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Abstract
Straube suggests that a model that reflects the bifunctional nature of the cycle enzyme uridylyltransferase/uridylyl-removing enzyme (UTase/UR) should be used, in which the UT and UR activities are distinct and reciprocally regulated activity states of the enzyme, and notes that if such a model is used, the effects of retroactivity at intermediate stimulation will be different. However, such a model does not accurately match the observed enzyme regulatory properties and fails to predict the ultrasensitive response obtained in the experiments. Here, we argue that modeling the UTase/UR enzyme as a bifunctional enzyme with reciprocally regulated activity states misses important aspects of the system.
