How does salt affect the enzyme activity of catalase breaking down H2O2?

Catalase is an enzyme which breaks down hydrogen peroxide into water and oxygen. It does so by way of a "decomposition reaction." Since catalase is an enzyme, it lowers the activation energy, or the amount of energy needed for a reaction to occur. At the same time, it releases energy as it decomposes hydrogen peroxide into its constituents by breaking bonds. That's why when you put hydrogen peroxide on an open wound, not only does it tickle when the bubbles start to form, but you also notice it gets warm, from the release of energy as heat.

Salt either interacts with amino acids that make up the enzyme or is coming in the way of the active site where H2O2 binds.

If the salt concentration is close to zero, the charged amino acid side chains of the enzyme molecules will attract each other. The enzyme will denature and form an inactive precipitate. If, on the other hand, the salt concentration is very high, normal interaction of charged groups will be blocked, new interactions will occur, and again the enzyme will precipitate. An intermediate salt concentration, such as that of human blood (0.9%) or cytoplasm, is the optimum for many enzymes.

^ What he said