Feed-forward stimulation

Stimulation of PK In liver, pyruvate kina.se is stimulated by fructose 1,6-bisphosphate (feed-forward stimulation). This is especially important during the transition from fasting (gluconeogenesis, PK inhibited) to hpogenesis (PK active) (Chapter 21). 

Thus, they concluded that ATP-sulfurylase was subject to feed-forward stimulation by a reaction product of nitrate reduction and that nitrate reductase was subject to an analogous feed forward by a product of sulfate, thus coordinating the first step of each of the two pathways. Some support for this mechanism has come fi om experiments with cell cultures of Ipomea (Zink, 1984) and from measurements of ATP-sulfurylase and nitrate reductase in the leaves of whole tobacco plants (Barney and Bush, 1985), though in the latter study the response of ATP-sulfurylase to nitrogen stress was far less pronounced than the response of nitrate reductase to sulfur stress. However, as discussed below, APS sulfo-transferase rather than ATP-sulfurylase appears to be the principal determining site for the feed-forward control of sulfate assimilation by nitrogen assimilation products in Lemna. 

Pyrimidine synthesis is controlled at the first committed step. ATP stimulates the aspartate transcarbamoylase reaction, while CTP inhibits it. CTP is a feedback inhibitor of the pathway, and ATP is a feed-forward activator. This regulation ensures that a balanced supply of purines and pyrimidines exists for RNA and synthesis. 

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