In Situ Behavior of Pyrimidine Pathway Enzymes in Saccharomyces cerevisiae 4. The Channeling of Carbamylphosphate to Aspartate Transcarbamylase and Its Partition in the Pyrimidine and Arginine Pathways

In Saccharomyces cerevisiae the two first reactions of the pyrimidine pathway are catalyzed by a multifunctional protein bearing carbamylphosphate synthetase and aspartate transcarbamylase activities. The present study shows that this complex exhibits channeling of the intermediary metabolite carbamylphosphate, although this channeling is not absolute. Transient time to attain steady state and concentration of this intermediary metabolite were determined under different conditions. It is shown that the process of channeling does not significantly affect the concentration of the intermediary product. This result is in agreement with the theoretical modeling made by Cornish Bowden et al., The lack of channeling increases the transient time necessary to reach the steady rate of reaction by only a factor of three. In addition, channeling has only a small effect on the partition of carbamylphosphate between the pyrimidine and arginine biosynthetic pathway in a mutant strain devoid of the carbamylphosphate synthetase specific to the arginine pathway. The use of a mutant form of the complex suggests that it is between the carbamylphosphate synthetase and aspartate transcarbamylase catalytic sites belonging to the same polypeptide chain that channeling occurs.