I am very confused about what one 'Katal' actually is. From Wikipedia,

"The katal is not used to express the rate of a reaction; that is expressed in units of concentration per second (or moles per liter per second). Rather, it is used to express catalytic activity which is a property of the catalyst. The katal is invariant of the measurement procedure, but the numerical quantity value is not and depends on the experimental conditions. Therefore, in order to define the quantity of a catalyst, the rate of conversion of a defined chemical reaction is specified as mols reacted per second. One katal of trypsin, for example, is that amount of trypsin which breaks a mole of peptide bonds per second under specified conditions."

So if a Katal is a specific AMOUNT of trypsin, then why is it expressed in units of moles per second. This is not dimensionally consistent! An 'amount' could be expressed as moles or mass, but certainly not as moles per second which is like a rate of reaction multiplied by volume...? The units and definition just seem to be all wrong.

If I were to take it that the Katal is the amount of enzyme catalysing one mole of a reaction per second, then this would mean that an enzyme with a smaller number of Katals of activity would actually be more active because less of it is required to catalyse one mole of the reaction per second?


2 Answers 2


A katal is indeed a specific amount of enzyme (rather than a reaction rate), but one which is measured with respect to the catalytic activity of the enzyme, rather than with respect to the number of molecules, their weight, or their size.

Roughly speaking, one katal is the amount of of enzyme which can convert 1 mole in 1 second. (Just like one pascal is the amount of pressure which applies one newton of force to an area of one square meter.) Or to rephrase things slightly, just like you can take an object of a particular length and express that length as "X meters" (being the number of meter sticks you can line up next to that length), you can take a given amount of enzyme and express the catalytic activity of that quantity of enzyme as "X katals", being the number of moles the enzyme can convert in one second.

This is not an intrinsic property of the enzyme itself. (It's an "extensive", not an "intensive" property.) The number of katals varies based on how much enzyme you have - if you have twice the amount of enzyme, you have twice the number of katals. (Just like you have twice the number of grams or twice the number of moles.)

As such, you can't talk about "an enzyme with a smaller number of katals of activity". I can understand the confusion, though, given the use of "activity" in the name. A paper the Wikipedia article references talks about the fact that there was some argument for calling the quantity being measured ‘catalytic amount’ rather than ‘catalytic activity’

As to why measuring enzyme amounts in katals versus moles or grams, the argument is similar to why there are separate units for mass (gram) and number (mole). It expresses a feature of the "size" of the enzyme not captured by the other units.


Although this is old news, as @R.M. has been good enough to provide an answer, I feel I should add my own. The reason is not that I think his answer is incorrect in any way — it is technically quite correct and I am not going to repeat or rephrase his technical arguments — but I do not think it addresses what I regard as the misunderstanding in the poster’s argument — a misunderstanding that others might make, even if the OP has progressed to other things.

The problem is one of semantics — What do you understand by the word ‘amount’?

The OP is clear. She writes:

“An 'amount' could be expressed as moles or mass, but certainly not as moles per second…”

So her understanding of the word amount is tied to the (extrinsic) mass property of an enzyme, which she clearly feels is intimately related to the number of molecules.

It seems to me that the way to deal with this argument is to point out that it is a misconception — there are properties of molecules other than mass that can be, and are, used to quantify them, and for enzymes catalytic activity is one such property, and the one that must be used in practice.

In practice, quantitation of molecules (the process that produces an amount) has to be performed in terms of their measurable properties when we are unable to ‘count their number’ (as it were). Mass is actually indirect — to be of use we need to know the purity of the compound and its molecular weight. Likewise we may quantify molecules by e.g. their light absorbance at a particuar wavelength, but to do this we need a molar reference value which will differ, e.g. between NADH and the amino acid tryptophan. Quantification using enzyme activity may be thought of in this light.


You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .