A bit of help regarding the hydrolysis of activated carriers

[saab]

I have a question for you; which is on a past paper, I am doing for my biochem exam. It's question C, I have done the other parts.

High energy compounds, sometimes referred to as “activated carriers” (e.g. phosphoarginine) have large negative free energies of hydrolysis, indicating that their reactions with water are spontaneous and proceed almost to completion. Explain why millimolar concentrations of activated carriers like phosphoarginine are often present in cells.


The only reason I can think of is that reactions rarely go to completion, so the equilibrium lies very heavily towards completion, But I have a feeling that is not what they are looking for.

Phosphoargenine was mentioned earlier on in a previous question, so that is why it is mentioned here. The question is worth 10% of the total section, so I'm guessing they're looking for a short paragraph or so

If you could help me it would be very much appreciated,

Thanks Saab

[JackBean]

High energy compounds, sometimes referred to as “activated carriers” (e.g. phosphoarginine) have large negative free energies of hydrolysis, indicating that their reactions with water are spontaneous and proceed almost to completion. Explain why millimolar concentrations of activated carriers like phosphoarginine are often present in cells.

I think this is the first important clue. The second one is to what is phosphoarginine hydrolysed to? Are these compounds abundant in the cell?

[JorgeLobo]

Consider the context of shuttle or intermediate.

[saab]

Thanks for your reply,

Phosphoargenine -> Argenine and inorganic phosphate (Pi)

I'm unsure of their abundances within the cell, however, I have found some sample data

Phosphoargenine = 6.8mM
Argenine = 2.6mM
inorganic phosphate = 5mM

So both the substrate as well as the products exist in relatively similar concentrations which indicates there is something stopping phophoargenine going to completion. Is it because there isn't enough water to completely drive the reaction to completion? It's a long shot but I'm really stumped on this question.

As for shuttle or intermediate, I'm really unsure.

Any more feedback or help is very much appreciated.

[canalon]

Read the introduction of this for example...
http://jeb.biologists.org/content/204/6/1063.full.pdf

[JackBean]

I more thing. Of the issue could be also the reaction kinetics and dynamics (think about how stable is mix of oxygen and hydrogen if left alone and if burned)