I am working with a metabolic model of Saccharomyces cerevisiae and I am studying its growth on acetate and ammonia. I am performing Flux Balance Analysis to compute the growth rate and then I am exploring the flux distribution over all metabolic reactions.
The flux distribution shows that acetate is uptaken and transported to the mitochondria, where it produces Ac-CoA, which enters the TCA cycle. But also some acetate remains in the cytosol and produces Ac-CoA, which is used to synthesize fatty acids and to enter the glyoxylate cycle.
My question is the following: if acetate is the only carbon source supplied and ammonia is the nitrogen source, how can initially Ac-CoA enter the TCA or the glyoxylate cycles? In both cases oxalacetate is needed, which cannot be produced directly from Ac-CoA from other alternative pathways.
My only guess is that TCA cycle intermediates are being produced somehow and keep the cycle running, but as the nitrogen source is not even an aminoacid, I do not understand where this whole thing starts.
Thank you very much for your help!