2O2 --> H2O + O2 
Catalase does this extremely efficiently -- up to 200,000 reactions per second. The bubbles you see in the foam are pure oxygen bubbles being created by the catalase.








http://www.cyberlipid.org/perox/oxid0003.htm

2 - Hydrogen peroxide (H2O2)

Hydrogen peroxide is mainly produced by enzymatic reactions. These enzymes are located in microsomes, peroxysomes and mitochondria. Even in normoxia conditions, the hydrogen peroxide production in relatively important and leads to a constant cellular concentration between 10-9 and 10-7 M. 

In plant and animal cells, superoxide dismutase is able to produce H2O2 by dismutation of O2.- , thus contributing to the lowering of oxidative reactions. The natural combination of dismutase and catalase contributes to remove H2O2 and thus has a true cellular antioxidant activity. H2O2 is also able to diffuse easily through cellular membranes 








http://metallo.scr
ipps.edu/PROMISE/PEROXIDASES.html#reaction

Peroxidases are haem­containing enzymes that use hydrogen peroxide (H2O2) as the electron acceptor to catalyse a number of oxidative reactions. Most haem peroxidases follow the reaction scheme [1, 2]: 



In this mechanism, the enzyme reacts with one equivalent of H2O2 to give compound I, a porphyrin ­cation radical containing FeIV. This is a two­electron oxidation/reduction reaction where H2O2 is reduced to water and the enzyme is oxidised. One oxidising equivalent resides on iron, giving the oxyferryl (FeIV=O) intermediate. Compound I then oxidises an organic substrate to give a substrate radical (·AH). Compound I undergoes a second one­electron oxidation reaction yielding compound II, which contains an oxyferryl centre coordinated to a normal (dianionic) porphyrin ligand. Finally, compound II, is reduced back to the native ferric state with concomitant one­electron substrate oxidation. The overall charge on the resting state and compound I is +1, while compound II is neutral (cf. catalase and chloroperoxidase). 

Peroxidases are found in bacteria, fungi, plants and animals. On the basis of sequence similarity, haem peroxidases can be categorised into two superfamilies: fungal, plant and bacterial peroxidases form one superfamily [3] and animal enzymes form another [4].