A differential labelling model for determining the number of catalytically essential carboxyl groups in fumarase.

We have developed a mathematical model of the nonideal case in which enzymatic activity changes may also result from modification of non-essential groups. As an illustration of this method, the number of essential carboxyl groups in pig heart fumarase (L-malate hydro-lyase, ED 4.2.1.2) was determined by the differential labeling technique. Enzymatic activity was related to the number of modified carboxyl groups according to the model and the results were compatible with the existence of two essential carboxyl groups in fumarase.