Control analysis of systems having two steps catalyzed by the same protein molecule in unbranched chains.

The analysis of the control of a metabolic pathway having an enzyme catalyzing two different reactions (or a protein displaying two different activities) has been performed. For such systems although the summation theorems are valid, the flux and concentration connectivity theorems of the metabolic control analysis are not valid. Another general relationship of control analysis is shown to be more widely obeyed and holds in these systems. An exemplary case, where the enzyme catalyzes two irreversible reactions, demonstrates that the level of one internal intermediate is constant, i.e. it does not depend upon the independent variables of the system.