[Maxina]

I'm a biologist and my questions are about ELISA, but strictly from the chemical point of view.
1. When peroxidase reacts with acidic solution of TMB (3,3',5,5' tetramethylbenzidine) and H2O2, the color is changed from colorless lo blue. I assume that in the presense of peroxidase H2O2 turns to H2O and H+, and than H+ does something to TMB. Could you tell me the right mechanism of the reaction and which bonds are responsible for the color change of the solution (aromatic ring?).
2. The reaction is stopped by 0.5M HCl addition and the color of the solution changes from blue to yellow. Do you think HCl cause peroxidase denaturation or it reacts somehow with TMB/H2O2 solution in a way that the enzyme has no substrate to work on? And also the color change - why it changes from blue to yellow after HCl addition?