Let's think ProteinA can phosphorylate proteinB, proteinC and proteinD.

Condition1: All proteins are expressed and proteinA phosphorylates proteinB, proteinC and proteinD.

Condition2: Only proteinA and proteinB are expressed for that reason protein A phosphorylates only proteinB.

Which is more realistic?

1) In condition1; proteinB, proteinC and proteinD will be 3 times less phosphorylated in comparison to proteinB in condition2.

2) The proteins of both conditions will be phosphorylated same.



  • $\begingroup$ Is this a homework question? $\endgroup$
    – Chris
    Jul 17, 2015 at 14:07
  • $\begingroup$ No it is not, why? $\endgroup$ Jul 17, 2015 at 15:36
  • $\begingroup$ it is a simplified version of my question which I could not find a clear answer. $\endgroup$ Jul 17, 2015 at 16:30
  • $\begingroup$ I really do not understand why the people just vote for this question instead of giving a simple answer. if a person do not have any answer about a question then how he/she can vote for the question. First give an answer then be able to vote the question... $\endgroup$ Jul 17, 2015 at 20:00
  • $\begingroup$ @Cankutçubuk please take the tour and read through the help center to understand how this and all Stack Exchange sites work. Voting is nothing personal, it merely reflects the quality of the question or answer being voted on. In this case, the question is not in line with our homework policy, which is slightly mis-named, as it not only applies to homework but to all trivial and/or un-researched questions, especially those (like this one) that look very much like homework or exam problems. (...) $\endgroup$
    – MattDMo
    Jul 18, 2015 at 4:13

1 Answer 1


Don't be discouraged. At least in vitro, if there is ample chemical substrates (e.g. ATP) and protein A (i.e. kinase) remains active, the kinase will just keep phosphorylating its protein substrates (B alone or a mixture of B, C, D) until the substrates are depleted.

Thus, all substrates will be phosphorylated eventually. The contrived Condition 1 may occur only if you only supply enough ATP for A to phosphorylate one mol equivalent of its protein substrate AND A has exactly the same catalytic efficiency toward B, C, and D that are present in equal amount. Of course, any or all of B/C/D may have more than one phosphorylation sites, each of which has its own phosphorylation rate.

  • $\begingroup$ @SKY thank you very much for your precious answer. $\endgroup$ Jul 18, 2015 at 11:36

Not the answer you're looking for? Browse other questions tagged or ask your own question.