Re: will catalase in diff. types of potatoes have diff. reactions w/ H202

Excellent questions Jamie,

As you correctly suggest, not all catalase enzymes are identical. However, you are really asking two different questions. First, you ask if the reactions carried out by catalases derived from different types of potatoes will differ, and second, you ask if differences can be measured between catalases from different types of potatoes. The answers to these two questions can be different, so lets take a closer look at catalase enzymes and see why.

First of all, catalase enzymes all carry out the same reaction when presented with hydrogen peroxide (H2O2); 2 H2O2 --> 2 H2O + O2. Two hydrogen peroxide molecules react, generating two water molecules and one molecule of oxygen. This reaction is the same regardless of where the enzyme is from. However, some enzymes that can carry out the catalase reaction can also carry out a second reaction called a peroxidation reaction. This reaction uses hydrogen peroxide to oxidize a reduced substrate (R), as follows; H2O2 + RH2 --> 2H2O + R.

The catalase enzymes that can only carry out the catalase reaction, which generates an oxygen molecule, are called 'true', 'typical', or ' Hydroxyperoxidase type II (HPII)' catalases. Other catalase enzymes can carry out both the catalase reaction and the peroxidase reaction. These are called 'atypical' or 'Hydroxyperoxidase type I (HPI)' catalases, or simply catalase-peroxidases. Both true catalases and catalase-peroxidases require the iron-containing enzymatic co-factor heme in order to complete their reactions. In addition, there is a third type of catalase which uses manganese as a co-factor instead of heme. These are called 'non-heme' or 'manganese' catalases, or 'pseudocatalases'.

So, we can see that there are three broad categories of catalases. This is because the catalase reaction is important to the survival of the cell; hydrogen peroxide can cause significant damage to a cell if it is not converted into water. Because of this, all organisms have catalase enzymes, and some of them have developed different versions of catalase via evolution. The HPII or true catalases are found in all eukaryotes (plants, animals, and fungi) as well as a large number of Archaea and Bacteria. As far as I can tell, catalase-peroxidases and pseudocatalases are only found in Archaea and Bacteria.

Now we can see that the catalase enzymes from all plants carry out the same reaction; so it doesn't matter what sort of potato your catalase enzyme comes from as far as the reaction goes. However, we can describe the rate at which an enzyme can carry out a given reaction using the Michaelis-Menten equation, one of several equations we can use to describe enzyme activity.

From these equations, we can derive two numbers that serve as constants for a given enzyme; Km, the Michaelis-Menten constant, and kcat, that enzyme's catalytic constant. Km represents the concentration of an enzyme's substrate at which the enzyme operates at half of its maximum velocity. Kcat represents the maximum number of substrate molecules that are converted to products per unit time. These two constants tell us a lot about how well an enzyme is functioning -- this is how the enzyme reaction would be rated as in the second part of your question.

I've looked around for values of Km and kcat for catalase, and I have found two sets of values. Most of the sources I have queried hold that catalase's Km is 0.025 moles / liter, and that its kcat is 1 x 10^7 molecules / second. However, a subset of sources list a Km of about 1.1 moles / liter, and a kcat of 4 x 10^7 molecules / second. This suggests to me that most catalases would be rated the same regardless of the organism that they came from, and possibly that there are a few out there that would be rated differently. However, I have to point out that the differences in Km and kcat might just be a math error on someone's part; one important calculation that tells you about the efficiency of an enzyme is the ratio of kcat/Km. The ratio for these examples differs only by a factor of 10.

Compare the Km and kcat values for catalases with this table of Km and kcat values for catalase-peroxidases.

So, since the scope of your question only pertained to potatoes, I feel fairly confident in saying that all potato catalases carry out the same reaction, and that these reactions would all be rated the same way.