The Synthesizing Unit as model for the stoichiometric fusion and branching of metabolic fluxes.

The Synthesizing Unit (SU) binds given numbers of substrate molecules of several types of substrate to produce a product molecule or set of product molecules. Irreversible binding results in relatively simple and explicit expressions for the rate of product formation. Reversible binding can be implemented with relative ease in the carrier-SU complex, where the products of a set of carriers (a special type of SU) serve as substrate for an SU or set of SUs. A simple and parameter sparse approximation is presented for the production rate of a generalized compound, i.e. a rich mixture of compounds that does not change in composition. An analysis of Droop's data on the growth of a haptophyte on phosphate and vitamin B(12) reserves illustrates the application of SUs.