Sorry, but you cannot do a model based on wrong biochemistry. APS is reduced to free sulfite and not sulfoglutathione, this has been shown more than 10 years ago. If you looked into some sulfur literature, you would find plenty of papers describing flux through sulfate assimilation, which is nothing else but sulfate reduction rate.

Thank you for your response, Dr. Kopriva. You are right about the reduction of APS to free sulphite - the sulfugluthathione is merely an intermediate compound in the process. (please see attached an image summarizing the sulphate pathway by Leustek (1999)). Researchers show that this is the step that regulates the speed of the entire process. Hence, I was trying to find out literature on how fast this step occurs.

Sorry, but you are wrong. It is the other way round, first sulfite is formed and then it reacts with oxidised glutahione to make sulfoglutathione. That is actually explained in the Suter paper you cite. If you want to learn about S metabolism, there are plenty of more up-to-date papers than Leustek 1999, e.g. the Annual review from 2011.

The Suter paper says exactly what you said. Thanks a lot! I had one more question: would this step be considered the rate-determining step for sulphate assimilation?

I believe this is the paper that you mentioned.

http://www.annualreviews.org/doi/pdf/10.1146/annurev-arplant-042110-103921

Unfortunately, I do not have access to the full downloadable PDF version. Could you please give me a copy?

Of course the Suter paper says that, I was involved in the work.

As for the rate-determining step, we showed in Vauclare et al. 2002 that APS reductase has the highest control over the flux. There are more papers showing the role of this enzyme, but also of ATP sulfurylase and sulfite reductase. But this information is easily available in PubMed or Web of Science, just needs a little effort in reading.